[Golang] map, struct を要素に持つリストをソートする例

main.go

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"sort"
)

type Person struct {
	Id        int
	Firstname string
	Lastname  string
}

func (p *Person) String() string {
	// ナイーブな実装
	// return fmt.Sprintf("{Id: %d, Firstname: \"%s\", Lastname: \"%s\"}", p.Id, p.Firstname, p.Lastname)
	// Json encode して返す
	b, err := json.Marshal(p)
	if err != nil {
		panic(fmt.Sprintf("marshal error: %s", p.String()))
	}
	return string(b)
}

func main() {
	log.Println("--- sort maps ---")
	dict := map[string]any{"c": 1, "b": 10, "x": "s"}
	for k, v := range dict {
		log.Println(k, v)
	}

	log.Println("-- by key")
	keys := make([]string, 0, len(dict))
	for k := range dict {
		keys = append(keys, k)
	}
	sort.Strings(keys)
	for _, k := range keys {
		log.Println(k, dict[k])
	}

	log.Println("--- sort structs ---")
	persons := []Person{
		{Id: 2, Firstname: "aaa", Lastname: "zzz"},
		{Id: 1, Firstname: "bbb", Lastname: "yyy"},
		{Id: 3, Firstname: "ccc", Lastname: "xxx"},
	}

	log.Println("-- by Id")
	sort.Slice(persons, func(i, j int) bool {
		return persons[i].Id < persons[j].Id
	})
	for _, p := range persons {
		log.Println(p.String())
	}

	log.Println("-- by Firstname")
	sort.Slice(persons, func(i, j int) bool {
		return persons[i].Firstname < persons[j].Firstname
	})
	for _, p := range persons {
		log.Println(p.String())
	}

	log.Println("-- by Lastname")
	sort.Slice(persons, func(i, j int) bool {
		return persons[i].Lastname < persons[j].Lastname
	})
	for _, p := range persons {
		log.Println(p.String())
	}
}

$ go run main.go
2022/07/28 xx:xx:xx --- sort maps ---
2022/07/28 xx:xx:xx b 10
2022/07/28 xx:xx:xx x s
2022/07/28 xx:xx:xx c 1
2022/07/28 xx:xx:xx -- by key
2022/07/28 xx:xx:xx b 10
2022/07/28 xx:xx:xx c 1
2022/07/28 xx:xx:xx x s
2022/07/28 xx:xx:xx --- sort structs ---
2022/07/28 xx:xx:xx -- by Id
2022/07/28 xx:xx:xx {"Id":1,"Firstname":"bbb","Lastname":"yyy"}
2022/07/28 xx:xx:xx {"Id":2,"Firstname":"aaa","Lastname":"zzz"}
2022/07/28 xx:xx:xx {"Id":3,"Firstname":"ccc","Lastname":"xxx"}
2022/07/28 xx:xx:xx -- by Firstname
2022/07/28 xx:xx:xx {"Id":2,"Firstname":"aaa","Lastname":"zzz"}
2022/07/28 xx:xx:xx {"Id":1,"Firstname":"bbb","Lastname":"yyy"}
2022/07/28 xx:xx:xx {"Id":3,"Firstname":"ccc","Lastname":"xxx"}
2022/07/28 xx:xx:xx -- by Lastname
2022/07/28 xx:xx:xx {"Id":3,"Firstname":"ccc","Lastname":"xxx"}
2022/07/28 xx:xx:xx {"Id":1,"Firstname":"bbb","Lastname":"yyy"}
2022/07/28 xx:xx:xx {"Id":2,"Firstname":"aaa","Lastname":"zzz"}