Go - 切片 (Slices)

Go 切片是对 Go 数组的抽象。Go 数组允许你定义可以保存多种相同类型数据的变量，但它不提供任何内置方法来动态增加其大小或获取其自身的子数组。切片克服了这个限制。它提供了数组所需的许多实用函数，并且广泛用于 Go 编程。

定义切片

要定义切片，你可以像声明数组一样声明它，而不指定其大小。或者，你可以使用 `make` 函数创建一个切片。

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()` 函数

切片是对数组的抽象。它实际上使用数组作为底层结构。`len()` 函数返回切片中存在的元素个数，而 `cap()` 函数返回切片的容量（即它可以容纳多少个元素）。下面的例子解释了切片的使用：

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)
}

当以上代码编译并执行时，它会产生以下结果：

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

空切片 (Nil slice)

如果一个切片声明时没有输入，则默认情况下，它被初始化为空。它的长度和容量都为零。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)
}

当以上代码编译并执行时，它会产生以下结果：

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

子切片 (Subslicing)

切片允许指定下界和上界，以使用 `[下界:上界]` 获取其子切片。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)
}

当以上代码编译并执行时，它会产生以下结果：

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()` 函数

可以使用 `append()` 函数增加切片的容量。使用 `copy()` 函数，源切片的内容被复制到目标切片。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)
}

当以上代码编译并执行时，它会产生以下结果：

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]