# 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.
```go
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
```go
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。
```go
delete(map, key)
```
**檢查鍵是否存在於 map**
```go
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]
```
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