# Go 语言Map(集合)

Map 是一种无序的键值对的集合。Map 最重要的一点是通过 key 来快速检索数据，key 类似于索引，指向数据的值。

Map 是一种集合，所以我们可以像迭代数组和切片那样迭代它。不过，Map 是无序的，我们无法决定它的返回顺序，这是因为 Map 是使用 hash 表来实现的。

### 定义 Map

```/* 声明变量，默认 map 是 nil */
var map_variable map[key_data_type]value_data_type

/* 使用 make 函数 */
map_variable = make(map[key_data_type]value_data_type)
```

### 实例

```package main

import "fmt"

func main() {
var countryCapitalMap map[string]string
/* 创建集合 */
countryCapitalMap = make(map[string]string)

/* map 插入 key-value 对，各个国家对应的首都 */
countryCapitalMap["France"] = "Paris"
countryCapitalMap["Italy"] = "Rome"
countryCapitalMap["Japan"] = "Tokyo"
countryCapitalMap["India"] = "New Delhi"

/* 使用 key 输出 map 值 */
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}

/* 查看元素在集合中是否存在 */
captial, ok := countryCapitalMap["United States"]
/* 如果 ok 是 true, 则存在，否则不存在 */
if(ok){
fmt.Println("Capital of United States is", captial)
}else {
fmt.Println("Capital of United States is not present")
}
}
```

```Capital of France is Paris
Capital of Italy is Rome
Capital of Japan is Tokyo
Capital of India is New Delhi
Capital of United States is not present
```

## delete() 函数

delete() 函数用于删除集合的元素, 参数为 map 和其对应的 key。实例如下：

```package main

import "fmt"

func main() {
/* 创建 map */
countryCapitalMap := map[string] string {"France":"Paris","Italy":"Rome","Japan":"Tokyo","India":"New Delhi"}

fmt.Println("原始 map")

/* 打印 map */
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}

/* 删除元素 */
delete(countryCapitalMap,"France");
fmt.Println("Entry for France is deleted")

fmt.Println("删除元素后 map")

/* 打印 map */
for country := range countryCapitalMap {
fmt.Println("Capital of",country,"is",countryCapitalMap[country])
}
}
```

```原始 map
Capital of France is Paris
Capital of Italy is Rome
Capital of Japan is Tokyo
Capital of India is New Delhi
Entry for France is deleted

Capital of Italy is Rome
Capital of Japan is Tokyo
Capital of India is New Delhi
```