How to prepend int to slice

Question

I am fairly new to Go and thus my question might seem a bit naive.

I have a slice which I created using

var x []int;
for i := 2; i < 10; i += 2 {
    x = append(x, i);
}

I want to prepend an integer to this slice, something like

x = append(2, x)

but obviously it won't work since append needs a slice as the first argument.

I have tried this but it only works for strings and it's not working in my case.

Answer 1

Use a slice composite literal: []int{1}, For example:

package main  import (     "fmt" )  func main() {     var x []int     for i := 2; i < 10; i += 2 {         x = append(x, i)     }     fmt.Println(x)      x = append([]int{1}, x...)      fmt.Println(x) } 

Playground: https://play.golang.org/p/Yc87gO7gJlD

Output:

[2 4 6 8] [1 2 4 6 8] 

---

However, this more efficient version may make fewer allocations, An allocation is only necessary when there is no spare slice capacity.

package main  import (     "fmt" )  func main() {     var x []int     for i := 2; i < 10; i += 2 {         x = append(x, i)     }     fmt.Println(x)      x = append(x, 0)     copy(x[1:], x)     x[0] = 1      fmt.Println(x) } 

Playground: https://play.golang.org/p/fswXul_YfvD

Output:

[2 4 6 8] [1 2 4 6 8] 

---

Good code must be readable. In Go, we often hide implementaion details inside a function. Go compilers are optimizing compilers, small, simple functions (like prependInt) are inlined.

package main  import (     "fmt" )  func prependInt(x []int, y int) []int {     x = append(x, 0)     copy(x[1:], x)     x[0] = y     return x }  func main() {     var x []int     for i := 2; i < 10; i += 2 {         x = append(x, i)     }     fmt.Println(len(x), cap(x), x)      x = prependInt(x, 1)      fmt.Println(len(x), cap(x), x) } 

Playground: https://play.golang.org/p/wl6gvoXraKH

Output:

4 4 [2 4 6 8] 5 8 [1 2 4 6 8] 

---

See Go SliceTricks.

Answer 2

The current version is go1.14.11

Prepend without a for loop:

package main

import "fmt"
func main () {
  data := []int{1, 2, 3, 4}
  fmt.Println(data)
  data = append([]int{99}, data...)
  fmt.Println(data)
}

Example taken form: https://codingair.wordpress.com/2014/07/18/go-appendprepend-item-into-slice/

Works with integers: https://play.golang.org/p/gaLhB5_d1Iu