Golang: Append to Slice

Append to Slice

The append is a critical function in golang. It's used to append, prepend, join, slices, or delete elements in slice. You need to pay attention whether it returns a new slice or just modify the original.

append(slice_x, newItem1, newItem2, etc)

Append new items to a slice, returns a new slice only if the result is more than the original slice's capacity.

🛑 WARNING: if the original slice is a slice of slice, then result may not go beyond its capacity, therefore, the result shares data as original slice. Meaning, change to the result also changes original slice.

Example: Basic Append

package main

import "fmt"

func main() {

	var s1 = []int{3, 5}
	var s2 = append(s1, 8, 9)

	fmt.Println(s2) // [3 5 8 9]

	// original not changed, in this case
	fmt.Println(s1) // [3 5]

}

Example: Append May or May Not Share Original Slice Data

Append creates a new slice ONLY WHEN the result is beyond original capacity.

package main

import "fmt"

func main() {

	var x1 = []int{0, 1, 2, 3, 4, 5}
	var x2 = x1[:3] // [0 1 2]

	var x3 = append(x2, 22)

	fmt.Println(x3) // [0 1 2 22]

	// x1 is changed
	fmt.Println(x1) // [0 1 2 22 4 5]

	// --------------------------------------------------
	// if the append added more items than the capacity of x1, then x1 will not be changed
	// now we do the above again to see

	var y1 = []int{0, 1, 2, 3, 4, 5}
	var y2 = y1[:3] // [0 1 2]

	var y3 = append(y2, 21, 22, 23, 24, 25)

	fmt.Println(y3) // [0 1 2 21 22 23 24 25]

	// y1 is not changed
	fmt.Println(y1) // [0 1 2 3 4 5]

}

Join Slices

Use triple dots operator to turn a slice into function arguments, then use append.

append(slice_x, slice_y ...)

package main

import "fmt"

func main() {

	var s1 = []int{3, 5}
	var s2 = []int{6, 7}
	var s3 = append(s1, s2...)

	fmt.Println(s3) // [3 5 6 7]

}

Append String as Byte Slice to Slice

Use triple dots operator to turn a string into byte slice, then use append.

append(slice_x, string ...)