map 宣告與操作

映射 MAP

• 使用var m map[string]int 可以宣告一個nil的map，需注意未初始化就塞值會引發panic

• **使用內建的 make() 函數初始化 map，**該函數將返回已初始化及可以使用的 map。

  • 或是var map[string]int{}可以透過將大括號留空，建立一個空 map

  • 須注意如果使用new(map[string]string)會產生nil map，操作會引發panic

• 傳遞時為按引用傳遞的，對元素的修改將導致函式外的值也發生改變

宣告

import "fmt"

func main() {
	var m map[string]int //語法宣告 map ///map 的零值是 nil
	fmt.Println(m) map[]
	if m == nil {
		fmt.Println("m is nil")
	}
	// m["one hundred"] = 100 //panic: assignment to entry in nil map

	var m2 = make(map[string]int) //使用內建的 make() 函數初始化 map

	fmt.Println(m2)

	if m2 == nil {
		fmt.Println("m2 is nil")
	} else {
		fmt.Println("m2 is not nil")
	}

	m2["one hundred"] = 100
	fmt.Println(m2)

	var m3 = map[string]int{} //可以透過將大括號留空，使用 map 定數來建立一個空 map
	if m3 == nil {
		fmt.Println("m3 is nil")
	} else {
		fmt.Println("m3 is not nil")
	}
	m3["one hundred"] = 100
	fmt.Println(m3)
}

Map literals

Map literals are like struct literals, but the keys are required.

package main

import "fmt"

type Vertex struct {
	Lat, Long float64
}

var m = map[string]Vertex{
	"Bell Labs": Vertex{
		40.68433, -74.39967,
	},
	"Google": Vertex{
		37.42202, -122.08408,
	},
}

func main() {
	fmt.Println(m)
}

加入鍵值到 map

m[key] = value

從 map 中刪除鍵

delete() 函數不會返回任何值。另外，如果該鍵不存在於 map，它也不會執行任何操作。

delete(map, key)

檢查鍵是否存在於 map

value, ok := m[key]

迭代 map

• map 是沒有順序的集合，不能保證 map 的迭代順序都相同。

for key, value:= range m{
}

map 是參考型別

當你將 map 分派給新變數時，它們都參考相同的底層資料結構

範例:

package main

import "fmt"

func main() {
	var personAgeMap1 = map[string]int{
		"Apple": 25,
		"James": 32,
		"Sarah": 29,
	}
	age, ok := personAgeMap1["Apple"]
	fmt.Println("age:", age, ",ok", ok)

	var m2 = personAgeMap1 //，它們都參考相同的底層資料結構
	fmt.Println("--ori.---")
	fmt.Println("personAgeMap1 = ", personAgeMap1)
	fmt.Println("m2 = ", m2)
	fmt.Println("-modify personAgeMap1=18----")
	for name, age := range personAgeMap1 {
		fmt.Println(name, age)
		personAgeMap1[name] = 18
	}

	fmt.Println("personAgeMap1 = ", personAgeMap1)
	fmt.Println("m2 = ", m2)
	fmt.Println("-modify add m2["Amy"] = 10----")
	m2["Amy"] = 10
	fmt.Println("personAge = ", personAgeMap1)
	fmt.Println("m2 = ", m2)
	fmt.Println("--modifymap(personAgeMap1)---")
	modifymap(personAgeMap1)
	fmt.Println("personAge = ", personAgeMap1)
	fmt.Println("m2 = ", m2)
	
}

func modifymap(personAge map[string]int) {
	for name := range personAge {
		personAge[name] = 20
	}
	delete(personAge, "Apple")
}

結果:

age: 25 ,ok true
--ori.---
personAgeMap1 =  map[Apple:25 James:32 Sarah:29]
m2 =  map[Apple:25 James:32 Sarah:29]
-modify personAgeMap1=18----
Apple 25
James 32
Sarah 29
personAgeMap1 =  map[Apple:18 James:18 Sarah:18]
m2 =  map[Apple:18 James:18 Sarah:18]
-modify add m2[Amy] = 10----
personAge =  map[Amy:10 Apple:18 James:18 Sarah:18]
m2 =  map[Amy:10 Apple:18 James:18 Sarah:18]
--modifymap(personAgeMap1)---
personAge =  map[Amy:20 James:20 Sarah:20]
m2 =  map[Amy:20 James:20 Sarah:20]