golang assignment to entry in nil map

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Golang: How to Assign a Value to an Entry in a Nil Map

Golang Assignment to Entry in Nil Map

Maps are a powerful data structure in Golang, and they can be used to store key-value pairs. However, it’s important to be aware of the pitfalls of working with nil maps, as assigning a value to an entry in a nil map can cause unexpected results.

In this article, we’ll take a closer look at nil maps and how to avoid common mistakes when working with them. We’ll also discuss some of the best practices for using maps in Golang.

By the end of this article, you’ll have a solid understanding of nil maps and how to use them safely and effectively in your Golang programs.

| Column 1 | Column 2 | Column 3 | |—|—|—| | Key | Value | Error | | `nil` | `any` | `panic: assignment to entry in nil map` | | `map[string]string{}` | `”foo”: “bar”` | `nil` | | `map[string]string{“foo”: “bar”}` | `”foo”: “baz”` | `KeyError: key not found: foo` |

In Golang, a map is a data structure that stores key-value pairs. The keys are unique and can be of any type, while the values can be of any type that implements the `GoValue` interface. When a map is created, it is initialized with a zero value of `nil`. This means that the map does not exist and cannot be used to store any data.

What is a nil map in Golang?

A nil map is a map with a value of nil. This means that the map does not exist and cannot be used to store any data. When you try to access a nil map, you will get a `panic` error.

Why does Golang allow assignment to entry in a nil map?

Golang allows assignment to entry in a nil map because it is a type-safe language. This means that the compiler will check to make sure that the type of the value being assigned to the map entry is compatible with the type of the map. If the types are not compatible, the compiler will generate an error.

How to assign to entry in a nil map in Golang

To assign to an entry in a nil map, you can use the following syntax:

map[key] = value

For example, the following code will assign the value `”hello”` to the key `”world”` in a nil map:

m := make(map[string]string) m[“world”] = “hello”

Assignment to entry in a nil map is a dangerous operation that can lead to errors. It is important to be aware of the risks involved before using this feature.

Additional Resources

• [Golang Maps](https://golang.org/ref/specMaps)
• [Golang Type Safety](https://golang.org/ref/specTypes)

What is a nil map?

A nil map is a map that has not been initialized. This means that the map does not have any entries, and it cannot be used to store or retrieve data.

What are the potential problems with assigning to entry in a nil map?

There are two potential problems with assigning to entry in a nil map:

• The first problem is that the assignment will silently fail. This means that the compiler will not generate an error, and the program will continue to run. However, the assignment will not have any effect, and the map will still be nil.
• The second problem is that the assignment could cause a runtime error. This could happen if the program tries to access the value of the map entry. Since the map is nil, the access will cause a runtime error.

How to avoid problems with assigning to entry in a nil map?

There are two ways to avoid problems with assigning to entry in a nil map:

• The first way is to check if the map is nil before assigning to it. This can be done using the `len()` function. If the length of the map is 0, then the map is nil.
• The second way is to use the `make()` function to create a new map. This will ensure that the map is not nil.

Example of assigning to entry in a nil map

The following code shows an example of assigning to entry in a nil map:

package main

import “fmt”

func main() { // Create a nil map. m := make(map[string]int)

// Try to assign to an entry in the map. m[“key”] = 10

// Print the value of the map entry. fmt.Println(m[“key”]) }

This code will print the following output:

This is because the map is nil, and there is no entry for the key “key”.

Assigning to entry in a nil map can cause problems. To avoid these problems, you should always check if the map is nil before assigning to it. You can also use the `make()` function to create a new map.

Q: What happens when you assign a value to an entry in a nil map in Golang?

A: When you assign a value to an entry in a nil map in Golang, the map is created with the specified key and value. For example, the following code will create a map with the key “foo” and the value “bar”:

m := make(map[string]string) m[“foo”] = “bar”

Q: What is the difference between a nil map and an empty map in Golang?

A: A nil map is a map that has not been initialized, while an empty map is a map that has been initialized but does not contain any entries. In Golang, you can create a nil map by using the `make()` function with the `map` type and no arguments. For example, the following code creates a nil map:

m := make(map[string]string)

You can create an empty map by using the `make()` function with the `map` type and one argument, which specifies the number of buckets to use for the map. For example, the following code creates an empty map with 10 buckets:

m := make(map[string]string, 10)

Q: How can I check if a map is nil in Golang?

A: You can check if a map is nil in Golang by using the `nil` operator. For example, the following code checks if the map `m` is nil:

if m == nil { // The map is nil }

Q: How can I iterate over the entries in a nil map in Golang?

A: You cannot iterate over the entries in a nil map in Golang. If you try to iterate over a nil map, you will get a `panic` error.

Q: How can I avoid assigning a value to an entry in a nil map in Golang?

A: There are a few ways to avoid assigning a value to an entry in a nil map in Golang.

• Use the `if` statement to check if the map is nil before assigning a value to it. For example, the following code uses the `if` statement to check if the map `m` is nil before assigning a value to it:

if m != nil { m[“foo”] = “bar” }

• Use the `defer` statement to delete the entry from the map if it is nil. For example, the following code uses the `defer` statement to delete the entry from the map `m` if it is nil:

defer func() { if m != nil { delete(m, “foo”) } }()

m[“foo”] = “bar”

• Use the `with` statement to create a new map with the specified key and value. For example, the following code uses the `with` statement to create a new map with the key “foo” and the value “bar”:

with(map[string]string{ “foo”: “bar”, })

In this article, we discussed the Golang assignment to entry in nil map error. We first explained what a nil map is and why it cannot be assigned to. Then, we provided several examples of code that would result in this error. Finally, we offered some tips on how to avoid this error in your own code.

We hope that this article has been helpful. If you have any other questions about Golang, please feel free to contact us.

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Assignment to entry in nil map

Why does this program panic?

You have to initialize the map using the make function (or a map literal) before you can add any elements:

See Maps explained for more about maps.

Golang Tutorial

Golang reference, beego framework, golang error assignment to entry in nil map.

Map types are reference types, like pointers or slices, and so the value of rect is nil ; it doesn't point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don't do that.

What do you think will be the output of the following program?

The Zero Value of an uninitialized map is nil. Both len and accessing the value of rect["height"] will work on nil map. len returns 0 and the key of "height" is not found in map and you will get back zero value for int which is 0. Similarly, idx will return 0 and key will return false.

You can also make a map and set its initial value with curly brackets {}.

Most Helpful This Week

Dealing with ‘panic: assignment to entry in nil map’ Error in Go

If you’re a Go developer, you might have encountered the error panic: assignment to entry in nil map . This error is a common pitfall when working with maps, and it occurs when you try to assign a value to an uninitialized map. In this article, we will discuss the reasons behind this error, how to fix it, and some best practices for working with maps in Go.

Understanding the Error

A map in Go is an unordered collection of key-value pairs, where each key can be associated with a value . The value of an uninitialized map is nil and needs to be initialized before it can be used to store key-value pairs Source 9 .

When you try to assign a value to an uninitialized map, Go will raise a runtime error: panic: assignment to entry in nil map Source 10 .

For example, consider the following code:

This code will result in the error: panic: assignment to entry in nil map because the map s.m was not initialized before the assignment Source 10 .

How to Fix the Error

There are two common ways to fix the panic: assignment to entry in nil map error:

By initializing the map before assigning values to it, you can avoid the panic: assignment to entry in nil map error.

Best Practices for Working with Maps in Go

To prevent issues like the panic: assignment to entry in nil map error and ensure efficient use of maps in Go, consider the following best practices:

• Always initialize a map before assigning values to it. This can be done either using the make() function or a constructor function Source 10 .
• Check if a map is nil before using it. If it is nil , initialize it with make() or a constructor function Source 1 .
• Use the defer , panic , and recover mechanisms for error handling and resource cleanup. These mechanisms allow you to gracefully handle errors and ensure resources are properly released Source 6 .
• Keep your code concise and easy to read by using short variable names, simple logic, and clear comments.

By following these best practices, you can avoid common errors like panic: assignment to entry in nil map and write efficient, maintainable code in Go.

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Example error:

This panic occurs when you fail to initialize a map properly.

Initial Steps Overview

• Check the declaration of the map

Detailed Steps

1) check the declaration of the map.

If necessary, use the error information to locate the map causing the issue, then find where this map is first declared, which may be as below:

The block of code above specifies the kind of map we want ( string: int ), but doesn’t actually create a map for us to use. This will cause a panic when we try to assign values to the map. Instead you should use the make keyword as outlined in Solution A . If you are trying to create a series of nested maps (a map similar to a JSON structure, for example), see Solution B .

Solutions List

A) use ‘make’ to initialize the map.

B) Nested maps

Solutions Detail

Instead, we can use make to initialize a map of the specified type. We’re then free to set and retrieve key:value pairs in the map as usual.

B) Nested Maps

If you are trying to use a map within another map, for example when building JSON-like data, things can become more complicated, but the same principles remain in that make is required to initialize a map.

For a more convenient way to work with this kind of nested structure see Further Step 1 . It may also be worth considering using Go structs or the Go JSON package .

Further Steps

• Use composite literals to create map in-line

1) Use composite literals to create map in-line

Using a composite literal we can skip having to use the make keyword and reduce the required number of lines of code.

Further Information

https://yourbasic.org/golang/gotcha-assignment-entry-nil-map/ https://stackoverflow.com/questions/35379378/go-assignment-to-entry-in-nil-map https://stackoverflow.com/questions/27267900/runtime-error-assignment-to-entry-in-nil-map

The Go Blog

Go maps in action.

Andrew Gerrand 6 February 2013

Introduction

One of the most useful data structures in computer science is the hash table. Many hash table implementations exist with varying properties, but in general they offer fast lookups, adds, and deletes. Go provides a built-in map type that implements a hash table.

Declaration and initialization

A Go map type looks like this:

where KeyType may be any type that is comparable (more on this later), and ValueType may be any type at all, including another map!

This variable m is a map of string keys to int values:

Map types are reference types, like pointers or slices, and so the value of m above is nil ; it doesn’t point to an initialized map. A nil map behaves like an empty map when reading, but attempts to write to a nil map will cause a runtime panic; don’t do that. To initialize a map, use the built in make function:

The make function allocates and initializes a hash map data structure and returns a map value that points to it. The specifics of that data structure are an implementation detail of the runtime and are not specified by the language itself. In this article we will focus on the use of maps, not their implementation.

Working with maps

Go provides a familiar syntax for working with maps. This statement sets the key "route" to the value 66 :

This statement retrieves the value stored under the key "route" and assigns it to a new variable i:

If the requested key doesn’t exist, we get the value type’s zero value . In this case the value type is int , so the zero value is 0 :

The built in len function returns on the number of items in a map:

The built in delete function removes an entry from the map:

The delete function doesn’t return anything, and will do nothing if the specified key doesn’t exist.

A two-value assignment tests for the existence of a key:

In this statement, the first value ( i ) is assigned the value stored under the key "route" . If that key doesn’t exist, i is the value type’s zero value ( 0 ). The second value ( ok ) is a bool that is true if the key exists in the map, and false if not.

To test for a key without retrieving the value, use an underscore in place of the first value:

To iterate over the contents of a map, use the range keyword:

To initialize a map with some data, use a map literal:

The same syntax may be used to initialize an empty map, which is functionally identical to using the make function:

Exploiting zero values

It can be convenient that a map retrieval yields a zero value when the key is not present.

For instance, a map of boolean values can be used as a set-like data structure (recall that the zero value for the boolean type is false). This example traverses a linked list of Nodes and prints their values. It uses a map of Node pointers to detect cycles in the list.

The expression visited[n] is true if n has been visited, or false if n is not present. There’s no need to use the two-value form to test for the presence of n in the map; the zero value default does it for us.

Another instance of helpful zero values is a map of slices. Appending to a nil slice just allocates a new slice, so it’s a one-liner to append a value to a map of slices; there’s no need to check if the key exists. In the following example, the slice people is populated with Person values. Each Person has a Name and a slice of Likes. The example creates a map to associate each like with a slice of people that like it.

To print a list of people who like cheese:

To print the number of people who like bacon:

Note that since both range and len treat a nil slice as a zero-length slice, these last two examples will work even if nobody likes cheese or bacon (however unlikely that may be).

As mentioned earlier, map keys may be of any type that is comparable. The language spec defines this precisely, but in short, comparable types are boolean, numeric, string, pointer, channel, and interface types, and structs or arrays that contain only those types. Notably absent from the list are slices, maps, and functions; these types cannot be compared using == , and may not be used as map keys.

It’s obvious that strings, ints, and other basic types should be available as map keys, but perhaps unexpected are struct keys. Struct can be used to key data by multiple dimensions. For example, this map of maps could be used to tally web page hits by country:

This is map of string to (map of string to int ). Each key of the outer map is the path to a web page with its own inner map. Each inner map key is a two-letter country code. This expression retrieves the number of times an Australian has loaded the documentation page:

Unfortunately, this approach becomes unwieldy when adding data, as for any given outer key you must check if the inner map exists, and create it if needed:

On the other hand, a design that uses a single map with a struct key does away with all that complexity:

When a Vietnamese person visits the home page, incrementing (and possibly creating) the appropriate counter is a one-liner:

And it’s similarly straightforward to see how many Swiss people have read the spec:

Concurrency

Maps are not safe for concurrent use : it’s not defined what happens when you read and write to them simultaneously. If you need to read from and write to a map from concurrently executing goroutines, the accesses must be mediated by some kind of synchronization mechanism. One common way to protect maps is with sync.RWMutex .

This statement declares a counter variable that is an anonymous struct containing a map and an embedded sync.RWMutex .

To read from the counter, take the read lock:

To write to the counter, take the write lock:

Iteration order

When iterating over a map with a range loop, the iteration order is not specified and is not guaranteed to be the same from one iteration to the next. If you require a stable iteration order you must maintain a separate data structure that specifies that order. This example uses a separate sorted slice of keys to print a map[int]string in key order:

“nil slice” vs “nil map”

Slice and map are all reference types in Go , and their default values are nil :

The result is like this：

When a slice's value is nil , you could also do operations on it, such as append :

A caveat you should notice is the length of both nil and empty slice is 0 :

So you should compare the slice's value with nil to check whether it is a nil .

Accessing a nil map is OK, but storing a nil map cause program panic:

The result is like this:

So the best practice is to initialize a map before using it, like this:

BTW, you should use the following pattern to check whether there is an element in map or not:

Reference: The Go Programming Language .

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不要向nil map写入(panic: assignment to entry in nil map) #7

kevinyan815 commented Aug 4, 2019 • edited

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fanyingjie11 commented Apr 25, 2022

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