golang map changes value

The map in golang is a very commonly used data structure, which can obtain the corresponding value through key. In golang, the implementation of map is similar to a hash table, supporting fast search and insertion operations. But when using map, sometimes we need to modify its value. This article will introduce in detail how to change the value of map in golang.

1. map in golang

The map type in golang is defined as: map[keyType]valueType, where keyType is the key value type and valueType is the key corresponding to value type. For example:

var m map[string]int //定义一个string类型的键和一个int类型value的map

In golang, map initialization can be achieved through make or literals.

//make初始化
m1 := make(map[string]string)

//字面量初始化
m2 := map[string]string{
    "key1": "value1",
    "key2": "value2",
    "key3": "value3",
}

2. Change the value of map

In golang, map itself is a reference type, so when the map is operated on, the corresponding original map will also be modified. If we need to modify the value in the map, we can do it through the subscript operator.

m := map[string]int{
    "apple":  2,
    "banana": 3,
    "orange": 4,
}
m["apple"] = 10 //将apple的value改为10
fmt.Println(m)  //输出: map[apple:10 banana:3 orange:4]

In the above example, we modify the value of apple to 10 through m["apple"] = 10.

If the corresponding key does not exist, a new key-value pair will be automatically added.

m := map[string]int{
    "apple":  2,
    "banana": 3,
    "orange": 4,
}
m["pear"] = 5 //添加一个新的key-value对
fmt.Println(m) //输出: map[apple:2 banana:3 orange:4 pear:5]

3. Determine whether the key exists

Before modifying the value of the map, we need to first determine whether the corresponding key exists in the map.

You can use v, ok := m[k] to check whether there is a key-value pair with key k in the map.

m := map[string]int{
    "apple":  2,
    "banana": 3,
    "orange": 4,
}
if v, ok := m["apple"]; ok {
    m["apple"] = v + 1 //将apple的value加1
}
fmt.Println(m) //输出: map[apple:3 banana:3 orange:4]

If the key exists, the value of ok is true, and the value of v is the corresponding value.

If the corresponding key does not exist, the value of ok is false, and the value of v is zero value of value type.

When judging that the key exists, we can directly modify the value of the map.

4. Traverse the map and modify the value

Sometimes, we need to traverse the map and modify the value.

A common method is to use for range to loop through the map and then modify the value.

m := map[string]int{
    "apple":  2,
    "banana": 3,
    "orange": 4,
}
for k, v := range m {
    m[k] = v * 2 //将key对应的value值乘2
}
fmt.Println(m) //输出: map[apple:4 banana:6 orange:8]

In the above example, we traverse the map through the for range loop and multiply the value corresponding to the key by 2. This method is suitable for situations where all keys need to be modified.

If you only need to modify the value of the specified key, you can use the method described above to directly modify the value of the map.

5. Thread safety

Finally, it should be noted that map in golang is not thread-safe. In the case of concurrency, multiple coroutines modify the same map at the same time, which may cause data competition problems.

In order to solve this problem, you can use the sync.Map type provided in the sync package. It is a thread-safe map implementation.

var m sync.Map
m.Store("apple", 2)
m.Store("banana", 3)
m.Store("orange", 4)

//使用Load方法获取map中的value值
if v, ok := m.Load("apple"); ok {
    m.Store("apple", v.(int)+1) //将apple的value加1
}

By using the sync.Map type, you can avoid data competition problems in concurrent situations.

Summary

This article details how to change the value of map in golang. We learned how to modify the value of the map through the subscript operator, and also learned how to determine whether the corresponding key value exists in the map and how to traverse the map and modify the value. In addition, we also introduced the sync.Map type, which is a thread-safe map implementation. When using map, you need to pay attention to thread safety issues to avoid data competition issues.

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