golang 數組與切片

經過下面幾個問題來更好理解golang 的數組和切片

1. 類型

   數組是值類型，將一個數組賦值給另外一個數組時，傳遞的是一份拷貝。
   切片是引用類型，切片包裝的數組稱爲該切片的底層數組。
   咱們來看一段代碼

   //a是一個數組，注意數組是一個固定長度的，初始化時候必需要指定長度，不指定長度的話就是切片了
   a := [3]int{1, 2, 3}
   //b是數組，是a的一份拷貝
   b := a
   //c是切片，是引用類型，底層數組是a
   c := a[:]
   for i := 0; i < len(a); i++ {
     a[i] = a[i] + 1
   }
   //改變a的值後，b是a的拷貝，b不變，c是引用，c的值改變
   fmt.Println(a) //[2,3,4]
   fmt.Println(b) //[1 2 3]
   fmt.Println(c) //[2,3,4]

2. make
   make 只能用於slice, map 和 channel, 因此下面一段代碼生成了一個slice，是引用類型

   s1 := make([]int, 0, 3)
   
   for i := 0; i < cap(s1); i++ {
       s1 = append(s1, i)
   }
   s2 := s1
   for i := 0; i < len(a); i++ {
       s1[i] = s1[i] + 1
   }
   
   fmt.Println(s1)  //[1 2 3]
   fmt.Println(s2)  //[1 2 3]

3. 當對slice append 超出底層數組的界限時

   //n1是n2的底層數組
   n1 := [3]int{1, 2, 3}
   n2 := n1[0:3]
   fmt.Println("address of items in n1: ")
   for i := 0; i < len(n1); i++ {
       fmt.Printf("%p\n", &n1[i])
   }
   //address of items in n1:
   //0xc20801e160
   //0xc20801e168
   //0xc20801e170
   fmt.Println("address of items in n2: ")
   for i := 0; i < len(n2); i++ {
       fmt.Printf("%p\n", &n2[i])
   }
   //address of items in n2:
   //0xc20801e160
   //0xc20801e168
   //0xc20801e170
   
   //對n2執行append操做後，n2超出了底層數組n1的j
   n2 = append(n2, 1)
   fmt.Println("address of items in n1: ")
   for i := 0; i < len(n1); i++ {
       fmt.Printf("%p\n", &n1[i])
   }
   //address of items in n1:
   //0xc20801e160
   //0xc20801e168
   //0xc20801e170
   
   fmt.Println("address of items in n2: ")
   for i := 0; i < len(n2); i++ {
       fmt.Printf("%p\n", &n2[i])
   }
   //address of items in n2:
   //0xc20803a2d0
   //0xc20803a2d8
   //0xc20803a2e0
   //0xc20803a2e8

4. 引用「失效」
   實現了刪除slice最後一個item的函數

   func rmLast(a []int) {
       fmt.Printf("[rmlast] the address of a is %p", a)
       a = a[:len(a)-1]
       fmt.Printf("[rmlast] after remove, the address of a is %p", a)
   }

   調用此函數後，發現原來的slice並無改變

   func main() {
       xyz := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
       fmt.Printf("[main] the address of xyz is %p\n", xyz)
       rmLast(xyz)
       fmt.Printf("[main] after remove, the address of xyz is %p\n", xyz)
       fmt.Printf("%v", xyz) //[1 2 3 4 5 6 7 8 9]
   }

   打印出來的結果以下：

   [main] the address of xyz is 0xc2080365f0
   [rmlast] the address of a is 0xc2080365f0
   [rmlast] after remove, the address of a is 0xc2080365f0
   [main] after remove, the address of xyz is 0xc2080365f0
   [1 2 3 4 5 6 7 8 9]

   這裏直接打印了slice的指針值，由於slice是引用類型，因此指針值都是相同的，咱們換成打印slice的地址看下

   func rmLast(a []int) {
       fmt.Printf("[rmlast] the address of a is %p", &a)
       a = a[:len(a)-1]
       fmt.Printf("[rmlast] after remove, the address of a is %p", &a)
   }
   func main() {
       xyz := []int{1, 2, 3, 4, 5, 6, 7, 8, 9}
       fmt.Printf("[main] the address of xyz is %p\n", &xyz)
       rmLast(xyz)
       fmt.Printf("[main] after remove, the address of xyz is %p\n", &xyz)
       fmt.Printf("%v", xyz) //[1 2 3 4 5 6 7 8 9]
   }

   結果：

   [main] the address of xyz is 0xc20801e1e0
   [rmlast] the address of a is 0xc20801e200
   [rmlast] after remove, the address of a is 0xc20801e200
   [main] after remove, the address of xyz is 0xc20801e1e0
   [1 2 3 4 5 6 7 8 9]

   此次能夠看到slice做爲函數參數傳入函數時，實際上也是拷貝了一份slice，由於slice自己是個指針，因此從現象來看，slice是引用類型