The 3 ways to sort in Go

yourbasic.org/golang

Slice of ints, float64s or strings
Custom comparator
Custom data structures
Bonus: Sort a map by key or value
Performance and implementation

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