difference between assignment statement and expression

Statement vs Expression – What's the Difference in Programming?

Learning the syntax of a programming language is key if you want to use that language effectively. This is true for both new and experienced developers.

And one of the most important things to pay attention to while learning a programming language is whether the code you're dealing with is a statement or an expression.

It can sometimes be confusing to differentiate between statements and expressions in programming. So this article is meant to simplify the differences so that you can improve your programming skills and become a better developer.

What is an Expression in Programming?

An expression is any word or group of words or symbols that is a value. In programming, an expression is a value, or anything that executes and ends up being a value.

It is necessary to understand that a value is unique. For example, const , let , 2 , 4 , s , a , true , false , and world are values because each of them is unique in meaning or character.

Let's look at some code as an example:

Judging from the code above, const , price , = , and 500 are expressions because each of them has a definite and unique meaning or value. But if we take all of them together const price = 500 - then we have a statement.

Let's look at another example:

Looking at the code above, you can see an anonymous function is assigned to a variable. Oh, wait! You might know that any function is a statement. Can it also be an expression?

Yes! A "function" and a "class" are both statements and expressions because they can perform actions (do or not do tasks) and still execute to a value.

This brings us to statements – so what are they?

What is a Statement in Programming?

A statement is a group of expressions and/or statements that you design to carry out a task or an action.

Statements are two-sided – that is, they either do tasks or don't do them. Any statement that can return a value is automatically qualified to be used as an expression. That is why a function or class is a statement and also an expression in JavaScript.

If you look at the example of the function under the section on expressions, you can see it is assigned and execute to a value passed to a variable. That is why it is an expression in that case.

Examples of Statements in Programming

Inline statements.

The whole of the code above is a statement because it carries out the task of assigning $2000 to amount . It is safe to say a line of code is a statement because most compilers or interpreters don't execute any standalone expression.

Block statements

Look at the below if statement:

The if statement is a statement because it helps us check whether I love you or not. As I have said before, it is two-sided: this code finds out whether "I love you" or not, and that is why it is a statement. Also, it doesn't return any value but it can create side effects.

Here's a loop statement:

In short, any loop is a statement because if it can only do the tasks it is meant to do or not – does loop and doesn't loop. But a loop can't execute to a value in the end. They can only have side effects in JavaScript. Once they can execute to a value in a programming language, then they can also be used as an expression.

For example, you can use forloop and if statement as expressions in Python.

There is also an "IF" expression in Python. That means that something that is a statement in one language can be an expression (or both statement and expression) in another.

Look at the below function statement:

We declare the function add(firstNumber, secondNumber) and it returns a value. The function is called with two arguments as in add(2, 3) by declaration and so it is a statement. If you pay close attention, you will realize that calling the function as a statement is useless since it has no side effect.

Hey, stop! How can we turn it into an expression? Oh yeah, we can do it like this:

Though the function is now an expression the way it is called above, the whole of the code is still a statement.

Check out this class statement:

You can see that we declare the class "Person" and instantiate and assign it to "User" immediately. So, it is used as an expression.

Now, let's use it as a statement:

A class is similar to a function in the sense that it can be declared, assigned, or used as an operand just like a class. So, a class is a statement and/or an expression.

The Main Differences Between an Expression and a Statement in Programming

Expressions can be assigned or used as operands, while statements can only be declared.

Statements create side effects to be useful, while expressions are values or execute to values.

Expressions are unique in meaning, while statements are two-sided in execution. For example, 1 has a certain value while go( ) may be executed or not.

Statements are the whole structure, while expressions are the building blocks. For example, a line or a block of code is a statement.

Why You Should Know the Difference

First of all, understanding the difference between statements and expressions should make learning new programming languages less surprising. If you're used to JavaScript, you may be surprised by Python's ability to assign an if statement as a variable which is not possible in JavaScript.

Second, it makes it easy to use programming paradigms across different programming languages.

For example, a JavaScript "if statement" cannot be used as an expression because it can't execute to a value – it can only create side effects. Yet, you can use the ternary operator if you want to avoid the side effects of using an if statement in JavaScript.

For this reason, you can understand why some programmers avoid if statements by using the ternary operator in JavaScript. It is because they want to avoid side effects .

It also makes your realize why you have to be always careful about the scope of your variables whenever you use a statement. This is true because statements mostly have side effects to be useful, and it is reasonable to understand the scope of your variables and operations. For example,

Hey wait! What would be logged in the console if you ran the code above?

Tell yourself the answer first and then paste the code in the console to confirm. If you you're wrong, you need to learn more about scope and side effects. But if you're right, try to make those functions a bit better to avoid the confusion they may generate.

Knowing the difference also helps you to easily identify non-composable and composable syntaxes (functions, classes, modules, and so on) of a programming language. This makes porting your experience from one programming language to another more interesting and direct.

Wrapping Up

Now that you understand the difference between expressions and statements in programming, and you know why understanding the differences is important, you can identify pieces of code as expressions or statements while coding.

Next time, we'll go even further and help make learning a second programming language easier.

Go and get things done now! See you soon.

I am planning to share a lot about programming tips and tutorials in 2023. If you're struggling to build projects or you want to stay connected with my write-ups and videos, please join my list at YouTooCanCode or subscribe to my YouTube channel at You Too Can Code on YouTube .

Ayobami loves writing history with JavaScript(React) and PHP(Laravel). He has been making programming fun to learn for learners. Check him out on YouTube: https://bit.ly/3usOu3s

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Python Enhancement Proposals

• Python »
• PEP Index »

PEP 572 – Assignment Expressions

The importance of real code, exceptional cases, scope of the target, relative precedence of :=, change to evaluation order, differences between assignment expressions and assignment statements, specification changes during implementation, _pydecimal.py, datetime.py, sysconfig.py, simplifying list comprehensions, capturing condition values, changing the scope rules for comprehensions, alternative spellings, special-casing conditional statements, special-casing comprehensions, lowering operator precedence, allowing commas to the right, always requiring parentheses, why not just turn existing assignment into an expression, with assignment expressions, why bother with assignment statements, why not use a sublocal scope and prevent namespace pollution, style guide recommendations, acknowledgements, a numeric example, appendix b: rough code translations for comprehensions, appendix c: no changes to scope semantics.

This is a proposal for creating a way to assign to variables within an expression using the notation NAME := expr .

As part of this change, there is also an update to dictionary comprehension evaluation order to ensure key expressions are executed before value expressions (allowing the key to be bound to a name and then re-used as part of calculating the corresponding value).

During discussion of this PEP, the operator became informally known as “the walrus operator”. The construct’s formal name is “Assignment Expressions” (as per the PEP title), but they may also be referred to as “Named Expressions” (e.g. the CPython reference implementation uses that name internally).

Naming the result of an expression is an important part of programming, allowing a descriptive name to be used in place of a longer expression, and permitting reuse. Currently, this feature is available only in statement form, making it unavailable in list comprehensions and other expression contexts.

Additionally, naming sub-parts of a large expression can assist an interactive debugger, providing useful display hooks and partial results. Without a way to capture sub-expressions inline, this would require refactoring of the original code; with assignment expressions, this merely requires the insertion of a few name := markers. Removing the need to refactor reduces the likelihood that the code be inadvertently changed as part of debugging (a common cause of Heisenbugs), and is easier to dictate to another programmer.

During the development of this PEP many people (supporters and critics both) have had a tendency to focus on toy examples on the one hand, and on overly complex examples on the other.

The danger of toy examples is twofold: they are often too abstract to make anyone go “ooh, that’s compelling”, and they are easily refuted with “I would never write it that way anyway”.

The danger of overly complex examples is that they provide a convenient strawman for critics of the proposal to shoot down (“that’s obfuscated”).

Yet there is some use for both extremely simple and extremely complex examples: they are helpful to clarify the intended semantics. Therefore, there will be some of each below.

However, in order to be compelling , examples should be rooted in real code, i.e. code that was written without any thought of this PEP, as part of a useful application, however large or small. Tim Peters has been extremely helpful by going over his own personal code repository and picking examples of code he had written that (in his view) would have been clearer if rewritten with (sparing) use of assignment expressions. His conclusion: the current proposal would have allowed a modest but clear improvement in quite a few bits of code.

Another use of real code is to observe indirectly how much value programmers place on compactness. Guido van Rossum searched through a Dropbox code base and discovered some evidence that programmers value writing fewer lines over shorter lines.

Case in point: Guido found several examples where a programmer repeated a subexpression, slowing down the program, in order to save one line of code, e.g. instead of writing:

they would write:

Another example illustrates that programmers sometimes do more work to save an extra level of indentation:

This code tries to match pattern2 even if pattern1 has a match (in which case the match on pattern2 is never used). The more efficient rewrite would have been:

Syntax and semantics

In most contexts where arbitrary Python expressions can be used, a named expression can appear. This is of the form NAME := expr where expr is any valid Python expression other than an unparenthesized tuple, and NAME is an identifier.

The value of such a named expression is the same as the incorporated expression, with the additional side-effect that the target is assigned that value:

There are a few places where assignment expressions are not allowed, in order to avoid ambiguities or user confusion:

This rule is included to simplify the choice for the user between an assignment statement and an assignment expression – there is no syntactic position where both are valid.

Again, this rule is included to avoid two visually similar ways of saying the same thing.

This rule is included to disallow excessively confusing code, and because parsing keyword arguments is complex enough already.

This rule is included to discourage side effects in a position whose exact semantics are already confusing to many users (cf. the common style recommendation against mutable default values), and also to echo the similar prohibition in calls (the previous bullet).

The reasoning here is similar to the two previous cases; this ungrouped assortment of symbols and operators composed of : and = is hard to read correctly.

This allows lambda to always bind less tightly than := ; having a name binding at the top level inside a lambda function is unlikely to be of value, as there is no way to make use of it. In cases where the name will be used more than once, the expression is likely to need parenthesizing anyway, so this prohibition will rarely affect code.

This shows that what looks like an assignment operator in an f-string is not always an assignment operator. The f-string parser uses : to indicate formatting options. To preserve backwards compatibility, assignment operator usage inside of f-strings must be parenthesized. As noted above, this usage of the assignment operator is not recommended.

An assignment expression does not introduce a new scope. In most cases the scope in which the target will be bound is self-explanatory: it is the current scope. If this scope contains a nonlocal or global declaration for the target, the assignment expression honors that. A lambda (being an explicit, if anonymous, function definition) counts as a scope for this purpose.

There is one special case: an assignment expression occurring in a list, set or dict comprehension or in a generator expression (below collectively referred to as “comprehensions”) binds the target in the containing scope, honoring a nonlocal or global declaration for the target in that scope, if one exists. For the purpose of this rule the containing scope of a nested comprehension is the scope that contains the outermost comprehension. A lambda counts as a containing scope.

The motivation for this special case is twofold. First, it allows us to conveniently capture a “witness” for an any() expression, or a counterexample for all() , for example:

Second, it allows a compact way of updating mutable state from a comprehension, for example:

However, an assignment expression target name cannot be the same as a for -target name appearing in any comprehension containing the assignment expression. The latter names are local to the comprehension in which they appear, so it would be contradictory for a contained use of the same name to refer to the scope containing the outermost comprehension instead.

For example, [i := i+1 for i in range(5)] is invalid: the for i part establishes that i is local to the comprehension, but the i := part insists that i is not local to the comprehension. The same reason makes these examples invalid too:

While it’s technically possible to assign consistent semantics to these cases, it’s difficult to determine whether those semantics actually make sense in the absence of real use cases. Accordingly, the reference implementation [1] will ensure that such cases raise SyntaxError , rather than executing with implementation defined behaviour.

This restriction applies even if the assignment expression is never executed:

For the comprehension body (the part before the first “for” keyword) and the filter expression (the part after “if” and before any nested “for”), this restriction applies solely to target names that are also used as iteration variables in the comprehension. Lambda expressions appearing in these positions introduce a new explicit function scope, and hence may use assignment expressions with no additional restrictions.

Due to design constraints in the reference implementation (the symbol table analyser cannot easily detect when names are re-used between the leftmost comprehension iterable expression and the rest of the comprehension), named expressions are disallowed entirely as part of comprehension iterable expressions (the part after each “in”, and before any subsequent “if” or “for” keyword):

A further exception applies when an assignment expression occurs in a comprehension whose containing scope is a class scope. If the rules above were to result in the target being assigned in that class’s scope, the assignment expression is expressly invalid. This case also raises SyntaxError :

(The reason for the latter exception is the implicit function scope created for comprehensions – there is currently no runtime mechanism for a function to refer to a variable in the containing class scope, and we do not want to add such a mechanism. If this issue ever gets resolved this special case may be removed from the specification of assignment expressions. Note that the problem already exists for using a variable defined in the class scope from a comprehension.)

See Appendix B for some examples of how the rules for targets in comprehensions translate to equivalent code.

The := operator groups more tightly than a comma in all syntactic positions where it is legal, but less tightly than all other operators, including or , and , not , and conditional expressions ( A if C else B ). As follows from section “Exceptional cases” above, it is never allowed at the same level as = . In case a different grouping is desired, parentheses should be used.

The := operator may be used directly in a positional function call argument; however it is invalid directly in a keyword argument.

Some examples to clarify what’s technically valid or invalid:

Most of the “valid” examples above are not recommended, since human readers of Python source code who are quickly glancing at some code may miss the distinction. But simple cases are not objectionable:

This PEP recommends always putting spaces around := , similar to PEP 8 ’s recommendation for = when used for assignment, whereas the latter disallows spaces around = used for keyword arguments.)

In order to have precisely defined semantics, the proposal requires evaluation order to be well-defined. This is technically not a new requirement, as function calls may already have side effects. Python already has a rule that subexpressions are generally evaluated from left to right. However, assignment expressions make these side effects more visible, and we propose a single change to the current evaluation order:

• In a dict comprehension {X: Y for ...} , Y is currently evaluated before X . We propose to change this so that X is evaluated before Y . (In a dict display like {X: Y} this is already the case, and also in dict((X, Y) for ...) which should clearly be equivalent to the dict comprehension.)

Most importantly, since := is an expression, it can be used in contexts where statements are illegal, including lambda functions and comprehensions.

Conversely, assignment expressions don’t support the advanced features found in assignment statements:

• Multiple targets are not directly supported: x = y = z = 0 # Equivalent: (z := (y := (x := 0)))
• Single assignment targets other than a single NAME are not supported: # No equivalent a [ i ] = x self . rest = []
• Priority around commas is different: x = 1 , 2 # Sets x to (1, 2) ( x := 1 , 2 ) # Sets x to 1
• Iterable packing and unpacking (both regular or extended forms) are not supported: # Equivalent needs extra parentheses loc = x , y # Use (loc := (x, y)) info = name , phone , * rest # Use (info := (name, phone, *rest)) # No equivalent px , py , pz = position name , phone , email , * other_info = contact
• Inline type annotations are not supported: # Closest equivalent is "p: Optional[int]" as a separate declaration p : Optional [ int ] = None
• Augmented assignment is not supported: total += tax # Equivalent: (total := total + tax)

The following changes have been made based on implementation experience and additional review after the PEP was first accepted and before Python 3.8 was released:

• for consistency with other similar exceptions, and to avoid locking in an exception name that is not necessarily going to improve clarity for end users, the originally proposed TargetScopeError subclass of SyntaxError was dropped in favour of just raising SyntaxError directly. [3]
• due to a limitation in CPython’s symbol table analysis process, the reference implementation raises SyntaxError for all uses of named expressions inside comprehension iterable expressions, rather than only raising them when the named expression target conflicts with one of the iteration variables in the comprehension. This could be revisited given sufficiently compelling examples, but the extra complexity needed to implement the more selective restriction doesn’t seem worthwhile for purely hypothetical use cases.

Examples from the Python standard library

env_base is only used on these lines, putting its assignment on the if moves it as the “header” of the block.

• Current: env_base = os . environ . get ( "PYTHONUSERBASE" , None ) if env_base : return env_base
• Improved: if env_base := os . environ . get ( "PYTHONUSERBASE" , None ): return env_base

Avoid nested if and remove one indentation level.

• Current: if self . _is_special : ans = self . _check_nans ( context = context ) if ans : return ans
• Improved: if self . _is_special and ( ans := self . _check_nans ( context = context )): return ans

Code looks more regular and avoid multiple nested if. (See Appendix A for the origin of this example.)

• Current: reductor = dispatch_table . get ( cls ) if reductor : rv = reductor ( x ) else : reductor = getattr ( x , "__reduce_ex__" , None ) if reductor : rv = reductor ( 4 ) else : reductor = getattr ( x , "__reduce__" , None ) if reductor : rv = reductor () else : raise Error ( "un(deep)copyable object of type %s " % cls )
• Improved: if reductor := dispatch_table . get ( cls ): rv = reductor ( x ) elif reductor := getattr ( x , "__reduce_ex__" , None ): rv = reductor ( 4 ) elif reductor := getattr ( x , "__reduce__" , None ): rv = reductor () else : raise Error ( "un(deep)copyable object of type %s " % cls )

tz is only used for s += tz , moving its assignment inside the if helps to show its scope.

• Current: s = _format_time ( self . _hour , self . _minute , self . _second , self . _microsecond , timespec ) tz = self . _tzstr () if tz : s += tz return s
• Improved: s = _format_time ( self . _hour , self . _minute , self . _second , self . _microsecond , timespec ) if tz := self . _tzstr (): s += tz return s

Calling fp.readline() in the while condition and calling .match() on the if lines make the code more compact without making it harder to understand.

• Current: while True : line = fp . readline () if not line : break m = define_rx . match ( line ) if m : n , v = m . group ( 1 , 2 ) try : v = int ( v ) except ValueError : pass vars [ n ] = v else : m = undef_rx . match ( line ) if m : vars [ m . group ( 1 )] = 0
• Improved: while line := fp . readline (): if m := define_rx . match ( line ): n , v = m . group ( 1 , 2 ) try : v = int ( v ) except ValueError : pass vars [ n ] = v elif m := undef_rx . match ( line ): vars [ m . group ( 1 )] = 0

A list comprehension can map and filter efficiently by capturing the condition:

Similarly, a subexpression can be reused within the main expression, by giving it a name on first use:

Note that in both cases the variable y is bound in the containing scope (i.e. at the same level as results or stuff ).

Assignment expressions can be used to good effect in the header of an if or while statement:

Particularly with the while loop, this can remove the need to have an infinite loop, an assignment, and a condition. It also creates a smooth parallel between a loop which simply uses a function call as its condition, and one which uses that as its condition but also uses the actual value.

An example from the low-level UNIX world:

Rejected alternative proposals

Proposals broadly similar to this one have come up frequently on python-ideas. Below are a number of alternative syntaxes, some of them specific to comprehensions, which have been rejected in favour of the one given above.

A previous version of this PEP proposed subtle changes to the scope rules for comprehensions, to make them more usable in class scope and to unify the scope of the “outermost iterable” and the rest of the comprehension. However, this part of the proposal would have caused backwards incompatibilities, and has been withdrawn so the PEP can focus on assignment expressions.

Broadly the same semantics as the current proposal, but spelled differently.

Since EXPR as NAME already has meaning in import , except and with statements (with different semantics), this would create unnecessary confusion or require special-casing (e.g. to forbid assignment within the headers of these statements).

(Note that with EXPR as VAR does not simply assign the value of EXPR to VAR – it calls EXPR.__enter__() and assigns the result of that to VAR .)

Additional reasons to prefer := over this spelling include:

• In if f(x) as y the assignment target doesn’t jump out at you – it just reads like if f x blah blah and it is too similar visually to if f(x) and y .
• import foo as bar
• except Exc as var
• with ctxmgr() as var

To the contrary, the assignment expression does not belong to the if or while that starts the line, and we intentionally allow assignment expressions in other contexts as well.

• NAME = EXPR
• if NAME := EXPR

reinforces the visual recognition of assignment expressions.

This syntax is inspired by languages such as R and Haskell, and some programmable calculators. (Note that a left-facing arrow y <- f(x) is not possible in Python, as it would be interpreted as less-than and unary minus.) This syntax has a slight advantage over ‘as’ in that it does not conflict with with , except and import , but otherwise is equivalent. But it is entirely unrelated to Python’s other use of -> (function return type annotations), and compared to := (which dates back to Algol-58) it has a much weaker tradition.

This has the advantage that leaked usage can be readily detected, removing some forms of syntactic ambiguity. However, this would be the only place in Python where a variable’s scope is encoded into its name, making refactoring harder.

Execution order is inverted (the indented body is performed first, followed by the “header”). This requires a new keyword, unless an existing keyword is repurposed (most likely with: ). See PEP 3150 for prior discussion on this subject (with the proposed keyword being given: ).

This syntax has fewer conflicts than as does (conflicting only with the raise Exc from Exc notation), but is otherwise comparable to it. Instead of paralleling with expr as target: (which can be useful but can also be confusing), this has no parallels, but is evocative.

One of the most popular use-cases is if and while statements. Instead of a more general solution, this proposal enhances the syntax of these two statements to add a means of capturing the compared value:

This works beautifully if and ONLY if the desired condition is based on the truthiness of the captured value. It is thus effective for specific use-cases (regex matches, socket reads that return '' when done), and completely useless in more complicated cases (e.g. where the condition is f(x) < 0 and you want to capture the value of f(x) ). It also has no benefit to list comprehensions.

Advantages: No syntactic ambiguities. Disadvantages: Answers only a fraction of possible use-cases, even in if / while statements.

Another common use-case is comprehensions (list/set/dict, and genexps). As above, proposals have been made for comprehension-specific solutions.

This brings the subexpression to a location in between the ‘for’ loop and the expression. It introduces an additional language keyword, which creates conflicts. Of the three, where reads the most cleanly, but also has the greatest potential for conflict (e.g. SQLAlchemy and numpy have where methods, as does tkinter.dnd.Icon in the standard library).

As above, but reusing the with keyword. Doesn’t read too badly, and needs no additional language keyword. Is restricted to comprehensions, though, and cannot as easily be transformed into “longhand” for-loop syntax. Has the C problem that an equals sign in an expression can now create a name binding, rather than performing a comparison. Would raise the question of why “with NAME = EXPR:” cannot be used as a statement on its own.

As per option 2, but using as rather than an equals sign. Aligns syntactically with other uses of as for name binding, but a simple transformation to for-loop longhand would create drastically different semantics; the meaning of with inside a comprehension would be completely different from the meaning as a stand-alone statement, while retaining identical syntax.

Regardless of the spelling chosen, this introduces a stark difference between comprehensions and the equivalent unrolled long-hand form of the loop. It is no longer possible to unwrap the loop into statement form without reworking any name bindings. The only keyword that can be repurposed to this task is with , thus giving it sneakily different semantics in a comprehension than in a statement; alternatively, a new keyword is needed, with all the costs therein.

There are two logical precedences for the := operator. Either it should bind as loosely as possible, as does statement-assignment; or it should bind more tightly than comparison operators. Placing its precedence between the comparison and arithmetic operators (to be precise: just lower than bitwise OR) allows most uses inside while and if conditions to be spelled without parentheses, as it is most likely that you wish to capture the value of something, then perform a comparison on it:

Once find() returns -1, the loop terminates. If := binds as loosely as = does, this would capture the result of the comparison (generally either True or False ), which is less useful.

While this behaviour would be convenient in many situations, it is also harder to explain than “the := operator behaves just like the assignment statement”, and as such, the precedence for := has been made as close as possible to that of = (with the exception that it binds tighter than comma).

Some critics have claimed that the assignment expressions should allow unparenthesized tuples on the right, so that these two would be equivalent:

(With the current version of the proposal, the latter would be equivalent to ((point := x), y) .)

However, adopting this stance would logically lead to the conclusion that when used in a function call, assignment expressions also bind less tight than comma, so we’d have the following confusing equivalence:

The less confusing option is to make := bind more tightly than comma.

It’s been proposed to just always require parentheses around an assignment expression. This would resolve many ambiguities, and indeed parentheses will frequently be needed to extract the desired subexpression. But in the following cases the extra parentheses feel redundant:

Frequently Raised Objections

C and its derivatives define the = operator as an expression, rather than a statement as is Python’s way. This allows assignments in more contexts, including contexts where comparisons are more common. The syntactic similarity between if (x == y) and if (x = y) belies their drastically different semantics. Thus this proposal uses := to clarify the distinction.

The two forms have different flexibilities. The := operator can be used inside a larger expression; the = statement can be augmented to += and its friends, can be chained, and can assign to attributes and subscripts.

Previous revisions of this proposal involved sublocal scope (restricted to a single statement), preventing name leakage and namespace pollution. While a definite advantage in a number of situations, this increases complexity in many others, and the costs are not justified by the benefits. In the interests of language simplicity, the name bindings created here are exactly equivalent to any other name bindings, including that usage at class or module scope will create externally-visible names. This is no different from for loops or other constructs, and can be solved the same way: del the name once it is no longer needed, or prefix it with an underscore.

(The author wishes to thank Guido van Rossum and Christoph Groth for their suggestions to move the proposal in this direction. [2] )

As expression assignments can sometimes be used equivalently to statement assignments, the question of which should be preferred will arise. For the benefit of style guides such as PEP 8 , two recommendations are suggested.

• If either assignment statements or assignment expressions can be used, prefer statements; they are a clear declaration of intent.
• If using assignment expressions would lead to ambiguity about execution order, restructure it to use statements instead.

The authors wish to thank Alyssa Coghlan and Steven D’Aprano for their considerable contributions to this proposal, and members of the core-mentorship mailing list for assistance with implementation.

Appendix A: Tim Peters’s findings

Here’s a brief essay Tim Peters wrote on the topic.

I dislike “busy” lines of code, and also dislike putting conceptually unrelated logic on a single line. So, for example, instead of:

instead. So I suspected I’d find few places I’d want to use assignment expressions. I didn’t even consider them for lines already stretching halfway across the screen. In other cases, “unrelated” ruled:

is a vast improvement over the briefer:

The original two statements are doing entirely different conceptual things, and slamming them together is conceptually insane.

In other cases, combining related logic made it harder to understand, such as rewriting:

as the briefer:

The while test there is too subtle, crucially relying on strict left-to-right evaluation in a non-short-circuiting or method-chaining context. My brain isn’t wired that way.

But cases like that were rare. Name binding is very frequent, and “sparse is better than dense” does not mean “almost empty is better than sparse”. For example, I have many functions that return None or 0 to communicate “I have nothing useful to return in this case, but since that’s expected often I’m not going to annoy you with an exception”. This is essentially the same as regular expression search functions returning None when there is no match. So there was lots of code of the form:

I find that clearer, and certainly a bit less typing and pattern-matching reading, as:

It’s also nice to trade away a small amount of horizontal whitespace to get another _line_ of surrounding code on screen. I didn’t give much weight to this at first, but it was so very frequent it added up, and I soon enough became annoyed that I couldn’t actually run the briefer code. That surprised me!

There are other cases where assignment expressions really shine. Rather than pick another from my code, Kirill Balunov gave a lovely example from the standard library’s copy() function in copy.py :

The ever-increasing indentation is semantically misleading: the logic is conceptually flat, “the first test that succeeds wins”:

Using easy assignment expressions allows the visual structure of the code to emphasize the conceptual flatness of the logic; ever-increasing indentation obscured it.

A smaller example from my code delighted me, both allowing to put inherently related logic in a single line, and allowing to remove an annoying “artificial” indentation level:

That if is about as long as I want my lines to get, but remains easy to follow.

So, in all, in most lines binding a name, I wouldn’t use assignment expressions, but because that construct is so very frequent, that leaves many places I would. In most of the latter, I found a small win that adds up due to how often it occurs, and in the rest I found a moderate to major win. I’d certainly use it more often than ternary if , but significantly less often than augmented assignment.

I have another example that quite impressed me at the time.

Where all variables are positive integers, and a is at least as large as the n’th root of x, this algorithm returns the floor of the n’th root of x (and roughly doubling the number of accurate bits per iteration):

It’s not obvious why that works, but is no more obvious in the “loop and a half” form. It’s hard to prove correctness without building on the right insight (the “arithmetic mean - geometric mean inequality”), and knowing some non-trivial things about how nested floor functions behave. That is, the challenges are in the math, not really in the coding.

If you do know all that, then the assignment-expression form is easily read as “while the current guess is too large, get a smaller guess”, where the “too large?” test and the new guess share an expensive sub-expression.

To my eyes, the original form is harder to understand:

This appendix attempts to clarify (though not specify) the rules when a target occurs in a comprehension or in a generator expression. For a number of illustrative examples we show the original code, containing a comprehension, and the translation, where the comprehension has been replaced by an equivalent generator function plus some scaffolding.

Since [x for ...] is equivalent to list(x for ...) these examples all use list comprehensions without loss of generality. And since these examples are meant to clarify edge cases of the rules, they aren’t trying to look like real code.

Note: comprehensions are already implemented via synthesizing nested generator functions like those in this appendix. The new part is adding appropriate declarations to establish the intended scope of assignment expression targets (the same scope they resolve to as if the assignment were performed in the block containing the outermost comprehension). For type inference purposes, these illustrative expansions do not imply that assignment expression targets are always Optional (but they do indicate the target binding scope).

Let’s start with a reminder of what code is generated for a generator expression without assignment expression.

• Original code (EXPR usually references VAR): def f (): a = [ EXPR for VAR in ITERABLE ]
• Translation (let’s not worry about name conflicts): def f (): def genexpr ( iterator ): for VAR in iterator : yield EXPR a = list ( genexpr ( iter ( ITERABLE )))

Let’s add a simple assignment expression.

• Original code: def f (): a = [ TARGET := EXPR for VAR in ITERABLE ]
• Translation: def f (): if False : TARGET = None # Dead code to ensure TARGET is a local variable def genexpr ( iterator ): nonlocal TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Let’s add a global TARGET declaration in f() .

• Original code: def f (): global TARGET a = [ TARGET := EXPR for VAR in ITERABLE ]
• Translation: def f (): global TARGET def genexpr ( iterator ): global TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Or instead let’s add a nonlocal TARGET declaration in f() .

• Original code: def g (): TARGET = ... def f (): nonlocal TARGET a = [ TARGET := EXPR for VAR in ITERABLE ]
• Translation: def g (): TARGET = ... def f (): nonlocal TARGET def genexpr ( iterator ): nonlocal TARGET for VAR in iterator : TARGET = EXPR yield TARGET a = list ( genexpr ( iter ( ITERABLE )))

Finally, let’s nest two comprehensions.

• Original code: def f (): a = [[ TARGET := i for i in range ( 3 )] for j in range ( 2 )] # I.e., a = [[0, 1, 2], [0, 1, 2]] print ( TARGET ) # prints 2
• Translation: def f (): if False : TARGET = None def outer_genexpr ( outer_iterator ): nonlocal TARGET def inner_generator ( inner_iterator ): nonlocal TARGET for i in inner_iterator : TARGET = i yield i for j in outer_iterator : yield list ( inner_generator ( range ( 3 ))) a = list ( outer_genexpr ( range ( 2 ))) print ( TARGET )

Because it has been a point of confusion, note that nothing about Python’s scoping semantics is changed. Function-local scopes continue to be resolved at compile time, and to have indefinite temporal extent at run time (“full closures”). Example:

This document has been placed in the public domain.

Source: https://github.com/python/peps/blob/main/peps/pep-0572.rst

Last modified: 2023-10-11 12:05:51 GMT

• Table of Contents
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• Scratch ActiveCode
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• Instructors Guide
• About Runestone
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• 1.1 Preface
• 1.2 Why Programming? Why Java?
• 1.3 Variables and Data Types
• 1.4 Expressions and Assignment Statements
• 1.5 Compound Assignment Operators
• 1.6 Casting and Ranges of Variables
• 1.7 Java Development Environments (optional)
• 1.8 Unit 1 Summary
• 1.9 Unit 1 Mixed Up Code Practice
• 1.10 Unit 1 Coding Practice
• 1.11 Multiple Choice Exercises
• 1.12 Lesson Workspace
• 1.3. Variables and Data Types" data-toggle="tooltip">
• 1.5. Compound Assignment Operators' data-toggle="tooltip" >

1.4. Expressions and Assignment Statements ¶

In this lesson, you will learn about assignment statements and expressions that contain math operators and variables.

1.4.1. Assignment Statements ¶

Remember that a variable holds a value that can change or vary. Assignment statements initialize or change the value stored in a variable using the assignment operator = . An assignment statement always has a single variable on the left hand side of the = sign. The value of the expression on the right hand side of the = sign (which can contain math operators and other variables) is copied into the memory location of the variable on the left hand side.

Figure 1: Assignment Statement (variable = expression) ¶

Instead of saying equals for the = operator in an assignment statement, say “gets” or “is assigned” to remember that the variable on the left hand side gets or is assigned the value on the right. In the figure above, score is assigned the value of 10 times points (which is another variable) plus 5.

The following video by Dr. Colleen Lewis shows how variables can change values in memory using assignment statements.

As we saw in the video, we can set one variable to a copy of the value of another variable like y = x;. This won’t change the value of the variable that you are copying from.

Click on the Show CodeLens button to step through the code and see how the values of the variables change.

The program is supposed to figure out the total money value given the number of dimes, quarters and nickels. There is an error in the calculation of the total. Fix the error to compute the correct amount.

Calculate and print the total pay given the weekly salary and the number of weeks worked. Use string concatenation with the totalPay variable to produce the output Total Pay = $3000 . Don’t hardcode the number 3000 in your print statement.

Assume you have a package with a given height 3 inches and width 5 inches. If the package is rotated 90 degrees, you should swap the values for the height and width. The code below makes an attempt to swap the values stored in two variables h and w, which represent height and width. Variable h should end up with w’s initial value of 5 and w should get h’s initial value of 3. Unfortunately this code has an error and does not work. Use the CodeLens to step through the code to understand why it fails to swap the values in h and w.

1-4-7: Explain in your own words why the ErrorSwap program code does not swap the values stored in h and w.

Swapping two variables requires a third variable. Before assigning h = w , you need to store the original value of h in the temporary variable. In the mixed up programs below, drag the blocks to the right to put them in the right order.

The following has the correct code that uses a third variable named “temp” to swap the values in h and w.

The code is mixed up and contains one extra block which is not needed in a correct solution. Drag the needed blocks from the left into the correct order on the right, then check your solution. You will be told if any of the blocks are in the wrong order or if you need to remove one or more blocks.

After three incorrect attempts you will be able to use the Help Me button to make the problem easier.

Fix the code below to perform a correct swap of h and w. You need to add a new variable named temp to use for the swap.

1.4.2. Incrementing the value of a variable ¶

If you use a variable to keep score you would probably increment it (add one to the current value) whenever score should go up. You can do this by setting the variable to the current value of the variable plus one (score = score + 1) as shown below. The formula looks a little crazy in math class, but it makes sense in coding because the variable on the left is set to the value of the arithmetic expression on the right. So, the score variable is set to the previous value of score + 1.

Click on the Show CodeLens button to step through the code and see how the score value changes.

1-4-11: What is the value of b after the following code executes?

• It sets the value for the variable on the left to the value from evaluating the right side. What is 5 * 2?
• Correct. 5 * 2 is 10.

1-4-12: What are the values of x, y, and z after the following code executes?

• x = 0, y = 1, z = 2
• These are the initial values in the variable, but the values are changed.
• x = 1, y = 2, z = 3
• x changes to y's initial value, y's value is doubled, and z is set to 3
• x = 2, y = 2, z = 3
• Remember that the equal sign doesn't mean that the two sides are equal. It sets the value for the variable on the left to the value from evaluating the right side.
• x = 1, y = 0, z = 3

1.4.3. Operators ¶

Java uses the standard mathematical operators for addition ( + ), subtraction ( - ), multiplication ( * ), and division ( / ). Arithmetic expressions can be of type int or double. An arithmetic operation that uses two int values will evaluate to an int value. An arithmetic operation that uses at least one double value will evaluate to a double value. (You may have noticed that + was also used to put text together in the input program above – more on this when we talk about strings.)

Java uses the operator == to test if the value on the left is equal to the value on the right and != to test if two items are not equal. Don’t get one equal sign = confused with two equal signs == ! They mean different things in Java. One equal sign is used to assign a value to a variable. Two equal signs are used to test a variable to see if it is a certain value and that returns true or false as you’ll see below. Use == and != only with int values and not doubles because double values are an approximation and 3.3333 will not equal 3.3334 even though they are very close.

Run the code below to see all the operators in action. Do all of those operators do what you expected? What about 2 / 3 ? Isn’t surprising that it prints 0 ? See the note below.

When Java sees you doing integer division (or any operation with integers) it assumes you want an integer result so it throws away anything after the decimal point in the answer, essentially rounding down the answer to a whole number. If you need a double answer, you should make at least one of the values in the expression a double like 2.0.

With division, another thing to watch out for is dividing by 0. An attempt to divide an integer by zero will result in an ArithmeticException error message. Try it in one of the active code windows above.

Operators can be used to create compound expressions with more than one operator. You can either use a literal value which is a fixed value like 2, or variables in them. When compound expressions are evaluated, operator precedence rules are used, so that *, /, and % are done before + and -. However, anything in parentheses is done first. It doesn’t hurt to put in extra parentheses if you are unsure as to what will be done first.

In the example below, try to guess what it will print out and then run it to see if you are right. Remember to consider operator precedence .

1-4-15: Consider the following code segment. Be careful about integer division.

What is printed when the code segment is executed?

• 0.666666666666667
• Don't forget that division and multiplication will be done first due to operator precedence.
• Yes, this is equivalent to (5 + ((a/b)*c) - 1).
• Don't forget that division and multiplication will be done first due to operator precedence, and that an int/int gives an int result where it is rounded down to the nearest int.

1-4-16: Consider the following code segment.

What is the value of the expression?

• Dividing an integer by an integer results in an integer
• Correct. Dividing an integer by an integer results in an integer
• The value 5.5 will be rounded down to 5

1-4-17: Consider the following code segment.

• Correct. Dividing a double by an integer results in a double
• Dividing a double by an integer results in a double

1-4-18: Consider the following code segment.

• Correct. Dividing an integer by an double results in a double
• Dividing an integer by an double results in a double

1.4.4. The Modulo Operator ¶

The percent sign operator ( % ) is the mod (modulo) or remainder operator. The mod operator ( x % y ) returns the remainder after you divide x (first number) by y (second number) so 5 % 2 will return 1 since 2 goes into 5 two times with a remainder of 1. Remember long division when you had to specify how many times one number went into another evenly and the remainder? That remainder is what is returned by the modulo operator.

Figure 2: Long division showing the whole number result and the remainder ¶

In the example below, try to guess what it will print out and then run it to see if you are right.

The result of x % y when x is smaller than y is always x . The value y can’t go into x at all (goes in 0 times), since x is smaller than y , so the result is just x . So if you see 2 % 3 the result is 2 .

1-4-21: What is the result of 158 % 10?

• This would be the result of 158 divided by 10. modulo gives you the remainder.
• modulo gives you the remainder after the division.
• When you divide 158 by 10 you get a remainder of 8.

1-4-22: What is the result of 3 % 8?

• 8 goes into 3 no times so the remainder is 3. The remainder of a smaller number divided by a larger number is always the smaller number!
• This would be the remainder if the question was 8 % 3 but here we are asking for the reminder after we divide 3 by 8.
• What is the remainder after you divide 3 by 8?

1.4.5. FlowCharting ¶

Assume you have 16 pieces of pizza and 5 people. If everyone gets the same number of slices, how many slices does each person get? Are there any leftover pieces?

In industry, a flowchart is used to describe a process through symbols and text. A flowchart usually does not show variable declarations, but it can show assignment statements (drawn as rectangle) and output statements (drawn as rhomboid).

The flowchart in figure 3 shows a process to compute the fair distribution of pizza slices among a number of people. The process relies on integer division to determine slices per person, and the mod operator to determine remaining slices.

Figure 3: Example Flow Chart ¶

A flowchart shows pseudo-code, which is like Java but not exactly the same. Syntactic details like semi-colons are omitted, and input and output is described in abstract terms.

Complete the program based on the process shown in the Figure 3 flowchart. Note the first line of code declares all 4 variables as type int. Add assignment statements and print statements to compute and print the slices per person and leftover slices. Use System.out.println for output.

1.4.6. Storing User Input in Variables ¶

Variables are a powerful abstraction in programming because the same algorithm can be used with different input values saved in variables.

Figure 4: Program input and output ¶

A Java program can ask the user to type in one or more values. The Java class Scanner is used to read from the keyboard input stream, which is referenced by System.in . Normally the keyboard input is typed into a console window, but since this is running in a browser you will type in a small textbox window displayed below the code. The code below shows an example of prompting the user to enter a name and then printing a greeting. The code String name = scan.nextLine() gets the string value you enter as program input and then stores the value in a variable.

Run the program a few times, typing in a different name. The code works for any name: behold, the power of variables!

Run this program to read in a name from the input stream. You can type a different name in the input window shown below the code.

Try stepping through the code with the CodeLens tool to see how the name variable is assigned to the value read by the scanner. You will have to click “Hide CodeLens” and then “Show in CodeLens” to enter a different name for input.

The Scanner class has several useful methods for reading user input. A token is a sequence of characters separated by white space.

Run this program to read in an integer from the input stream. You can type a different integer value in the input window shown below the code.

A rhomboid (slanted rectangle) is used in a flowchart to depict data flowing into and out of a program. The previous flowchart in Figure 3 used a rhomboid to indicate program output. A rhomboid is also used to denote reading a value from the input stream.

Figure 5: Flow Chart Reading User Input ¶

Figure 5 contains an updated version of the pizza calculator process. The first two steps have been altered to initialize the pizzaSlices and numPeople variables by reading two values from the input stream. In Java this will be done using a Scanner object and reading from System.in.

Complete the program based on the process shown in the Figure 5 flowchart. The program should scan two integer values to initialize pizzaSlices and numPeople. Run the program a few times to experiment with different values for input. What happens if you enter 0 for the number of people? The program will bomb due to division by zero! We will see how to prevent this in a later lesson.

The program below reads two integer values from the input stream and attempts to print the sum. Unfortunately there is a problem with the last line of code that prints the sum.

Run the program and look at the result. When the input is 5 and 7 , the output is Sum is 57 . Both of the + operators in the print statement are performing string concatenation. While the first + operator should perform string concatenation, the second + operator should perform addition. You can force the second + operator to perform addition by putting the arithmetic expression in parentheses ( num1 + num2 ) .

More information on using the Scanner class can be found here https://www.w3schools.com/java/java_user_input.asp

1.4.7. Programming Challenge : Dog Years ¶

In this programming challenge, you will calculate your age, and your pet’s age from your birthdates, and your pet’s age in dog years. In the code below, type in the current year, the year you were born, the year your dog or cat was born (if you don’t have one, make one up!) in the variables below. Then write formulas in assignment statements to calculate how old you are, how old your dog or cat is, and how old they are in dog years which is 7 times a human year. Finally, print it all out.

Calculate your age and your pet’s age from the birthdates, and then your pet’s age in dog years. If you want an extra challenge, try reading the values using a Scanner.

1.4.8. Summary ¶

Arithmetic expressions include expressions of type int and double.

The arithmetic operators consist of +, -, * , /, and % (modulo for the remainder in division).

An arithmetic operation that uses two int values will evaluate to an int value. With integer division, any decimal part in the result will be thrown away, essentially rounding down the answer to a whole number.

An arithmetic operation that uses at least one double value will evaluate to a double value.

Operators can be used to construct compound expressions.

During evaluation, operands are associated with operators according to operator precedence to determine how they are grouped. (*, /, % have precedence over + and -, unless parentheses are used to group those.)

An attempt to divide an integer by zero will result in an ArithmeticException to occur.

The assignment operator (=) allows a program to initialize or change the value stored in a variable. The value of the expression on the right is stored in the variable on the left.

During execution, expressions are evaluated to produce a single value.

The value of an expression has a type based on the evaluation of the expression.

Understanding the Difference – Statements vs. Expressions Explained

Introduction.

Understanding the difference between statements and expressions is crucial in programming. These two concepts are fundamental building blocks of any programming language and play a significant role in how code is executed and produces results. In this blog post, we will delve into statements and expressions, their definitions, characteristics, and provide examples in various programming languages.

Statements in programming are units of code that perform actions or operations. They are the instructions that tell the computer what to do. Statements can range from simple assignment statements to complex control flow structures like if/else statements, switch statements, and loops. Additionally, function or method calls are also considered statements. In programming languages, statements are typically terminated with a semicolon (;) to indicate their completion. The use of semicolons allows the compiler or interpreter to distinguish between different statements and ensures proper execution order. Let’s look at some examples of common types of statements: – Assignment statements: These statements assign a value to a variable. For example, in Java, we can have `int x = 5;` where the value 5 is assigned to the variable `x`. – Control flow statements: These statements determine the flow of the program based on different conditions. Examples include if/else statements, switch statements, and loops. For instance, in Python, we can have an if/else statement like this: “`python if x > 0: print(“Positive number”) else: print(“Negative number”) “` – Function/method calls: These statements invoke a function or method with certain arguments. In JavaScript, we can have a function call like this: `console.log(“Hello, World!”);` where the `console.log()` function is called, and the string “Hello, World!” is passed as an argument. Statements are an essential part of any programming language as they allow us to control the flow of execution and perform actions on data.

Expressions

Expressions, on the other hand, are units of code that produce values. Unlike statements that perform actions, expressions are evaluated to obtain a result. Expressions can be as simple as a single variable or can involve complex mathematical calculations and logical operations. In programming, expressions can be categorized into several types: – Arithmetic expressions: These expressions involve mathematical calculations like addition, subtraction, multiplication, and division. For instance, in C++, we can have an arithmetic expression like this: `int sum = x + y;` where the sum of variables `x` and `y` is assigned to the variable `sum`. – Relational expressions: These expressions compare values and produce a boolean result (true or false). For example, in Python, we can have a relational expression like this: `x > 5` which checks if `x` is greater than 5 and returns either `True` or `False`. – Boolean expressions: These expressions evaluate to a boolean value (`true` or `false`). They can involve logical operators such as AND, OR, and NOT. In Java, we can have a boolean expression like this: `isTrue = x > 10 && y and if `y` is less than 20. Expressions play a crucial role in programming as they allow us to manipulate data and make decisions based on the evaluated result.

Key Differences between Statements and Expressions

While statements and expressions are both essential components of programming, they have several key differences that are important to understand. Let’s explore these differences: Conceptual differences: 1. Statements are executed for their side effects, while expressions are evaluated for their resulting values. Statements perform actions like assigning values, controlling program flow, or invoking functions. Expressions, on the other hand, produce values that can be used further in the code. 2. Statements cannot be nested within expressions, but expressions can be nested within statements. Statements are standalone units of code and cannot be used as part of an expression. In contrast, expressions can be embedded within statements to perform calculations or evaluations. Syntax differences: 1. Statements usually end with a semicolon, while expressions may or may not. As mentioned earlier, statements in most programming languages are terminated with a semicolon to indicate their completion. Expressions, however, do not always require a semicolon at the end, depending on the specific programming language syntax. 2. Statements have a specific structure defined by the programming language, while expressions can be more flexible. Statements follow a predetermined structure dictated by the programming language. Expressions, on the other hand, can vary in structure and complexity depending on the specific requirements of the program. Practical implications of understanding the differences: 1. Understanding the differences between statements and expressions helps developers avoid errors and confusion when writing code. Mixing up statements and expressions or using them inappropriately can lead to unexpected results or syntax errors. For example, trying to assign a value to an expression instead of a variable may result in a syntax error. 2. Having a clear understanding of statements and expressions improves code readability and maintainability. By appropriately using statements for actions and expressions for calculations, code becomes more organized and easier for other developers to understand and modify.

In conclusion, statements and expressions are fundamental concepts in programming, and understanding the difference between them is crucial for writing effective code. Statements perform actions or operations, while expressions produce values. Recognizing the conceptual and syntax differences between statements and expressions allows developers to write error-free code and maximize code readability and maintainability. I encourage you to deepen your understanding of statements and expressions by exploring further examples and practicing their usage in different programming languages. Mastery of these fundamental concepts will undoubtedly enhance your programming skills and make you a more proficient developer.

Related posts:

• Mastering Python Single Line If-Else Statements – A Comprehensive Guide
• Mastering Kotlin – Unleashing the Power of else if Statements
• Mastering if-else Statements in Python – Unlock the Power of One-Liners
• Mastering the Basics of Conditional Statements – A Guide to If-Else in GoLang
• Mastering Rust Else If – Simplifying Conditional Statements in Rust Programming

Learning Python by doing

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Variables, Expressions, and Assignments

Variables, expressions, and assignments 1 #, introduction #.

In this chapter, we introduce some of the main building blocks needed to create programs–that is, variables, expressions, and assignments. Programming related variables can be intepret in the same way that we interpret mathematical variables, as elements that store values that can later be changed. Usually, variables and values are used within the so-called expressions. Once again, just as in mathematics, an expression is a construct of values and variables connected with operators that result in a new value. Lastly, an assignment is a language construct know as an statement that assign a value (either as a constant or expression) to a variable. The rest of this notebook will dive into the main concepts that we need to fully understand these three language constructs.

Values and Types #

A value is the basic unit used in a program. It may be, for instance, a number respresenting temperature. It may be a string representing a word. Some values are 42, 42.0, and ‘Hello, Data Scientists!’.

Each value has its own type : 42 is an integer ( int in Python), 42.0 is a floating-point number ( float in Python), and ‘Hello, Data Scientists!’ is a string ( str in Python).

The Python interpreter can tell you the type of a value: the function type takes a value as argument and returns its corresponding type.

Observe the difference between type(42) and type('42') !

Expressions and Statements #

On the one hand, an expression is a combination of values, variables, and operators.

A value all by itself is considered an expression, and so is a variable.

When you type an expression at the prompt, the interpreter evaluates it, which means that it calculates the value of the expression and displays it.

In boxes above, m has the value 27 and m + 25 has the value 52 . m + 25 is said to be an expression.

On the other hand, a statement is an instruction that has an effect, like creating a variable or displaying a value.

The first statement initializes the variable n with the value 17 , this is a so-called assignment statement .

The second statement is a print statement that prints the value of the variable n .

The effect is not always visible. Assigning a value to a variable is not visible, but printing the value of a variable is.

Assignment Statements #

We have already seen that Python allows you to evaluate expressions, for instance 40 + 2 . It is very convenient if we are able to store the calculated value in some variable for future use. The latter can be done via an assignment statement. An assignment statement creates a new variable with a given name and assigns it a value.

The example in the previous code contains three assignments. The first one assigns the value of the expression 40 + 2 to a new variable called magicnumber ; the second one assigns the value of π to the variable pi , and; the last assignment assigns the string value 'Data is eatig the world' to the variable message .

Programmers generally choose names for their variables that are meaningful. In this way, they document what the variable is used for.

Do It Yourself!

Let’s compute the volume of a cube with side \(s = 5\) . Remember that the volume of a cube is defined as \(v = s^3\) . Assign the value to a variable called volume .

Well done! Now, why don’t you print the result in a message? It can say something like “The volume of the cube with side 5 is \(volume\) ”.

Beware that there is no checking of types ( type checking ) in Python, so a variable to which you have assigned an integer may be re-used as a float, even if we provide type-hints .

Names and Keywords #

Names of variable and other language constructs such as functions (we will cover this topic later), should be meaningful and reflect the purpose of the construct.

In general, Python names should adhere to the following rules:

It should start with a letter or underscore.

It cannot start with a number.

It must only contain alpha-numeric (i.e., letters a-z A-Z and digits 0-9) characters and underscores.

They cannot share the name of a Python keyword.

If you use illegal variable names you will get a syntax error.

By choosing the right variables names you make the code self-documenting, what is better the variable v or velocity ?

The following are examples of invalid variable names.

These basic development principles are sometimes called architectural rules . By defining and agreeing upon architectural rules you make it easier for you and your fellow developers to understand and modify your code.

If you want to read more on this, please have a look at Code complete a book by Steven McConnell [ McC04 ] .

Every programming language has a collection of reserved keywords . They are used in predefined language constructs, such as loops and conditionals . These language concepts and their usage will be explained later.

The interpreter uses keywords to recognize these language constructs in a program. Python 3 has the following keywords:

False class finally is return

None continue for lambda try

True def from nonlocal while

and del global not with

as elif if or yield

assert else import pass break

except in raise

Reassignments #

It is allowed to assign a new value to an existing variable. This process is called reassignment . As soon as you assign a value to a variable, the old value is lost.

The assignment of a variable to another variable, for instance b = a does not imply that if a is reassigned then b changes as well.

You have a variable salary that shows the weekly salary of an employee. However, you want to compute the monthly salary. Can you reassign the value to the salary variable according to the instruction?

Updating Variables #

A frequently used reassignment is for updating puposes: the value of a variable depends on the previous value of the variable.

This statement expresses “get the current value of x , add one, and then update x with the new value.”

Beware, that the variable should be initialized first, usually with a simple assignment.

Do you remember the salary excercise of the previous section (cf. 13. Reassignments)? Well, if you have not done it yet, update the salary variable by using its previous value.

Updating a variable by adding 1 is called an increment ; subtracting 1 is called a decrement . A shorthand way of doing is using += and -= , which stands for x = x + ... and x = x - ... respectively.

Order of Operations #

Expressions may contain multiple operators. The order of evaluation depends on the priorities of the operators also known as rules of precedence .

For mathematical operators, Python follows mathematical convention. The acronym PEMDAS is a useful way to remember the rules:

Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3 - 1) is 4 , and (1 + 1)**(5 - 2) is 8 . You can also use parentheses to make an expression easier to read, even if it does not change the result.

Exponentiation has the next highest precedence, so 1 + 2**3 is 9 , not 27 , and 2 * 3**2 is 18 , not 36 .

Multiplication and division have higher precedence than addition and subtraction . So 2 * 3 - 1 is 5 , not 4 , and 6 + 4 / 2 is 8 , not 5 .

Operators with the same precedence are evaluated from left to right (except exponentiation). So in the expression degrees / 2 * pi , the division happens first and the result is multiplied by pi . To divide by 2π, you can use parentheses or write: degrees / 2 / pi .

In case of doubt, use parentheses!

Let’s see what happens when we evaluate the following expressions. Just run the cell to check the resulting value.

Floor Division and Modulus Operators #

The floor division operator // divides two numbers and rounds down to an integer.

For example, suppose that driving to the south of France takes 555 minutes. You might want to know how long that is in hours.

Conventional division returns a floating-point number.

Hours are normally not represented with decimal points. Floor division returns the integer number of hours, dropping the fraction part.

You spend around 225 minutes every week on programming activities. You want to know around how many hours you invest to this activity during a month. Use the \(//\) operator to give the answer.

The modulus operator % works on integer values. It computes the remainder when dividing the first integer by the second one.

The modulus operator is more useful than it seems.

For example, you can check whether one number is divisible by another—if x % y is zero, then x is divisible by y .

String Operations #

In general, you cannot perform mathematical operations on strings, even if the strings look like numbers, so the following operations are illegal: '2'-'1' 'eggs'/'easy' 'third'*'a charm'

But there are two exceptions, + and * .

The + operator performs string concatenation, which means it joins the strings by linking them end-to-end.

The * operator also works on strings; it performs repetition.

Speedy Gonzales is a cartoon known to be the fastest mouse in all Mexico . He is also famous for saying “Arriba Arriba Andale Arriba Arriba Yepa”. Can you use the following variables, namely arriba , andale and yepa to print the mentioned expression? Don’t forget to use the string operators.

Asking the User for Input #

The programs we have written so far accept no input from the user.

To get data from the user through the Python prompt, we can use the built-in function input .

When input is called your whole program stops and waits for the user to enter the required data. Once the user types the value and presses Return or Enter , the function returns the input value as a string and the program continues with its execution.

Try it out!

You can also print a message to clarify the purpose of the required input as follows.

The resulting string can later be translated to a different type, like an integer or a float. To do so, you use the functions int and float , respectively. But be careful, the user might introduce a value that cannot be converted to the type you required.

We want to know the name of a user so we can display a welcome message in our program. The message should say something like “Hello \(name\) , welcome to our hello world program!”.

Script Mode #

So far we have run Python in interactive mode in these Jupyter notebooks, which means that you interact directly with the interpreter in the code cells . The interactive mode is a good way to get started, but if you are working with more than a few lines of code, it can be clumsy. The alternative is to save code in a file called a script and then run the interpreter in script mode to execute the script. By convention, Python scripts have names that end with .py .

Use the PyCharm icon in Anaconda Navigator to create and execute stand-alone Python scripts. Later in the course, you will have to work with Python projects for the assignments, in order to get acquainted with another way of interacing with Python code.

This Jupyter Notebook is based on Chapter 2 of the books Python for Everybody [ Sev16 ] and Think Python (Sections 5.1, 7.1, 7.2, and 5.12) [ Dow15 ] .

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Expression statement

An expression statement is an expression used in a place where a statement is expected. The expression is evaluated and its result is discarded — therefore, it makes sense only for expressions that have side effects, such as executing a function or updating a variable.

An arbitrary expression to be evaluated. There are certain expressions that may be ambiguous with other statements and are thus forbidden.

Description

Apart from the dedicated statement syntaxes , you can also use almost any expression as a statement on its own. The expression statement syntax requires a semicolon at the end, but the automatic semicolon insertion process may insert one for you if the lack of a semicolon results in invalid syntax.

Because the expression is evaluated and then discarded, the result of the expression is not available. Therefore, the expression must have some side effect for it to be useful. Expression statements are commonly:

• Function calls ( console.log("Hello"); , [1, 2, 3].forEach((i) => console.log(i)); )
• Tagged template literals
• Assignment expressions , including compound assignments
• Increment and decrement operators
• yield and yield*

Others may also have side effects if they invoke getters or trigger type coercions .

Forbidden expressions

In order for an expression to be used as a statement, it must not be ambiguous with other statement syntaxes. Therefore, the expression must not start with any of the following tokens:

• function : which would be a function declaration or function* declaration , not a function expression or function* expression
• async function : which would be an async function declaration or async function* declaration , not an async function expression or async function* expression
• class : which would be a class declaration , not a class expression
• let[ : which would be a let declaration with array destructuring , not a property accessor on a variable called let ( let can only be an identifier in non-strict mode )
• { : which would be a block statement , not an object literal

Therefore, all of the following are invalid:

More dangerously, sometimes the code happens to be valid syntax, but is not what you intend.

To avoid these problems, you can use parentheses, so that the statement is unambiguously an expression statement.

Avoiding control flow statements

You can avoid almost all use of control flow statements using expression statements. For example, if...else can be replaced with ternary operators and logical operators . Iterative statements like for or for...of can be replaced with array methods .

Warning: This only demonstrates a capability of the language. Excessive use of expression statements as a substitute for control-flow statements can make code much less readable.

Specifications

• Statements and declarations
• Expressions and operators

2. Expressions and Assignment Statements

Recall that an assignment statement is used to store a value in a variable, and looks like this:

When I first introduced assignment statements, I told you that C# requires that the data type of the expression be compatible with the data type of the variable (on the left side). Thus, if x is an int variable, x = 5 is legal, but x = "Jon" is not.

We need to dig into this rule a little bit, because until you understand it well, you will have difficulty when you are working with expressions that include variables of different data types, which happens all the time in C#. There are two parts to consider: "the data type of the expression" and "compatible with the data type of the variable".

2.1. Determining the Data Type of an Expression

An expression, as you know, computes a value, and that value has a data type. By "data type of an expression," I am referring to the data type of the value produced by the expression. For example, the expression 5 + 5 yields the int value 10, and the expression 2.0 * 3.0 yields the double value 6.0.

It's pretty easy to determine the data type of a simple expressions, such as a literal or a single variable. But what happens when you start mixing types? More complicated expressions can present a challenge, but if you learn to tackle them in a systematic way, you can easily analyze them to determine their type. First, here are the two rules governing numeric expressions:

A mathematical expression that consists only of integer types will produce an int result. When a long value is involved, the result is long.

A mathematical expression that contains at least one double or float value will always produce a floating point result. The resulting data type depends on the "largest" data type in the expression. For example, if a double value is involved, then the result is a double. If only float values (and possibly integer values) are used, then the result is a float.

An expression that contains at least one string value will always produce a string result.

Let's take a look at some examples. We'll use the following variables:

Here is the first one:

int1 + int2 - 5

This one is easy. Only int values are involved; the result is an int.

int1 + long1

Still only integer types, but since a long is involved, the result is a long.

long1 + float1 - int2

Since a float is involved, the result must be floating point. No double value, so the result is a float.

int1 + int2 - (long1 * float1) / double2 * float2

Again, since floats and doubles are involved, the result must be a floating point type. Since a double is involved, the result is double.

"Fred weighs " + double1 + " pounds."

Since a string is involved, the result is a string.

2.2. Data Type Compatibility

In an assignment statement, the data type of the expression must be compatible with the data type of the variable being changed. I use the word "compatible" because the two types don't have to be identical. Let me give you an example to explain what I mean.

In this fragment, the compiler would permit the first assignment statement, but not the second. Here's why. An int expression is "assignment compatible" with a double variable, but a double expression is not "assignment compatible" with an int variable. The reason that one is allowed and the other is not has to do with information loss. Any int value can be safely stored in a double variable with no loss of information. However, a double value cannot be safely stored in an int variable, because an int variable cannot hold digits after the decimal point. In other words, C# allows automatic data type conversions any time that information loss is avoided.

Perhaps you have encountered an error message like this:

This is the error that my compiler issued for the second assignment statement above. To avoid errors like this, you must observe the rules on assignment compatibility:

A smaller integer-type expression can be stored in a larger integer-type variable. For example, an int expression can be stored in a long variable.

Any integer-type expression (byte, short, int, long) can be stored in a float or double variable.

A float expression can be stored in a double variable.

Other combinations are not legal.

2.3. Forcing Square Pegs into Round Holes

There are times when you have an expression that produces a result whose type is not compatible with the variable you want to hold the result. In other words, there are legitimate reasons to want to convert data from one type to another. In these cases, you need a way to tell C# to force the conversion, losing information if necessary. You do this with something called a cast. In our example, here's how it would work:

i = (int) d;

When you write a data type in parenthesis in front of an expression, C# computes the result of the expression as it normally would, and then after the result is computed, converts it to the indicated type. For example, if d were 3.2532, the (int) cast would convert the expression to 3, so it can be stored in the int variable. Note that the cast does not affect any variables in the expression; it only affects the result of the expression. So d's value is not changed.

The cast is actually an operator. It has a very high precedence -- higher than all the arithmetic operators. When your expression is more complicated than a single variable, you should enclose the expression in parenthesis. Take the following as an example:

This assignment statement is invalid, because the type of the expression is double. You might try to correct the problem by adding a cast:

i = (int) i + d;

But the compiler will still complain that the expression produces a double. What's going on here? To understand the problem, take a look at the assignment statement rewritten with the expression fully parenthesized:

i = ((int) i) + d;

This statement is equivalent to the previous one, and highlights the problem. The cast (int) has higher precedence than the + operator, so it affects only the value of i, not the value of (i + d). In effect, the cast does nothing. There are two ways to fix the problem.

Parenthesize the expression like this:

i = (int) (i + d);

Now the cast applies to the double value produced by (i + d).

Change the order of the operands:

i = (int) d + i;

Here, the double value of d is converted to an int before the addition occurs, thus ensuring an int result.

I prefer the first technique, because it is more obvious. In fact, to increase your chances of using the cast correctly, I suggest that you always parenthesize the expression being cast, whether it's needed or not.

2.4. More on Casting

The cast can occur in places other than the beginning of an expression. For example, consider this expression:

(total / num - 2) * 2

If total and num are both int variables, the result is an int. The integer division will likely yield undesirable results. You might think adding a cast at the beginning would solve the problem:

(double) (total / num - 2) * 2

But it doesn't. To understand why it doesn't help, fully parenthesize the expression according to operator precedence:

((double) ((total / num) - 2)) * 2

Now, follow the steps the computer would follow when evaluating the expression:

Compute total / num, yielding an int result (oops).

Subtract 2, yielding an int

Convert the result to a double

Multiply by 2, yielding a double

The problem occurred in the very first step. We want that division to yield a double, not an int. To fix the problem, we must move the cast inside the parentheses:

((double) total / num - 2) * 2

Now, fully parenthesized, the expression looks like this:

((((double) total) / num) - 2) * 2

And the computer evaluates it like this:

Take the value of total and convert to a double.

Divide the result by num, yielding a double.

Subtract 2, yielding a double

2.5. String Conversions

There's an important limitation on casts: you cannot use a cast to convert between a string type and a numeric type. For example, the following won't work:

To convert between string and numeric values, you must use methods in the Convert class. Here's a fragment that shows how to do it:

When you have a numeric value that you need to convert to a string, use the Convert.ToString( ) method. It accepts a numeric expression and returns the string equivalent.

Converting from Strings to numeric types requires the use of the ToDouble and ToInt32 methods in the Convert class. Note that conversion will fail with a runtime error if the string contains any spaces or other non-numeric characters. I will discuss how to handle this problem gracefully in a later chapter.

2.6. Increment, Decrement, and Modulus Operations

You'll find that adding 1 to a variable is an extremely common operation in programming. Subtracting 1 from a variable is also pretty common. You might perform the operation of adding 1 to a variable with assignment statements such as:

The effect of the assignment statement x = x + 1 is to take the old value of the variable x, compute the result of adding 1 to that value, and store the answer as the new value of x. The same operation can be accomplished by writing x++ (or, if you prefer, ++x). This actually changes the value of x, so that it has the same effect as writing "x = x + 1". The statement above could be written

Similarly, you could write x-- (or --x) to subtract 1 from x. That is, x-- performs the same computation as x = x - 1. Adding 1 to a variable is called incrementing that variable, and subtracting 1 is called decrementing. The operators ++ and -- are called the increment operator and the decrement operator, respectively. These operators can be used on variables belonging to any of the numerical types and also on variables of type char.

Sometimes you want to increment or decrement a variable by more than 1. The += and -= operations come in handy for this purpose. Here's an example of what you can do:

counter += 2;

This is basically an abbreviation for

counter = counter + 2;

You can use any expression on the right-hand side of the += operator. For example,

counter += (num * 5) + 2;

would be equivalent to writing

counter = counter + ((num * 5) + 2);

All of the binary arithmetic operators, including +, -, *, /, and the modulo operator (introduced next) can be used in this way. For example, to multiply a number by 3, you could write:

goals *= 3;

In addition to the standard +, -, *, and / operators, C# also has an operator for computing the remainder when one integer is divided by another. This operator is the modulo operator, indicated by %. If A and B are integers, then A % B represents the remainder when A is divided by B. For example, 7 % 2 is 1, while 34577 % 100 is 77, and 50 % 8 is 2. A common use of % is to test whether a given integer is even or odd. N is even if N % 2 is zero, and it is odd if N % 2 is 1. More generally, you can check whether an integer N is evenly divisible by an integer M by checking whether N % M is zero.

2.7. Precedence Rules

If you use several operators in one expression, and if you don't use parentheses to explicitly indicate the order of evaluation, then you have to worry about the precedence rules that determine the order of evaluation. (Advice: don't confuse yourself or the reader of your program; use parentheses liberally.) Here is a listing of the operators discussed in this section (along with some we haven't discussed yet), listed in order from highest precedence (evaluated first) to lowest precedence (evaluated last):

Table 4.4. Operator Precedence in C#

Operators on the same line (like * and /) have the same precedence. When they occur together, unary operators and assignment operators are evaluated right-to-left, and the remaining operators are evaluated left-to-right. For example, A*B/C means (A*B)/C, while A=B=C means A=(B=C). (Tip: You can write things like X = Y = 0 to assign several values the same value with one statement.)

The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases. See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.

Expressions, Statements, and Blocks

Now that you understand variables and operators, it's time to learn about expressions , statements , and blocks . Operators may be used in building expressions, which compute values; expressions are the core components of statements; statements may be grouped into blocks.

Expressions

An expression is a construct made up of variables, operators, and method invocations, which are constructed according to the syntax of the language, that evaluates to a single value. You've already seen examples of expressions, illustrated in bold below:

The data type of the value returned by an expression depends on the elements used in the expression. The expression cadence = 0 returns an int because the assignment operator returns a value of the same data type as its left-hand operand; in this case, cadence is an int . As you can see from the other expressions, an expression can return other types of values as well, such as boolean or String .

The Java programming language allows you to construct compound expressions from various smaller expressions as long as the data type required by one part of the expression matches the data type of the other. Here's an example of a compound expression:

In this particular example, the order in which the expression is evaluated is unimportant because the result of multiplication is independent of order; the outcome is always the same, no matter in which order you apply the multiplications. However, this is not true of all expressions. For example, the following expression gives different results, depending on whether you perform the addition or the division operation first:

You can specify exactly how an expression will be evaluated using balanced parenthesis: ( and ). For example, to make the previous expression unambiguous, you could write the following:

If you don't explicitly indicate the order for the operations to be performed, the order is determined by the precedence assigned to the operators in use within the expression. Operators that have a higher precedence get evaluated first. For example, the division operator has a higher precedence than does the addition operator. Therefore, the following two statements are equivalent:

When writing compound expressions, be explicit and indicate with parentheses which operators should be evaluated first. This practice makes code easier to read and to maintain.

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon ( ; ).

• Assignment expressions
• Any use of ++ or --
• Method invocations
• Object creation expressions

Such statements are called expression statements . Here are some examples of expression statements.

In addition to expression statements, there are two other kinds of statements: declaration statements and control flow statements . A declaration statement declares a variable. You've seen many examples of declaration statements already:

Finally, control flow statements regulate the order in which statements get executed. You'll learn about control flow statements in the next section, Control Flow Statements

A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed. The following example, BlockDemo , illustrates the use of blocks:

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Expressions vs. statements

In programming language terminology, an “expression” is a combination of values and functions that are combined and interpreted by the compiler to create a new value, as opposed to a “statement” which is just a standalone unit of execution and doesn’t return anything. One way to think of this is that the purpose of an expression is to create a value (with some possible side-effects), while the sole purpose of a statement is to have side-effects.

C# and most imperative languages make a distinction between expressions and statements and have rules about where each kind can be used. But as should be apparent, a truly pure functional language cannot support statements at all, because in a truly pure language, there would be no side-effects.

Even though F# is not pure, it does follow the same principle. In F# everything is an expression. Not just values and functions, but also control flows (such as if-then-else and loops), pattern matching, and so on.

There are some subtle benefits to using expressions over statements. First, unlike statements, smaller expressions can be combined (or “composed”) into larger expressions. So if everything is an expression, then everything is also composable.

Second, a series of statements always implies a specific order of evaluation, which means that a statement cannot be understood without looking at prior statements. But with pure expressions, the subexpressions do not have any implied order of execution or dependencies.

So in the expression a+b , if both the ‘ a ’ and ‘ b ’ parts are pure, then the ‘ a ’ part can be isolated, understood, tested and evaluated on its own, as can the ‘ b ’ part. This “isolatibility” of expressions is another beneficial aspect of functional programming.

Note that the F# interactive window also relies on everything being an expression. It would be much harder to use a C# interactive window.

Expressions are safer and more compact

Using expressions consistently leads to code that is both safer and more compact. Let’s see what I mean by this.

First, let’s look at a statement based approach. Statements don’t return values, so you have to use temporary variables that are assigned to from within statement bodies. Here are some examples using a C-like language (OK, C#) rather than F#:

Because the “if-then” is a statement, the result variable must be defined outside the statement and but assigned to inside the statement, which leads to some issues:

• The result variable has to be set up outside the statement itself. What initial value should it be set to?
• What if I forget to assign to the result variable in the if statement? The purpose of the “if “statement is purely to have side effects (the assignment to the variables). This means that the statements are potentially buggy, because it would be easy to forget to do an assignment in one branch. And because the assignment was just a side effect, the compiler could not offer any warning. Since the result variable has already been defined in scope, I could easily use it, unaware that it was invalid.
• What is the value of the result variable in the “else” case? In this case, I haven’t specified a value. Did I forget? Is this a potential bug?
• Finally, the reliance on side-effects to get things done means that the statements are not easily usable in another context (for example, extracted for refactoring, or parallelizing) because they have a dependency on a variable that is not part of the statement itself.

Note: the code above will not compile in C# because the compiler will complain if you use an unassigned local variable like this. But having to define some default value for result before it is even used is still a problem.

For comparison, here is the same code, rewritten in an expression-oriented style:

In the expression-oriented version, none of the earlier issues even apply!

• The result variable is declared at the same time that it is assigned. No variables have to be set up “outside” the expression and there is no worry about what initial value they should be set to.
• The “else” is explicitly handled. There is no chance of forgetting to do an assignment in one of the branches.
• And I cannot possibly forget to assign to result , because then the variable would not even exist!

In F#, the two examples would be written as:

The “ mutable ” keyword is considered a code smell in F#, and is discouraged except in certain special cases. It should be avoided at all cost while you are learning!

In the expression based version, the mutable variable has been eliminated and there is no reassignment anywhere.

Once we have the if statement converted into an expression, it is now trivial to refactor it and move the entire subexpression to a different context without introducing errors.

Here’s the refactored version in C#:

Statements vs. expressions for loops

Going back to C# again, here is a similar example of statements vs. expressions using a loop statement

I’ve used an old-style “for” statement, where the index variables are declared outside the loop. Many of the issues discussed earlier apply to the loop index “ i ” and the max value “ length ”, such as: can they be used outside the loop? And what happens if they are not assigned to?

A more modern version of a for-loop addresses these issues by declaring and assigning the loop variables in the “for” loop itself, and by requiring the “ sum ” variable to be initialized:

This more modern version follows the general principle of combining the declaration of a local variable with its first assignment.

But of course, we can keep improving by using a foreach loop instead of a for loop:

Each time, not only are we condensing the code, but we are reducing the likelihood of errors.

But taking that principle to its logical conclusion leads to a completely expression based approach! Here’s how it might be done using LINQ:

Note that I could have used LINQ’s built-in “sum” function, but I used Aggregate in order to show how the sum logic embedded in a statement can be converted into a lambda and used as part of an expression.

In the next post, we’ll look at the various kinds of expressions in F#.

• ← 1. Expressions and syntax: Introduction
• 3. Overview of F# expressions →

The "Expressions and syntax" series

• Expressions and syntax: Introduction How to code in F#
• Expressions vs. statements Why expressions are safer and make better building blocks
• Overview of F# expressions Control flows, lets, dos, and more
• Binding with let, use, and do How to use them
• F# syntax: indentation and verbosity Understanding the offside rule
• Parameter and value naming conventions a, f, x and friends
• Control flow expressions And how to avoid using them
• Exceptions Syntax for throwing and catching
• Match expressions The workhorse of F#
• Formatted text using printf Tips and techniques for printing and logging
• Worked example: Parsing command line arguments Pattern matching in practice
• Worked example: Roman numerals More pattern matching in practice

1.7 Java | Assignment Statements & Expressions

An assignment statement designates a value for a variable. An assignment statement can be used as an expression in Java.

After a variable is declared, you can assign a value to it by using an assignment statement . In Java, the equal sign = is used as the assignment operator . The syntax for assignment statements is as follows:

An expression represents a computation involving values, variables, and operators that, when taking them together, evaluates to a value. For example, consider the following code:

You can use a variable in an expression. A variable can also be used on both sides of the =  operator. For example:

In the above assignment statement, the result of x + 1  is assigned to the variable x . Let’s say that x is 1 before the statement is executed, and so becomes 2 after the statement execution.

To assign a value to a variable, you must place the variable name to the left of the assignment operator. Thus the following statement is wrong:

Note that the math equation  x = 2 * x + 1  ≠ the Java expression x = 2 * x + 1

Which is equivalent to:

And this statement

is equivalent to:

Note: The data type of a variable on the left must be compatible with the data type of a value on the right. For example, int x = 1.0 would be illegal, because the data type of x is int (integer) and does not accept the double value 1.0 without Type Casting .

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Statements, Expressions and Operators

If the embed above does not work here is a link to the full version of the video

We need to write code that does something

• create a variable
• assign a value to a variable
• run a separate piece of code elsewhere
• print something out to the screen
• pass some value on to another piece of code

statements are the building blocks for our code that do these things. Any piece of code we write that has an effect is a statement.

An assignment statement will assign a value to a variable

A more complex assignment statement will evaluate an expression to assign a value to a variable

Multiple statements combine to create more complex functionality

Statements follow each other, and are carried out in order

Later, we’ll look at complex statements that contain other statements and have their own functionality

• loop statements that repeat code, or iterate (for, while)
• branching statements that allow code to make decisions (if, if-else)

the names of these complex statements (for, while, if, if-else) are often reserved. This means that they cannot be used for other purposes in our code - for example we usally cannot use ‘for’ as a variable name.

Expressions

Expressions are pieces of code that will be evaluated to determine their value

• a mathematical operation
• that applies operators to the values stored in variables
• and provides a result

Expressions do not have effects, they merely produce a value. Statements can contain expressions, which means that the results of expressions can be used in our statements.

The operators that we can use in expressions will depend on the programming language we are using, but will typically include:

• basic mathematical operators, for example +, -, *, /
• other mathematical operators, for example ^, %
• logical operators, for example |, &, !
• comparison operators, for example ==, <, >

Operators can be classified by how many things they operate on. Most (but not all) operators are binary, they operate on two values (or ‘operands’). Unary operators only operate on a single value. Ternary operators operate on 3 operands.

Operators can be described by where they are placed. Prefix operators appear before their operand(s). Infix operators appear between their operands. Postfix operators appear after their operands.

The mathematical operators (addition, subtraction, multiplication and division) are all binary operators and are typically (though not always) infix:

It is possible sometimes to use mathematical operators as unary operators, for example, a unary prefix ‘-’ operator

^ is sometimes used to raise a number to a higher power

though this also might be done using ‘**’, depending on the language

‘%’ is the modulus operator, it gives the result from integer division

E will contain the value 1, since 2 divides 5 twice, with 1 as a remainder.

The binary logical operators ‘and’ and ‘or’ allow you to combine boolean values (more on this later), while the unary logical operator ‘not’ allows you to negate a binary value (true becomes false, and vice versa).

Comparison operators allow you to compare values. We can test for equality (is A equal to B?) or whether a value is less than or greater than (or less than or equal to or greater than or equal to) another value.

You may occasionally see a ternary operator ‘?’, though this is not present in all languages. This has three operands, a boolean expression, a value if the expression is true and a value if the expression is false

Do not worry too much about this one at the moment, we’ll see it again later.

Some languages (but not all!) include additional operators that can shorthand some operations, for example ‘++’ or ‘–’ as increment and decrement operators. These can typically be used postfix or prefix, and are shortcuts for ‘this value +1’ or ‘this value -1’

in many languages is the same as:

Whether these operators are used as prefix or postfix determines whether they perform their function before or after expression evaluation. For example:

The prefix ‘++’ operator will be applied to A (increasing the value by 1 to 2) and then the result of that expression stored in B, so both A and B will contain the value ‘2’.

The postfix ‘++’ operator will be applied to A after the value of A has been read and stored in B, so in this example A will have the value 2, but B will contain the original value of A before the postfix increment operator was applied, so will contain ‘1’.

If an operator sits between two values, we call it ...

No, sorry, that's incorrect. A postfix operator comes *after* its operand(s), not between them.

No, sorry, that's incorrect. A prefix operator comes *before* its operand(s), not between them.

No, sorry, that's incorrect. I just made that word up. Do you like it?

Yes, correct! An operator that operates on two operands while sitting in between them is called an 'infix' operator

Any code we write that has some __effect__ is a ...

Yes, well done, that's correct.

No, sorry, that's incorrect. Expressions are evaluated to produce a result.

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Different Forms of Assignment Statements in Python

We use Python assignment statements to assign objects to names. The target of an assignment statement is written on the left side of the equal sign (=), and the object on the right can be an arbitrary expression that computes an object.

There are some important properties of assignment in Python :-

• Assignment creates object references instead of copying the objects.
• Python creates a variable name the first time when they are assigned a value.
• Names must be assigned before being referenced.
• There are some operations that perform assignments implicitly.

Assignment statement forms :-

1. Basic form:

This form is the most common form.

2. Tuple assignment:

When we code a tuple on the left side of the =, Python pairs objects on the right side with targets on the left by position and assigns them from left to right. Therefore, the values of x and y are 50 and 100 respectively.

3. List assignment:

This works in the same way as the tuple assignment.

4. Sequence assignment:

In recent version of Python, tuple and list assignment have been generalized into instances of what we now call sequence assignment – any sequence of names can be assigned to any sequence of values, and Python assigns the items one at a time by position.

5. Extended Sequence unpacking:

It allows us to be more flexible in how we select portions of a sequence to assign.

Here, p is matched with the first character in the string on the right and q with the rest. The starred name (*q) is assigned a list, which collects all items in the sequence not assigned to other names.

This is especially handy for a common coding pattern such as splitting a sequence and accessing its front and rest part.

6. Multiple- target assignment:

In this form, Python assigns a reference to the same object (the object which is rightmost) to all the target on the left.

7. Augmented assignment :

The augmented assignment is a shorthand assignment that combines an expression and an assignment.

There are several other augmented assignment forms:

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• Assignment Statement

An Assignment statement is a statement that is used to set a value to the variable name in a program .

Assignment statement allows a variable to hold different types of values during its program lifespan. Another way of understanding an assignment statement is, it stores a value in the memory location which is denoted by a variable name.

The symbol used in an assignment statement is called as an operator . The symbol is ‘=’ .

Note: The Assignment Operator should never be used for Equality purpose which is double equal sign ‘==’.

The Basic Syntax of Assignment Statement in a programming language is :

variable = expression ;

variable = variable name

expression = it could be either a direct value or a math expression/formula or a function call

Few programming languages such as Java, C, C++ require data type to be specified for the variable, so that it is easy to allocate memory space and store those values during program execution.

data_type variable_name = value ;

In the above-given examples, Variable ‘a’ is assigned a value in the same statement as per its defined data type. A data type is only declared for Variable ‘b’. In the 3 rd line of code, Variable ‘a’ is reassigned the value 25. The 4 th line of code assigns the value for Variable ‘b’.

Assignment Statement Forms

This is one of the most common forms of Assignment Statements. Here the Variable name is defined, initialized, and assigned a value in the same statement. This form is generally used when we want to use the Variable quite a few times and we do not want to change its value very frequently.

Tuple Assignment

Generally, we use this form when we want to define and assign values for more than 1 variable at the same time. This saves time and is an easy method. Note that here every individual variable has a different value assigned to it.

(Code In Python)

Sequence Assignment

(Code in Python)

Multiple-target Assignment or Chain Assignment

In this format, a single value is assigned to two or more variables.

Augmented Assignment

In this format, we use the combination of mathematical expressions and values for the Variable. Other augmented Assignment forms are: &=, -=, **=, etc.

Browse more Topics Under Data Types, Variables and Constants

• Concept of Data types
• Built-in Data Types
• Constants in Programing Language 
• Access Modifier
• Variables of Built-in-Datatypes
• Declaration/Initialization of Variables
• Type Modifier

Few Rules for Assignment Statement

Few Rules to be followed while writing the Assignment Statements are:

• Variable names must begin with a letter, underscore, non-number character. Each language has its own conventions.
• The Data type defined and the variable value must match.
• A variable name once defined can only be used once in the program. You cannot define it again to store other types of value.
• If you assign a new value to an existing variable, it will overwrite the previous value and assign the new value.

FAQs on Assignment Statement

Q1. Which of the following shows the syntax of an  assignment statement ?

• variablename = expression ;
• expression = variable ;
• datatype = variablename ;
• expression = datatype variable ;

Answer – Option A.

Q2. What is an expression ?

• Same as statement
• List of statements that make up a program
• Combination of literals, operators, variables, math formulas used to calculate a value
• Numbers expressed in digits

Answer – Option C.

Q3. What are the two steps that take place when an  assignment statement  is executed?

• Evaluate the expression, store the value in the variable
• Reserve memory, fill it with value
• Evaluate variable, store the result
• Store the value in the variable, evaluate the expression.

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Data Types, Variables and Constants

• Variables in Programming Language
• Concept of Data Types
• Declaration of Variables
• Type Modifiers
• Access Modifiers
• Constants in Programming Language

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Difference between declaration statement and assignment statement in C? [duplicate]

I am new to programming and trying to learn C. I am reading a book where I read about these statements but could not understand their meaning.

2 Answers 2

Declaration:

Assignment:

Declaration and assignment in one statement:

Declaration says, "I'm going to use a variable named " a " to store an integer value." Assignment says, "Put the value 3 into the variable a ."

(As @delnan points out, my last example is technically initialization , since you're specifying what value the variable starts with, rather than changing the value. Initialization has special syntax that also supports specifying the contents of a struct or array.)

• 6 The third isn't an assignment. It's initalization. –  user395760 Commented Apr 22, 2014 at 18:33
• 6 It matters in C insofar you can do things in initialization that you can't do in assignment. For example int a[2] = {}; works but int a[2]; a = {}; doesn't. –  user395760 Commented Apr 22, 2014 at 18:41

Declaring a variable sets it up to be used at a later point in the code. You can create variables to hold numbers, characters, strings (an array of characters), etc.

You can declare a variable without giving it a value. But, until a variable has a value, it's not very useful.

You declare a variable like so: char myChar; NOTE: This variable is not initialized.

Once a variable is declared, you can assign a value to it, like: myChar = 'a'; NOTE: Assigning a value to myChar initializes the variable.

To make things easier, if you know what a variable should be when you declare it, you can simply declare it and assign it a value in one statement: char myChar = 'a'; NOTE: This declares and initializes the variable.

So, once your myChar variable has been given a value, you can then use it in your code elsewhere. Example:

This prints the value of myChar and myOtherChar to stdout (the console), and looks like:

If you had declared char myChar; without assigning it a value, and then attempted to print myChar to stdout, you would receive an error telling you that myChar has not been initialized.

• You mostly describe the difference between declaring a variable and not initializing it vs. declaring a variable and initializing it. –  user395760 Commented Apr 22, 2014 at 18:42

Not the answer you're looking for? Browse other questions tagged c difference or ask your own question .

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