Golang 使用 sort 對切片排序須要注意的一個點

最近用 sort.Slice 方法對 slice 作排序功能的時候遇到了一個小坑

先看一段代碼

package main

import (
	"fmt"
	"sort"
)

func main() {
	people := []struct {
		Name string
		Age  int
	}{
		{"Alice", 25},
		{"Elizabeth", 75},
		{"Alice", 75},
		{"Bob", 75},
		{"Alice", 75},
		{"Bob", 25},
		{"Colin", 25},
		{"Elizabeth", 25},
	}
	sort.Slice(people, func(i, j int) bool { return people[i].Name < people[j].Name })
	fmt.Println("By name:", people)

	sort.Slice(people, func(i, j int) bool { return people[i].Age < people[j].Age })
	fmt.Println("By age:", people)
}

輸出

By name: [{Alice 25} {Alice 75} {Alice 75} {Bob 75} {Bob 25} {Colin 25} {Elizabeth 75} {Elizabeth 25}]
By age: [{Alice 25} {Elizabeth 25} {Bob 25} {Colin 25} {Alice 75} {Bob 75} {Elizabeth 75} {Alice 75}]

有沒有注意到一個點，按照 Age 排序的地方，相等的元素老的切片中的順序變了，通常狀況下，這種狀況沒什麼影響，可是若是業務需求對順序有強要求的話，這就有問題了。

解決方案，用 sort.SliceStable 方法，sort.SliceStable 能夠保持元素順序和以前切片中的同樣。

SliceStable sorts the provided slice given the provided less function > > while keeping the original order of equal elements.

The function panics if the provided interface is not a slice.

新的代碼以下

package main

import (
	"fmt"
	"sort"
)

func main() {

	people := []struct {
		Name string
		Age  int
	}{
		{"Alice", 25},
		{"Elizabeth", 75},
		{"Alice", 75},
		{"Bob", 75},
		{"Alice", 75},
		{"Bob", 25},
		{"Colin", 25},
		{"Elizabeth", 25},
	}

	// Sort by name, preserving original order
	sort.SliceStable(people, func(i, j int) bool { return people[i].Name < people[j].Name })
	fmt.Println("By name:", people)

	// Sort by age preserving name order
	sort.SliceStable(people, func(i, j int) bool { return people[i].Age < people[j].Age })
	fmt.Println("By age,name:", people)

}

此次的結果，按照 Age 排序，相等的元素依然保持着和以前同樣的排序

By name: [{Alice 25} {Alice 75} {Alice 75} {Bob 75} {Bob 25} {Colin 25} {Elizabeth 75} {Elizabeth 25}]
By age,name: [{Alice 25} {Bob 25} {Colin 25} {Elizabeth 25} {Alice 75} {Alice 75} {Bob 75} {Elizabeth 75}]

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