assignment to entry in nil map go

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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.

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

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

Go gotcha: Why can't I add elements to my map?

Why does this code give a run-time error?

You have to initialize the map using the make function before you can add any elements:

Panic: assignment to entry in nil map for complex struct

Complete Code : https://play.golang.org/p/bctvsy7ARqS

package main

import ( “fmt” )

type Plan struct { BufferMeasures map[string]*ItemSiteMeasure } type ItemSiteMeasure struct { itemtest string }

func main() {

func (p *Plan) AddBuffer() { fmt.Println(“method start”) p.BufferMeasures[“itmsit1”] = &ItemSiteMeasure{itemtest: “item1”} fmt.Println("obj values : ", p.BufferMeasures) fmt.Println(“method end”) }

Error /output:

start method start panic: assignment to entry in nil map

goroutine 1 [running]: main.(*Plan).AddBuffer(0xc000068f50) /tmp/sandbox633108119/prog.go:29 +0xe8 main.main() /tmp/sandbox633108119/prog.go:20 +0x90

To initialize a map, use the built in make function The Go Blog: Go maps in action

. start method start obj values : map[itmsit1:0xc000010230] method end end

Thanks Petrus, issue resolved.

This topic was automatically closed 90 days after the last reply. New replies are no longer allowed.

Panic: assignment to entry in nil map

When doing docker login in the command prompt / powershell, I the error posted below. Though when doing this, docker desktop gets logged in just fine.

login Authenticating with existing credentials… panic: assignment to entry in nil map

goroutine 1 [running]: github.com/docker/cli/cli/config/credentials.(*fileStore).Store (0xc0004d32c0, {{0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0x149b9b7, …}, …})

Although powershell is available for Linux and macOS as well, I assume you installed Docker Desktop on Windows, right?

I thing so because I have the same problem and I certainly installed on Windows… Do you have a solution? QVQ

I believe I’m experiencing the same issue - new laptop, new docker desktop for windows install. can’t login via command line:

goroutine 1 [running]: github.com/docker/cli/cli/config/credentials.(*fileStore).Store (0xc0004d4600, {{0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0x0, 0x0}, {0xc00003c420, …}, …}) /go/src/github.com/docker/cli/cli/config/credentials/file_store.go:55 +0x49

I’m experiencing the same issue with my new windows laptop with fresh installation of docker.

Sorry, shortly after posting, I came to think about this very important info. You are of course absolutely correct. This is on a freshly installed Windows 11, latest docker-desktop.

I could try, if wanted. To do a fresh install on a Linux box and see if I experience the same issue there?

I have the same issue, works for me when I use WT and ubuntu, but not from cmd, git bash or powershell

If it is not a problem for you, that coud help to find out if it is only a Windows issue, but since so many of you had the same issue on the same day, it is very likely to be a bug. Can you share this issue on GitHub?

I tried it on my Windows even though I don’t use Docker Desktop on Windows only when I try to help someone, and it worked for me but it doesn’t mean that it’s not a bug.

If you report the bug on GitHub and share the link here, everyone can join the conversation there too.

In the meantime everyone could try to rename the .docker folder in the \Users\USERNAME folder and try the docke rlogin command again. If the error was something in that folder, that can fix it, but even if it is the case, it shouldn’t have happened.

you cloud try to run docker logout and then docker login ,it works for me .

That’s a good idea too.

I can verify that this did help on my PC too. I have created en issue here:

Hi all, a fix for this will be tracked on the docker/cli issue tracker: Nil pointer dereference on loading the config file · Issue #4414 · docker/cli · GitHub

I was using “az acr login” to do an azure registry docker login and getting this error, but I followed your advice and did a “docker logout” and that cleaned up my issue.

worked for my on my box (latest docker - Docker version 24.0.2, build cb74dfc) on W11. thx for solution.

its work for me. Recommend!

This solution works for me

“docker logout” works for me. Thank you!

Logout worked here too!

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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 Loading

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assignment to nil map golang

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assignment to entry in nil map , initialization map doesn

The code below gives me "assignement to entry in nil map" error, I searched this error, and many answer says I need to initialize my map, I tried to initialize the map need as "need := make(map[string]Item)" it still gives me the same error, b.ingredients returns a type of map[string]Item, what is my mistake here ?

• Please post the Bread map structure. It is not clear what is b.ingredients –  Himanshu Commented Apr 8, 2018 at 7:28
• @Himanshu I've posted it, thanks for noticing –  YuanZheng Hu Commented Apr 8, 2018 at 7:45
• @YuanZhengHu Please also post how you're initializing Bread (and it's ingredients field specifically). –  Emile Pels Commented Apr 8, 2018 at 7:46
• insufficient details –  user4466350 Commented Apr 8, 2018 at 7:58
• What is left you are using and also on which line you are getting here mention the comment in code where you are getting error –  Himanshu Commented Apr 8, 2018 at 8:00

You are using named return values in your ShoppingList function, so difference and left are declared : they are set to the types' zero-values. For maps, this is nil ( see spec ) - naturally, appending an item to a nil map results in a panic.

So, before appending any items to them, do the following to assign them a value:

• Thanks! Is there any differnce between initialize the map with make and assinging the map variable with ":=" ? I'm new to Go –  YuanZheng Hu Commented Apr 8, 2018 at 8:25
• 1 @YuanZhengHu, I strongly suggest you take the Tour . This is covered in "Short variable declarations". –  Peter Commented Apr 8, 2018 at 9:36
• @YuanZhengHu Please note you should use = instead of := here. Taking the tour like Peter suggests will tell you why :) –  Emile Pels Commented Apr 8, 2018 at 16:33

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