arraysgosliceprepend

How to prepend int to slice


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.


Solution

  • 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.