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.