如何使用範圍關鍵字在GO中的頻道上迭代？

在Go中使用range遍歷通道時，必須由發送方關閉通道以避免panic。具體步驟如下：1. 創建通道並啟動一個goroutine向其發送數據；2. 發送完成後使用close函數關閉通道；3. 使用for range循環接收數據，循環會在通道關閉後自動結束。需要注意不要多次關閉通道或向已關閉的通道發送數據，這會導致運行時錯誤。若存在多個發送者，應通過sync.WaitGroup或額外的信號通道協調關閉操作，以確保程序安全與穩定。

In Go, the range keyword is commonly used to iterate over channels when you want to receive values until the channel is closed. This approach is clean and avoids manually managing loop conditions for receiving.

Here's how it works in practice.

How to Use range with a Channel

When you use range on a channel in a for loop, it will keep pulling values from the channel until it's closed. You don't need to check for a second return value like you might with maps or slices — the loop simply stops once the channel is closed.

 ch := make(chan int)
go func() {
    ch <- 1
    ch <- 2
    ch <- 3
    close(ch)
}()

for val := range ch {
    fmt.Println(val)
}

This will print:

 1
2
3

The loop ends after the channel is closed. Note that you must close the channel ; otherwise, the loop will hang forever waiting for more input.

When to Close the Channel

Closing a channel signals that no more values will be sent. It's important to ensure only the sending side closes the channel — not the receiver. Closing from the receiver side can lead to panic if another sender is still active.

A common pattern:

• One or more goroutines send data.
• Once all sends are done, the sender closes the channel.
• The receiver uses range to process all incoming data safely.

For example:

 ch := make(chan string)
go func() {
    names := []string{"Alice", "Bob", "Charlie"}
    for _, name := range names {
        ch <- name
    }
    close(ch)
}()

for name := range ch {
    fmt.Println("Received:", name)
}

This ensures the loop exits cleanly after all names are printed.

Common Pitfalls to Avoid

Here are a few gotchas when using range over channels:

• Don't close a channel multiple times. Doing so will cause a runtime panic.
• Don't send on a closed channel. This also causes a panic.
• If you're unsure whether a channel should be closed, consider using a sync.WaitGroup instead to coordinate completion without closing the channel.

If you're working with multiple senders, coordinating closure gets trickier. In those cases, you may want to use a separate signal (like a done channel) or reference counting via sync.WaitGroup .

Summary

Using range on a channel is a neat way to consume all values sent on it until it's closed. Just remember to close the channel from the sender side, avoid common mistakes like double-closing or sending on a closed channel, and you'll be good to go.

That's basically how it works — straightforward but easy to misuse if you're not careful.

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