Go 切片

Go切片（Slice）是对Go数组的抽象。Go数组允许您定义可以容纳多个相同种类的数据项的变量，但它不提供任何内置方法来动态增加其大小或获取自身的子数组。切片克服了这个限制。它提供了许多在数组上所需的实用功能，并在Go编程中广泛使用。

定义一个切片

要定义一个切片，可以将其声明为一个没有指定大小的数组。或者，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

切片是对数组的一种抽象方式。实际上，它使用数组作为底层结构。len()函数返回切片中的元素数量，而cap()函数返回切片的容量（即能够容纳多少元素）。以下示例解释了切片的用法 –

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)
}

当上述代码被编译并执行时，会产生以下结果 –

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

空切片

如果一个切片没有输入，那么默认情况下，它会被初始化为nil。它的长度和容量都是零。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)
}

当上述代码被编译和执行时，它产生以下结果−

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

子切片

切片允许指定下限和上限来获取其子切片，使用 [下限:上限] 。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)
}

当上述代码被编译和执行时，它产生以下结果−

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slice的容量。使用 copy() 函数，将源slice的内容复制到目标slice。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)
}

当上述代码被编译并执行时，会产生如下结果−

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]