loops - 为什么可以通过赋值来更改 slice ，而不能通过for循环来更改？

Question

我需要在接收 slice 指针的函数中将每个分数更改为其他一些值，我可以通过赋值来更改值，但是如果在for循环中执行此操作则不会更改任何值。这是为什么？

package main

import (
    "fmt"
    "sync"
    "time"
)

type TestType struct {
    id    int
    score float64
}

func worker(index int, wg *sync.WaitGroup, listScores *[][]TestType) {
    defer wg.Done()
    time.Sleep(1000 * time.Millisecond)

    // It works by assigning.
    (*listScores)[index] = []TestType{{index + 1, 2.22},
        {index + 1, 2.22},
        {index + 1, 2.22},}
    // It doesn't work in a for loop.
    //for _, score := range (*listScores)[index] {
    //  score.score = 2.22
    //}
}

func main() {
    scoresList := [][]TestType{
        {{1, 0.0},
            {1, 0.0},
            {1, 0.0},},
        {{2, 0.0},
            {2, 0.0},
            {2, 0.0},
        },}

    fmt.Println(scoresList)

    var wg sync.WaitGroup
    for i, _ := range scoresList {
        wg.Add(1)
        go worker(i, &wg, &scoresList)
    }
    wg.Wait()

    fmt.Println(scoresList)
}

通过为它分配一个新的整体 slice ，可以将分数更改为2.22:

[[{1 0} {1 0} {1 0}] [{2 0} {2 0} {2 0}]]
[[{1 2.22} {1 2.22} {1 2.22}] [{2 2.22} {2 2.22} {2 2.22}]]

但是，如果我在注释中的for循环中执行此操作，则输出如下所示:

[[{1 0} {1 0} {1 0}] [{2 0} {2 0} {2 0}]]
[[{1 0} {1 0} {1 0}] [{2 0} {2 0} {2 0}]]

Answer 1

因为range提供了两个元素:

索引

如果迭代 slice ，则将复制到元素的。

无需使用线程即可将代码更改为以下代码:

package main

import (
    "fmt"
)

type testType struct {
    id    int
    score float64
}

func main() {
    scoresList := [][]testType{
        {{1, 0.0}, {1, 0.0}, {1, 0.0}},
        {{2, 0.0}, {2, 0.0}, {2, 0.0}}}
    fmt.Println(scoresList)
    for i := range scoresList {
        scoresList[i] = []testType{{i + 1, 2.22},
            {i + 1, 2.22},
            {i + 1, 2.22}}
    }
    fmt.Println(scoresList)
}

如果要使用线程，则可以使用以下线程:

package main

import (
    "fmt"
    "sync"
    "time"
)

type TestType struct {
    id    int
    score float64
}

func worker(index int, wg *sync.WaitGroup, listScores *[][]TestType) {
    defer wg.Done()
    time.Sleep(1000 * time.Millisecond)

    for i := range (*listScores)[index] {
        (*listScores)[index][i].score = 2.22
    }
}

func main() {
    scoresList := [][]TestType{
        {{1, 0.0},
            {1, 0.0},
            {1, 0.0}},
        {{2, 0.0},
            {2, 0.0},
            {2, 0.0},
        }}

    fmt.Println(scoresList)

    var wg sync.WaitGroup
    for i, _ := range scoresList {
        wg.Add(1)
        go worker(i, &wg, &scoresList)
    }
    wg.Wait()

    fmt.Println(scoresList)
}