golang error assignment to entry in nil map

golang panic: assignment to entry in nil map（map赋值前要先初始化 - map的初始化及使用 ）

• 执行 test1 提示
• 执行 test2 也提示

因为 没有初始化 ， map 不像 array 和 基础类型 在你 定义 就会给你 初始化一个默认值

1）、test1 打印

2）、test2 打印.

请填写红包祝福语或标题

1.余额是钱包充值的虚拟货币，按照1:1的比例进行支付金额的抵扣。 2.余额无法直接购买下载，可以购买VIP、付费专栏及课程。

HatchJS.com

Cracking the Shell of Mystery

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.

Author Profile

Latest entries

• December 26, 2023 Error Fixing User: Anonymous is not authorized to perform: execute-api:invoke on resource: How to fix this error
• December 26, 2023 How To Guides Valid Intents Must Be Provided for the Client: Why It’s Important and How to Do It
• December 26, 2023 Error Fixing How to Fix the The Root Filesystem Requires a Manual fsck Error
• December 26, 2023 Troubleshooting How to Fix the `sed unterminated s` Command

Similar Posts

Ansible: how to read a file into a variable.

Ansible: Reading a File into a Variable One of the most common tasks you’ll need to perform when using Ansible is reading a file into a variable. This can be useful for a variety of purposes, such as getting the contents of a configuration file, passing data to a playbook, or storing sensitive information. In…

How to Straighten Warped 2x4s: A Step-by-Step Guide

How to Straighten Warped 2x4s Have you ever tried to build something with a warped 2×4 and ended up with a crooked project? If so, you’re not alone. Warped 2x4s are a common problem, but there are a few simple steps you can take to straighten them out. In this article, we’ll show you how…

How to Add an Empty Column to a DataFrame in Python

Adding an empty column to a dataframe Dataframes are a powerful tool for storing and manipulating data. They are essentially two-dimensional tables, with rows representing observations and columns representing variables. In this article, we will show you how to add an empty column to a dataframe. This can be useful for a variety of purposes,…

How to Use the PROC SQL FORMAT DATE Function

Proc SQL Format Date: A Guide for Data Scientists Data scientists often need to work with dates and times. This can be a challenge, as dates and times can be represented in a variety of formats. The `proc sql format date` statement can be used to format dates and times in a consistent and readable…

VS Code: How to Find Files by Name

VS Code Find by File Name: A Quick and Easy Guide Visual Studio Code is a powerful code editor that can be used for a variety of development tasks. One of its most useful features is the ability to find files by name. This can be a great time-saver, especially when you’re working on a…

Ubuntu Bluetooth Driver 22.04: How to Install and Troubleshoot

Ubuntu Bluetooth Driver 22.04: A Guide for New Users Bluetooth is a wireless technology that allows you to connect devices such as your phone, headphones, and speakers to your computer. In Ubuntu 22.04, the Bluetooth driver has been updated to improve performance and stability. This guide will show you how to install and use the…

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

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

• Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers
• Advertising & Talent Reach devs & technologists worldwide about your product, service or employer brand
• OverflowAI GenAI features for Teams
• OverflowAPI Train & fine-tune LLMs
• Labs The future of collective knowledge sharing
• About the company Visit the blog

Collectives™ on Stack Overflow

Find centralized, trusted content and collaborate around the technologies you use most.

Q&A for work

Connect and share knowledge within a single location that is structured and easy to search.

Get early access and see previews of new features.

golang map of interface - panic: assignment to entry in nil map [duplicate]

I am new to golang and I am trying to create a map of type map[string]interface{}.

But when I try to create a new key when it doesn't exists, I get a runtime error "panic: assignment to entry in nil map". Can anyone tell me what I am doing wrong please?

Go PlayGround: https://play.golang.org/p/vIEE0T11yl

Here is my code:

• I didn't knew if I was missing an initialization when I asked this question, so I don't know if this is technically a duplication. Otherwise, i knew how to initialize a map within struct or in this case map of interface slice. –  bn00d Commented Sep 18, 2015 at 15:24

You need to initialise the map itself: https://play.golang.org/p/wl4mMGjmRP

You could also use a constructor for your struct type - e.g. NewBuffer(...) *Buffer - that initialises the field as well, but it's good practice to check for nil before using it. Same goes for accessing map keys.

• I knew I was doing something really silly mistake. Thanks a lot, you saved me lot of time –  bn00d Commented Sep 18, 2015 at 5:15

Not the answer you're looking for? Browse other questions tagged go hashmap slice or ask your own question .

• The Overflow Blog
• Community Products Roadmap Update, October 2024
• Meet the AI native developers who build software through prompt engineering
• Featured on Meta
• Preventing unauthorized automated access to the network
• Upcoming initiatives on Stack Overflow and across the Stack Exchange network...
• Feedback Requested: How do you use the tagged questions page?
• Proposed designs to update the homepage for logged-in users

Hot Network Questions

• Producing solo work
• SwiftUI TimeUnits-Converter App
• How could tusked dragons eat?
• How does the Push weapon mastery interact with occupied spaces?
• Is it possible to climb to the outer side of Sigil?
• Is the aboleth's mucus cloud ability supposed to hinder affected PCs? It seems beneficial
• Would adapting grounding notation to musical notation make the concept seem more distinctive and also clearer/more useful?
• how can I solve equation of multi-variables raised to themselves?
• How can I draw a simple house?
• Definition of vector
• Is LetsEncrypt activity Public?
• Finding a formula which involves iteration
• Firefox snap won't save websites
• Why can I define a std::string instance that is constinit? Isn't constinit forbidden if an object requires dynamic initialization?
• Are seaplanes with floats more aerodynamically efficient (less drag) than planes with tires?
• How to center a series of text width a fixed width that can automatically linebreak and underline it?
• Why does my iPhone 16 Pro have iOS 18.0 but I can’t find a way to activate developer mode anywhere?
• Concocting a fourth spatial dimension that can support wormhole-like travel and not mess up life?
• What is Iran's long-term objective in the Middle East?
• When did the distinction between "pure" and "applied" mathematics become common?
• My institution only counts two publications in the same journal for promotion; I have four. Can I retract them and republish somewhere else?
• jq create object with property name from variable
• Multiplication in Peter-Weyl theorem
• Airport security confiscated umbrella

Map is apparently nil even though I already assigned to it

Go version: 1.12.2 Error text: panic: assignment to entry in nil map

I’m aware that assigning to just a var myMap map[string]int leads to an issue with assignment to a nil map and that you have to do myMap := make(map[string]int) but I seem to be having the same issue with a map that I’ve already assigned to. Below is the stripped down code. Beware that I’ve added a bunch of superfluous stuff as a means of troubleshooting (though I tried to rip a lot of that out):

I added “tmpRect” in an attempt to make sure that detMap is actually not nil. ImageFrames.Sublimate() is below:

I get the same issue if I just do tmpImg, _, _, _, detMap, sublError := imageFrames.Sublimate(frm) without creating the map beforehand.

Hi @ashinnv Try to send the current value of the frm to the goroutine. Example:

Or replace “input[i]” with the variable “i”. Otherwise maybe you have a concurrency problem.

Which line here is that panic coming from? That should indicate which line has the map access where the map is nil.

Which line?

I tried that but still ended up with the same error. I’ve re-written the function to include the lines from Sublimate(), but I’m still getting the issue. I completely removed the anonymous function, as well, so that this is now just a serial loop with just an input chan and an output chan. The new, unstripped version is below:

This is just weird. I’m sure it’s some stupid little thing I’m completely overlooking, but still, geez.

The issue was coming from creating the “Frame” type. I create them in a completely different process from images taken from the webcam. These are then encoded and transmitted down a pipeline. When I originally created the Frame type, I created a function that spits out a new frame to be used across the whole project for when I want to create frames, but at this time, I didn’t have the map of opencv detections included.

When I added computer vision to the project, I didn’t update the function that creates blank frames, instead only adding the struct element. This meant that following that, all new frames that were created would have a nil element where the detection map should be.

That frame with a nil detection map would be encoded and sent down the pipeline, where it caused problems in this process. Thanks for all the help and suggestions, guys.

Is this nil? You create retMap in the Sublimate function, but then overwrite it with whatever is in inFrm.Detections and then return that.

EDIT : Oops. I missed that you found out already. Never mind!

That came from the thrashing I’d given the source in trying to figure out what was going on. I kept adding and removing stuff in an attempt to get an idea of where the issue was happening. Never considered it wasn’t even in this process at all.