How Can I Break Out of a For Loop in Go from an External Function?

Breaking a For Loop Execution from Outside: A Go Programming Solution

In programming, it's often necessary to have control over the flow of execution, especially when managing loops. In Go, one may encounter situations where an infinite for loop needs to be terminated from an external source. This article addresses this specific scenario by providing a comprehensive solution.

Problem Description:

Consider an infinite for loop with a label, running concurrently with a scheduled function. The goal is to break the loop when a specific condition is met from within the scheduled function. Below is an example of such an attempt, which fails due to scope limitations:

<code class="go">package main

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

func main() {
  count := 0
  var wg sync.WaitGroup
  wg.Add(1)
  t := time.NewTicker(time.Second * 1)

  go func() {
    for {
      fmt.Println("I will print every second", count)
      count++
      if count > 5 {
        break myLoop; // Issue due to scope error: 'myLoop' not visible
        wg.Done()
      }
      <-t.C
    }
  }()

  i := 1

  myLoop:
  for {
    fmt.Println("iteration", i)
    i++
  }

  wg.Wait()
  fmt.Println("I will execute at the end")
}

Solution:

To achieve this desired functionality, we employ a signaling channel. Here's a step-by-step breakdown:

1. Create a Signaling Channel:

We create a signaling channel quit of type chan struct{}. This channel acts as a signal for when the loop should terminate.

<code class="go">quit := make(chan struct{})

2. Close the Channel to Signal Break:

When the condition is met within the scheduled function, we close the signaling channel. This indicates that the for loop should break.

<code class="go">go func() {
    for {
        fmt.Println("I will print every second", count)
        count++
        if count > 5 {
          close(quit)
          wg.Done()
          return
        }
        <-t.C
    }  
  }()</code>

3. Check for Channel Closure to Break the Loop:

In the for loop, we read from the signaling channel using a select statement. When the channel is closed (signaling a break), the execution branches to the case <-quit: block, breaking the loop. Otherwise, the default case executes the iterations as normal.

<code class="go">myLoop:
  for {
    select {
    case <-quit:
      break myLoop
    default:
      fmt.Println("iteration", i)
      i++
    }
  }</code>

This solution effectively allows us to control the execution of a loop from outside its own scope, making it possible to terminate the loop when desired without worrying about scope limitations.

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