GeezerVison

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"
)

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

We implement a comparison function for string lengths. cmp.Compare is helpful for this.

    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},
    }

Sort people by age using slices.SortFunc.

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!

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

$ go run sorting-by-functions.go 
[kiwi peach banana]
[{TJ 25} {Jax 37} {Alex 72}]