Golang 使用 sort 对切片排序须要注意的一个点

最近用 sort.Slice 方法对 slice 作排序功能的时候遇到了一个小坑

先看一段代码

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)
}

输出

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}]

有没有注意到一个点，按照 Age 排序的地方，相等的元素老的切片中的顺序变了，通常状况下，这种状况没什么影响，可是若是业务需求对顺序有强要求的话，这就有问题了。

解决方案，用 sort.SliceStable 方法，sort.SliceStable 能够保持元素顺序和以前切片中的同样。

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.

新的代码以下

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)

}

此次的结果，按照 Age 排序，相等的元素依然保持着和以前同样的排序

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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