assignment 9.4 python for everybody

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9.4: Looping and Dictionaries

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• Chuck Severance
• University of Michigan

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If you use a dictionary as the sequence in a for statement, it traverses the keys of the dictionary. This loop prints each key and the corresponding value:

Code 9.4.1 (Python)

Here's what the output looks like:

Again, the keys are in no particular order.

We can use this pattern to implement the various loop idioms that we have described earlier. For example if we wanted to find all the entries in a dictionary with a value above ten, we could write the following code:

The for loop iterates through the keys of the dictionary, so we must use the index operator to retrieve the corresponding value for each key. Here's what the output looks like:

We see only the entries with a value above 10.

If you want to print the keys in alphabetical order, you first make a list of the keys in the dictionary using the keys method available in dictionary objects, and then sort that list and loop through the sorted list, looking up each key and printing out key-value pairs in sorted order as follows:

First you see the list of keys in unsorted order that we get from the keys method. Then we see the key-value pairs in order from the for loop.

Python for Everybody

Exercise 9.1, exercise 9.2, exercise 9.3, exercise 9.4, exercise 9.5.

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• 9.1 A list is a sequence
• 9.2 Lists are mutable
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9.5. List slices ¶

The slice operator also works on lists:

If you omit the first index, the slice starts at the beginning. If you omit the second, the slice goes to the end. So if you omit both, the slice is a copy of the whole list.

Q-2: What is printed by the following statements?

• [ [ ], 3.14, False]
• Yes, the slice starts at index 4 and goes up to and including the last item.
• [ [ ], 3.14]
• By leaving out the upper bound on the slice, we go up to and including the last item.
• [ [56, 57, "dog"], [ ], 3.14, False]
• Index values start at 0.

Q-3: What is printed by the following statements?

• An empty slice like this would not produce an empty list. Think about how the slice indexes.
• This would be correct if the slice was [:1]
• [3, 67, "cat", [56, 57, "dog"], [ ], 3.14, False]
• Omitting both indexes in a slice will create a copy of the whole list.

Since lists are mutable, it is often useful to make a copy before performing operations that fold, spindle, or mutilate lists.

A slice operator on the left side of an assignment can update multiple elements:

Q-5: What is printed by the following statements?

• The list begins with the second item of L and includes everything up to but not including the last item.
• Yes, there are 3 items in this slice.