Go - 切片 (Slices)

Go 切片是對 Go 陣列的抽象。Go 陣列允許你定義可以儲存多種相同型別資料的變數，但它不提供任何內建方法來動態增加其大小或獲取其自身的子陣列。切片克服了這個限制。它提供了陣列所需的許多實用函式，並且廣泛用於 Go 程式設計。

定義切片

要定義切片，你可以像宣告陣列一樣宣告它，而不指定其大小。或者，你可以使用 `make` 函式建立一個切片。

var numbers []int /* a slice of unspecified size */
/* numbers == []int{0,0,0,0,0}*/
numbers = make([]int,5,5) /* a slice of length 5 and capacity 5*/

`len()` 和 `cap()` 函式

切片是對陣列的抽象。它實際上使用陣列作為底層結構。`len()` 函式返回切片中存在的元素個數，而 `cap()` 函式返回切片的容量（即它可以容納多少個元素）。下面的例子解釋了切片的使用：

package main

import "fmt"

func main() {
   var numbers = make([]int,3,5)
   printSlice(numbers)
}
func printSlice(x []int){
   fmt.Printf("len=%d cap=%d slice=%v\n",len(x),cap(x),x)
}

當以上程式碼編譯並執行時，它會產生以下結果：

len = 3 cap = 5 slice = [0 0 0]

空切片 (Nil slice)

如果一個切片宣告時沒有輸入，則預設情況下，它被初始化為空。它的長度和容量都為零。例如：

package main

import "fmt"

func main() {
   var numbers []int
   printSlice(numbers)
   
   if(numbers == nil){
      fmt.Printf("slice is nil")
   }
}
func printSlice(x []int){
   fmt.Printf("len = %d cap = %d slice = %v\n", len(x), cap(x),x)
}

當以上程式碼編譯並執行時，它會產生以下結果：

len = 0 cap = 0 slice = []
slice is nil

子切片 (Subslicing)

切片允許指定下界和上界，以使用 `[下界:上界]` 獲取其子切片。例如：

package main

import "fmt"

func main() {
   /* create a slice */
   numbers := []int{0,1,2,3,4,5,6,7,8}   
   printSlice(numbers)
   
   /* print the original slice */
   fmt.Println("numbers ==", numbers)
   
   /* print the sub slice starting from index 1(included) to index 4(excluded)*/
   fmt.Println("numbers[1:4] ==", numbers[1:4])
   
   /* missing lower bound implies 0*/
   fmt.Println("numbers[:3] ==", numbers[:3])
   
   /* missing upper bound implies len(s)*/
   fmt.Println("numbers[4:] ==", numbers[4:])
   
   numbers1 := make([]int,0,5)
   printSlice(numbers1)
   
   /* print the sub slice starting from index 0(included) to index 2(excluded) */
   number2 := numbers[:2]
   printSlice(number2)
   
   /* print the sub slice starting from index 2(included) to index 5(excluded) */
   number3 := numbers[2:5]
   printSlice(number3)
   
}
func printSlice(x []int){
   fmt.Printf("len = %d cap = %d slice = %v\n", len(x), cap(x),x)
}

當以上程式碼編譯並執行時，它會產生以下結果：

len = 9 cap = 9 slice = [0 1 2 3 4 5 6 7 8]
numbers == [0 1 2 3 4 5 6 7 8]
numbers[1:4] == [1 2 3]
numbers[:3] == [0 1 2]
numbers[4:] == [4 5 6 7 8]
len = 0 cap = 5 slice = []
len = 2 cap = 9  slice = [0 1]
len = 3 cap = 7 slice = [2 3 4]

`append()` 和 `copy()` 函式

可以使用 `append()` 函式增加切片的容量。使用 `copy()` 函式，源切片的內容被複制到目標切片。例如：

package main

import "fmt"

func main() {
   var numbers []int
   printSlice(numbers)
   
   /* append allows nil slice */
   numbers = append(numbers, 0)
   printSlice(numbers)
   
   /* add one element to slice*/
   numbers = append(numbers, 1)
   printSlice(numbers)
   
   /* add more than one element at a time*/
   numbers = append(numbers, 2,3,4)
   printSlice(numbers)
   
   /* create a slice numbers1 with double the capacity of earlier slice*/
   numbers1 := make([]int, len(numbers), (cap(numbers))*2)
   
   /* copy content of numbers to numbers1 */
   copy(numbers1,numbers)
   printSlice(numbers1)   
}
func printSlice(x []int){
   fmt.Printf("len=%d cap=%d slice=%v\n",len(x),cap(x),x)
}

當以上程式碼編譯並執行時，它會產生以下結果：

len = 0 cap = 0 slice = []
len = 1 cap = 2 slice = [0]
len = 2 cap = 2 slice = [0 1]
len = 5 cap = 8 slice = [0 1 2 3 4]
len = 5 cap = 16 slice = [0 1 2 3 4]