Sorting And Go Slices

Word of caution for anyone passing Go slices to a function which will sort them. Doing so as is will modify the original slice. If you were to write this, for example:

package main

import (
	"fmt"
	"sort"
)

func printSorted(ys []int) {
	sort.Slice(ys, func(i, j int) bool { return ys[i] < ys[j] })
	fmt.Println(ys)
}

func main() {
	xs := []int{3, 1, 2}
	printSorted(xs)
	fmt.Println(xs)
}

You will find, when you run it, that both xs and ys will be sorted:

[1,2,3]
[1,2,3]

If this is not desired, the remedy would be make a copy of the slice prior to sorting it:

func printSorted(ys []int) {
	ysDup := make([]int, len(ys))
	copy(ysDup, ys)
	sort.Slice(ysDup, func(i, j int) bool { return ys[i] < ys[j] })
	fmt.Println(ysDup)
}

This make sense when you consider that the elements of a slice are more or less stored in a normal array. Slices add things like start and end items, which are stored as values within the slice struct, but the array itself is just a normal reference and it is this that sort.Slice will modify.

On the face of it, this is a pretty trivial thing to find out. But it’s worth noting here just so that I don’t have to remember it again.