Go - Slices

Go Slice is an abstraction over Go Array. As Go Array allows you to define type of variables that can hold several data items of the same kind but it do not provide any inbuilt method to increase size of it dynamically or get a sub-array of its own. Slices covers this limitation. It provides many utility functions required on Array and is widely used in Go programming.

Defining a slice

To define a slice, you can declare it as an array without specifying size or use make function to create the one.

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() and cap() functions

As slice is an abstraction over array. It actually uses array as an underlying structure.len() function returns the elements presents in the slice where cap() function returns the capacity of slice as how many elements it can be accomodate. Following is the example to explain usage of slice:

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)
}

When the above code is compiled and executed, it produces the following result:

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

Nil slice

If a slice is declared with no inputs the by default, it is initialized as nil. Its length and capacity are zero. Following is an example:

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)
}

When the above code is compiled and executed, it produces the following result:

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

sub-slicing

Slice allows lower-bound and upper bound to be specified to get the subslice of it using[lower-bound:upper-bound]. Following is the example:

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)
}

When the above code is compiled and executed, it produces the following result:

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() and copy() functions

Slice allows increasing the capacity of a slice using append() function.Using copy() function, contents of a source slice are copied to destination slice. Following is the example:

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)
}

When the above code is compiled and executed, it produces the following result:

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]