Go map

Map的声明要求

Map types
The value of an uninitialized map is nil.
A new, empty map value is made using the built-in function make, which takes the map type and an optional capacity hint as arguments:

make(map[string]int)
make(map[string]int, 100)

The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of nil maps. A nil map is equivalent to an empty map except that no elements may be added.

Map的创建与初始化

func main (
    //第一种
    //声明map 产生一个值为nil的map即零值map
    //值为nil的map等同于空map，但却不能增加值（与channel相同，slice则除外）
    var m1 map[string]interface{}
    //m1["tom"] = "cat" //报错：panic: assignment to entry in nil map
    fmt.Println(m1)

    //零值切片（通过var声明的切片）可以直接使用，无需make创建
    //所以，返回空slice，返回nil即可
    var s1 []int
    fmt.Println(s1)  //[]
    s1 = append(s1, 1)
    fmt.Println(s1)  //[1]

     //第二种
    //使用字面量
    m2 := map[string]int{}
    m2["tom"] = 12
    fmt.Println(m2)  //map[tom:12]
    //或者
    m3 := map[string]int{"tom":12}
    fmt.Println(m3)  //map[tom:12]

    //第三种
    //使用make
    m4 := make(map[string]string, 10)
    m4["tom"] = "cat"
    fmt.Println(m4)  //map[tom:cat]
)

Map的使用

    m5 := map[string]string{}
    //增加
    m5["tom"] = "dog"
    //修改
    m5["tom"] = "cat"
    //查询
    val, ok := m5["tom"]
    if !ok {
        fmt.Println("无此mao")
        return
    }
    fmt.Println(val)
    //删除
    delete(m5, "tom")
    fmt.Println(m5) //map[]

Map的赋值与值拷贝

    //map赋值
    //由于map是引用类型，在进行赋值时是不会将指向底层的数据一起拷贝的
    //所以，赋值后的数据进行更改，还是会改动底层的数据
    m6 := map[string]string{"tom":"cat"}
    fmt.Println(m6) //map[tom:cat]
    m7 := m6
    m7["tome"] = "dog"
    fmt.Println(m6) //map[tom:cat tome:dog]
    fmt.Println(m7) //map[tom:cat tome:dog]

    //map值拷贝
    m8 := map[string]string{"tom":"cat", "jack":"mouse"}
    m9 := make(map[string]string, len(m8))
    for index, value := range m8 {
        m9[index] = value
    }
    fmt.Println(m8) //map[jack:mouse tom:cat]
    fmt.Println(m9) //map[jack:mouse tom:cat]
    m9["tom"] = "dog"
    fmt.Println(m8) //map[jack:mouse tom:cat]
    fmt.Println(m9) //map[jack:mouse tom:dog]