go panic 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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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

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.

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:

Nillability and zero-values in go

Beeing a long time java-developer, I am obsessed with null-checking and handling null values. In golang, the story is somewhat different. In this post I will try to describe how nil and zero-values are used in golang.

non-nillable and nillable types

Types can be either nillable or non-nillable in go. The non-nillable types can never be nil and will never cause you a nil-panic (the java equivalent of nullpointerexception) But when are dealing with the nillable types, we have to take a bit of caution although not as much as in java(or other languages with nillable types).

The non-nillables basic types

In go, the basic types are not nillable. A statement like

does not compile because an int can never be nil. The default value of an unassigned int type is 0. Running the statement

will output the default value of int ; “ 0 ”. We call this the zero-value of the type.

The same way int defaults to 0, these are the other basic types with their zero-values:

Non-nillable structs

Composed struct types are also non-nillable, and the default value of a struct will contain the default value for all its fields.

Consider the code with the struct type Person,

will print [main.Person{Name:"", Age:0}] when run in main. You can test this on this snippet on The Go Playground.

The nillable types

More advanced types are nillable and can cause panic if they are not initialized.

The nillable types are functions, channels, slices, maps, interface-types and pointers .

However, nil-slices and nil-maps can still be used and does not have to be initialized before we start using them.

Maps will always return the zero-value of the value if it is nil, the same behaviour as if the key of the map is non-existent. The code

simply prints "" length 0 , the extracted value for key 99 is the zero value of string .

Assigning values to a nil-map, will however cause panic:

Refering to out-of-bounds on slices will cause panic, but operations like len() and cap() will not panic. They will simply return 0 , since both capacity and length is zero for an uninitialized slice. Append can be safely called on the nil-slice. So the code

Play with this example on the playground .

nillable pointers, functions and interface-types can cause panic

Pointers and interface-types are however nillable. Whenever dealing with these types, we have to consider if they are nil or not to avoid panics. These code-snippets for instance, will cause a panic:

nil channels blocks forever

Trying to read from a nil-channel or write to a nil-channel will block forever. Closing a nil-channel will cause panic.

nil is well defined in go. Knowing what can be nil and how to handle nil values of the different types increases your understanding of whats happening and can help you write better go-code.

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

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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`panic: assignment to entry in nil map` at nested maps

this code giving err panic: assignment to entry in nil map

is anything wrong in implementing the maps? i want to create a map key like 1,2,3.... like this, so imeplemented [string(rune(id+1))] now it giving error atfter implementing this one.

i want to create a map to convert it into json like this:

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I've got an error with map

error: panic: assignment to entry in nil map

goroutine 1 [running]: github.com/SpecterTeam/SpecterGO/utils.(*Config ).Set(…) /home/fris/go/src/github.com/SpecterTeam/SpecterGO/utils/Config.go:157 exit status 2

here is the code: const ( TypeJson = iota // .json TypeYaml // .yml & .yaml )

type( Content map[string]interface{}

func NewConfig(file string, configType int, defaults map[string]interface{}) Config { c := Config{} c.SetConfigType(configType) c.SetFile(file) if FileExists(file) { if ext := filepath.Ext(file); ExtMatchType(ext, configType) { c.SetConfig(c.Unmarshal()) } else { err := errors.New(“Ext of " + file + " doesn’t match the configType!”) HandleError(err) } } else { os.Create(file) } for key, value := range defaults { c.CheckDefault(key, value) } c.Save() return c }

func (c *Config) CheckDefault(key string, value interface{}) { if !c.Exist(key) { c.Set(key, value) } }

func ExtMatchType(ext string, configType int) bool { switch configType { case TypeJson: if ext == “json” { return true } else { return false } case TypeYaml: if ext == “yml” || ext == “yaml” { return true } else { return false } } return false }

func (c *Config) Marshal() ([]byte, error) { var b []byte

func (c *Config) Unmarshal() Content { var r Content switch c.ConfigType() { case TypeYaml: bts,_ := ioutil.ReadFile(c.File()) yaml.Unmarshal(bts,&r) case TypeJson: bts,_ := ioutil.ReadFile(c.File()) json.Unmarshal(bts,&r) } return r }

func (c *Config) Save() { bts, err := c.Marshal() if err != nil { HandleError(err) } else { ioutil.WriteFile(c.File(), bts, 0644) }

func (c *Config) ConfigType() int { return c.configType }

func (c *Config) SetConfigType(configType int) { c.configType = configType }

func (c *Config) Config() Content { return c.config }

func (c *Config) SetConfig(config Content) { c.config = config }

func (c *Config) File() string { return c.file }

func (c *Config) SetFile(file string) { c.file = file }

func (c *Config) Set(key string, value interface{}) { c.config[key] = value }

func (c *Config) Get(key string) *interface{} { i := c.config[key] return &i }

func (c *Config) Remove(key string) { config := c.Config() delete(config, key) } func (c *Config) Exist(key string) bool { config := c.Unmarshal() _,exist := config[key] return exist }

Consider the error message

and correlate with the information about how to use maps in, for example, the tour .

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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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fanyingjie11 commented Apr 25, 2022

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assignment to entry in nil map

I am trying to assign values to a map that is initialized in the init func.

But panic occurs: assignment to entry in nil map

https://play.golang.org/p/yOwXzDkWIo

• instantiation

2 Answers 2

The function takes Test as value, so it gets its own copy of it. All changes to test Test will be gone when the function returns. Take Test by pointer instead:

Note though, the struct Test is exported, the method init is not, therefore a user of your library could potentially create a Test but not init it properly. It seems like the go community has established the convention of a freestanding NewType method:

This ensures a user can only obtain a test by calling NewTest and it will be initialized as intended.

• Makes total sense, i can't believe I didnt realize this myself. Thanks for the answer and the extra pair of eyes. –  TjeerdJan Commented Aug 21, 2016 at 12:39

You should use a pointer receiver for the init method:

Without a pointer, you are initializing a map for a copy of the test object. The actual test object never gets an initialized map.

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