How to solve the 'cannot use x (type y) as type z in map index' error in golang?

During the development process using Golang, we often encounter the error message "cannot use x (type y) as type z in map index". This error message is generally because we did not pay attention to the type of key value when using the map type, but the specific situation varies from person to person, and the cause of the error may be more complicated. In this article, we will explain how to troubleshoot this error and resolve it.

First of all, we need to make it clear that the map type in Golang is a key-value pair structure. Key is an untyped type that only supports equality comparison, and Value is a mapped value that can be of any type. When declaring a map type variable, we need to specify its Key and Value types.

The following is a simple example:

// 使用 map 类型声明一个变量
var mp map[string]int

// 给 map 变量赋值
mp = make(map[string]int)

// 往 map 中添加键值对
mp["hello"] = 1000

In the above example, we use the make function to allocate a map storage space and add it to the map variable# A key-value pair has been added to ##mp.

We have found from practice that the reason for the "cannot use x (type y) as type z in map index" error is that we do not pay attention to the type of the key value when using map. The following are some situations that may cause this error:

Scenario 1: Key type mismatch

When we assign a value to a non-existent Key according to the type corresponding to Value, it will cause This error occurs. For example:

// 使用 map 类型声明一个变量
var mp map[string]int

// 给 map 变量赋值
mp = make(map[string]int)

// 如下的代码就会导致 "cannot use x (type y) as type z in map index"
// 因为 Key 类型不匹配，但是没有定义成员为 int 类型的 mp["hello"]
mp[100] = 1000

Solution:

Check the type of Key and make it consistent with the Key type of the map variable.

Scenario 2: The type of Value does not match

After the storage space has been allocated, if we assign the Value of type int to a Key of the corresponding type, there will be no problem. However, if the types we assign do not match, the system will error. For example:

// 使用 map 类型声明一个变量
var mp map[string]int

// 给 map 变量赋值
mp = make(map[string]int)

// 如下的代码就会导致 "cannot use x (type y) as type z in map index"
// 因为 Value 类型不匹配，其实际类型为 string，而不是 int
mp["hello"] = "world"

Solution:

Check the type of Value to make it consistent with the Value type of the map variable.

Scenario 3: The map variable is not initialized

When using a map type variable, if no storage space is allocated, then the variable is a nil map. If we try to assign a value to it, we get a "cannot use x (type y) as type z in map index" error. For example:

// 使用 map 类型声明一个变量
var mp map[string]int

// 如下的代码就会导致 "cannot use x (type y) as type z in map index"
// 因为 map 变量 mp 没有被初始化，所以在给它的 Key 赋值时出错了。
mp["hello"] = 1000

Solution:

Before using, allocate enough storage space, or use the

make function to initialize.

Scenario 4: Using a non-existent variable

Sometimes when using a map variable, due to spelling errors or other factors, we will use a non-existent map variable, for example:

// 定义名为 mp 的 map 类型变量
var mp map[string]int

// 定义名为 nm 的 string 类型变量
var nm string

// 如下的代码就会导致 "cannot use x (type y) as type z in map index"
// 因为 nm 完全不是一个 map 类型变量，所以使用其进行索引就会出现错误。
nm["hello"] = 1000

Solution:

Check whether the variable name is spelled correctly and of the correct type.

Summary:

The above are several common situations and solutions that lead to "cannot use x (type y) as type z in map index" errors. It is recommended to add more comments when writing code and write the code into an easy-to-understand form, so as to avoid some common mistakes.

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