What should this code print?

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package main

import (
  "fmt"
  "sync"
)

func main() {

  arr := []int {1, 2, 3}

  var wg sync.WaitGroup
  for i := 0; i < len(arr); i++ {
    wg.Add(1)
    // Start a thread to do some heavy work in the background
    go func() {
      fmt.Printf("i is %d\n", i)
      wg.Done()
    }()
  }
  // Wait till all the threads finish
  wg.Wait()
}

Instead of printing

Bash
1
2
3
i is 0
i is 1
i is 2

It generates

Bash
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2
3
i is 3
i is 3
i is 3

What happened?

Turns out that the reference to i is captured but not the value. So when the internal anonymous function runs, it gets the current value of i at the time of execution and not creation.

One general rule for this is to never access value from the outer function directly, instead pass those values as parameters.

Consider this.

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package main

import (
  "fmt"
  "sync"
)

func main() {

  arr := []int {1, 2, 3}

  var wg sync.WaitGroup
  for i := 0; i < len(arr); i++ {
    wg.Add(1)
    // Start a thread to do some heavy work in the background
    go func(j int) {
      fmt.Printf("i is %d\n", j)
      wg.Done()
    }(i)  // Capture the value at the time of thread creation
  }
  // Wait till all the threads finish
  wg.Wait()
}

And it produces

Bash
1
2
3
i is 2
i is 0
i is 1