goAsync race conditionMedium

Loop variable captured by reference in goroutine

Time: 0:00

BugFind the bug and fix the code

What it should do

Spawn worker for each integer in list and return collected values. Example: spawnLoopWorkers([1, 2, 3]) -> [1, 2, 3]

What happens instead

All goroutines capture the final loop variable value 3

package main

import (
	"fmt"
	"sync"
)

func spawnLoopWorkers(nums []int) []int {
	var wg sync.WaitGroup
	var mu sync.Mutex
	results := make([]int, len(nums))
	for i, val := range nums {
		wg.Add(1)
		go func() {
			defer wg.Done()
			mu.Lock()
			results[i] = val
			mu.Unlock()
		}()
	}
	wg.Wait()
	return results
}

func main() {
	fmt.Println(spawnLoopWorkers([]int{1, 2, 3}))
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

⏱️0:00

Hints

🏆 0 hints = Flawless Solve

Think & Test Cooldown

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Hint 2

Unlocks after Hint 1

Hint 3

Unlocks after Hint 2

This bug belongs to the async and race conditions pattern

A missing await, unbuffered channel deadlocks, ConcurrentModificationException, and data races under load.

Learn the async and race conditions pattern Step through your own code

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package main import ( "fmt" "sync" ) func spawnLoopWorkers(nums []int) []int { var wg sync.WaitGroup var mu sync.Mutex results := make([]int, len(nums)) for i, val := range nums { wg.Add(1) go func() { defer wg.Done() mu.Lock() results[i] = val mu.Unlock() }() } wg.Wait() return results } func main() { fmt.Println(spawnLoopWorkers([]int{1, 2, 3})) }

Loop variable captured by reference in goroutine

What it should do

What happens instead

More async and race conditions to practise

Loop variable captured by reference in goroutine

What it should do

What happens instead

More async and race conditions to practise