Sorting by Functions

Sometimes we'll want to sort a collection by something other than its natural order. For example, suppose we wanted to sort strings by their length instead of alphabetically. Here's an example of custom sorts in Go.

package main

import (
	"cmp"
	"fmt"
	"slices"
	"strings"
)

func main() {
	fruits := []string{"peach", "banana", "kiwi"}

We implement a comparison function for string lengths. cmp.Compare is helpful for this: it returns a negative number, zero, or a positive number.

	lenCmp := func(a, b string) int {
		return cmp.Compare(len(a), len(b))
	}

Now we can call slices.SortFunc with this custom comparison function to sort fruits by name length.

	slices.SortFunc(fruits, lenCmp)
	fmt.Println(fruits)

We can use the same technique to sort a slice of values that aren't built-in types.

	type Person struct {
		name string
		age  int
	}

	people := []Person{
		Person{name: "Jax", age: 37},
		Person{name: "TJ", age: 25},
		Person{name: "Alex", age: 72},
		Person{name: "Bea", age: 25},
	}

Sort people by age using slices.SortFunc.

	slices.SortFunc(people,
		func(a, b Person) int {
			return cmp.Compare(a.age, b.age)
		})
	fmt.Println(people)

cmp.Or chains comparisons: by age, then by name for equal ages. Swapping a and b sorts in descending order.

	slices.SortFunc(people, func(a, b Person) int {
		return cmp.Or(
			cmp.Compare(b.age, a.age),
			strings.Compare(a.name, b.name),
		)
	})
	fmt.Println(people)

SortStableFunc keeps equal elements in their original order.

	words := []string{"bb", "a", "cc", "d"}
	slices.SortStableFunc(words, lenCmp)
	fmt.Println(words)

Note: if the Person struct is large, you may want the slice to contain *Person instead and adjust the sorting function accordingly. If in doubt, benchmark!

$ go run ./53_sorting_by_functions
[kiwi peach banana]
[{TJ 25} {Bea 25} {Jax 37} {Alex 72}]
[{Alex 72} {Jax 37} {Bea 25} {TJ 25}]
[a d bb cc]

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