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Go Slice Operations

Slice operations change or check the items in a slice. Append adds items, copy duplicates them, and Equal compares two slices.

Append a whole slice

Append first adds one item on its own. Three dots then spread a second slice into the append call.

package main

import "fmt"

func main() {
	nums := []int{1, 2}
	nums = append(nums, 3)
	fmt.Println(nums)
	extra := []int{4, 5}
	nums = append(nums, extra...)
	fmt.Println(nums)
}

Copy a slice

Copy duplicates the items into a fresh slice. A later change to the original leaves the copy alone.

package main

import "fmt"

func main() {
	src := []int{1, 2, 3}
	dst := make([]int, len(src))
	copy(dst, src)
	src[0] = 99
	fmt.Println(dst)
	fmt.Println(src)
}

Compare two slices

Equal reports whether two slices hold the same items in the same order. The second pair differs in the last item.

package main

import "fmt"
import "slices"

func main() {
	a := []int{1, 2, 3}
	b := []int{1, 2, 3}
	c := []int{1, 2, 4}
	fmt.Println(slices.Equal(a, b))
	fmt.Println(slices.Equal(a, c))
}

Delete the middle item

The idiom appends the tail onto the front part. The item at index 1 drops out.

package main

import "fmt"

func main() {
	nums := []int{10, 20, 30, 40}
	nums = append(nums[:1], nums[2:]...)
	fmt.Println(nums)
}

Example

package main

import "fmt"

func main() {
	songs := []string{"Sun", "Moon"}
	fmt.Println(songs)
	songs = append(songs, "Stars")
	fmt.Println(songs)
	songs = append(songs[:0], songs[1:]...)
	fmt.Println(songs)
}

Key points

  • The three dots after a slice spread its items into an append call.
  • Use copy to duplicate items into another slice.
  • A copy holds its own items after the copy runs.
  • Use slices.Equal to test whether two slices match.
  • The delete idiom appends the tail of a slice onto its front part.

In this example, songs starts with two titles. Append adds Stars at the end, and the slice prints with three titles. The delete idiom then drops the first title. The last print line shows the two titles that remain.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Go Playground

Exercise

What does copy(dst, src) do?