遍历通道时出现错误 "too many variables in range"

Question

我有点迷路了，我试图让一个 goroutine 添加到数组中，并让另一个 goroutine 从中读取，我怀疑这与我下面的内容有些接近，但我需要尝试一下与 wait().

但是，我收到错误 prog.go:19:14: too many variables in range，第 19 行是 for _, v := range c { 我无法在线找到该问题的答案，我在这里做什么或不做什么？

package main

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

func hello(wg *sync.WaitGroup, s []int, c chan int) {
    for _, v := range s {
        c <- v
    }
    fmt.Println("Finished adding to channel")
    wg.Done()
}

func hello2(wg *sync.WaitGroup, c chan int) {
    fmt.Println("Channel",c)
    for _, v := range c {
        fmt.Println("Received",v)   
    }
    fmt.Println("Finished taking from channel")
    wg.Done()
}

func main() {
    s := []int{1, 2, 3, 4, 5}
    var c = make(chan int, 5)


    var wg sync.WaitGroup
    wg.Add(1)
    go hello(&wg, s, c)
    wg.Wait()
    wg.Add(1)
    go hello2(&wg, c)
    wg.Wait()
    //time.Sleep(1 * time.Second)
    fmt.Println("main function")
}

Answer 1

当您遍历通道时，迭代只会产生一个值，即在通道上发送的值。没有像切片或映射那样的索引或键值。

所以你必须使用：

for v := range c {
    fmt.Println("Received", v)   
}

这在Spec: For statements:

中有详细说明

If the range expression is a channel, at most one iteration variable is permitted, otherwise there may be up to two.

并且：

For channels, the iteration values produced are the successive values sent on the channel until the channel is closed. If the channel is nil, the range expression blocks forever.

还有：

Function calls on the left are evaluated once per iteration. For each iteration, iteration values are produced as follows if the respective iteration variables are present:
Range expression                          1st value          2nd value

array or slice  a  [n]E, *[n]E, or []E    index    i  int    a[i]       E
string          s  string type            index    i  int    see below  rune
map             m  map[K]V                key      k  K      m[k]       V
channel         c  chan E, <-chan E       element  e  E

Answer 2

我知道你没有intending/wishing为你的通道使用迭代器来进行范围循环，但是如果你确实希望有一个迭代器，它仍然可以通过在循环之外自己定义迭代器，并在每次读取通道中的值时递增它。

    var wg sync.WaitGroup
    var iterator = 0
    for v := range c1 {
        wg.Add(1)
        iterator += 1
        go func(v2, i2 int) {
            c2 <- timeConsumingWork(v2)
            fmt.Println("item ", i2, " done")
            wg.Done()
        }(v, iterator)
    }
    wg.Wait()
    close(c2)

}

在这里您可以看到 go func 正在接受通道值 (v2) 和迭代器值 (i2)，同时每次从通道读取值时递增它。

此外，我看到您使用缓冲等待组来避免阻塞两个方法的顺序调用。理想的 Go 代码应该尽可能避免缓冲通道，而是以互锁的方式调用发送和接收函数，这样它们就不需要分开了。

遍历通道时出现错误 "too many variables in range"

Error "too many variables in range" when iterating over a channel

for-loop

channel

go