The 3 ways to sort in Go

Sort a slice of ints, float64s or strings

Use one of the functions

s := []int{4, 2, 3, 1}
sort.Ints(s)
fmt.Println(s) // [1 2 3 4]

Package radix contains a drop-in replacement for sort.Strings, which can be more than twice as fast in some settings.

Sort with custom comparator

• Use the function sort.Slice. It sorts a slice using a provided function less(i, j int) bool.
• To sort the slice while keeping the original order of equal elements, use sort.SliceStable instead.

family := []struct {
    Name string
    Age  int
}{
    {"Alice", 23},
    {"David", 2},
    {"Eve", 2},
    {"Bob", 25},
}

// Sort by age, keeping original order or equal elements.
sort.SliceStable(family, func(i, j int) bool {
    return family[i].Age < family[j].Age
})
fmt.Println(family) // [{David 2} {Eve 2} {Alice 23} {Bob 25}]

Sort custom data structures

• Use the generic sort.Sort and sort.Stable functions.
• They sort any collection that implements the sort.Interface interface.

type Interface interface {
        // Len is the number of elements in the collection.
        Len() int
        // Less reports whether the element with
        // index i should sort before the element with index j.
        Less(i, j int) bool
        // Swap swaps the elements with indexes i and j.
        Swap(i, j int)
}

Here’s an example.

type Person struct {
    Name string
    Age  int
}

// ByAge implements sort.Interface based on the Age field.
type ByAge []Person

func (a ByAge) Len() int           { return len(a) }
func (a ByAge) Less(i, j int) bool { return a[i].Age < a[j].Age }
func (a ByAge) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

func main() {
    family := []Person{
        {"Alice", 23},
        {"Eve", 2},
        {"Bob", 25},
    }
    sort.Sort(ByAge(family))
    fmt.Println(family) // [{Eve 2} {Alice 23} {Bob 25}]
}

Bonus: Sort a map by key or value

A map is an unordered collection of key-value pairs. If you need a stable iteration order, you must maintain a separate data structure.

This code example uses a slice of keys to sort a map in key order.

m := map[string]int{"Alice": 2, "Cecil": 1, "Bob": 3}

keys := make([]string, 0, len(m))
for k := range m {
    keys = append(keys, k)
}
sort.Strings(keys)

for _, k := range keys {
    fmt.Println(k, m[k])
}
// Output:
// Alice 2
// Bob 3
// Cecil 1

Also, starting with Go 1.12, the fmt package prints maps in key-sorted order to ease testing.

Performance and implementation

All algorithms in the Go sort package make O(n log n) comparisons in the worst case, where n is the number of elements to be sorted.

Most of the functions are implemented using an optimized version of quicksort.

More code examples

Go blueprints: code for com­mon tasks is a collection of handy code examples.

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