goAsync race conditionMedium

Unexpected zero value in concurrent Go program

Time: 0:00

BugFind the bug and fix the code

What it should do

Process data concurrently and collect results. Example: processConcurrentResults(data) -> []int

What happens instead

The program unexpectedly returns zero values in the result slice.

package main 

import (
	"fmt"
	"sync"
)

func processConcurrentResults(data []int) []int {
	resultChan := make(chan int)
	var wg sync.WaitGroup
	for _, d := range data {
		wg.Add(1)
		go func(d int) {
			defer wg.Done()
			resultChan <- d * 2
		}(d)
	}
	wg.Wait()
	close(resultChan)
	result := make([]int, len(data))
	for i := 0; i < len(data); i++ {
		result[i] = <-resultChan
	}
	return result
}

func main() {
	data := []int{1, 2, 3}
	result := processConcurrentResults(data)
	fmt.Println(result)
}

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

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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 processConcurrentResults(data []int) []int { resultChan := make(chan int) var wg sync.WaitGroup for _, d := range data { wg.Add(1) go func(d int) { defer wg.Done() resultChan <- d * 2 }(d) } wg.Wait() close(resultChan) result := make([]int, len(data)) for i := 0; i < len(data); i++ { result[i] = <-resultChan } return result } func main() { data := []int{1, 2, 3} result := processConcurrentResults(data) fmt.Println(result) }

Unexpected zero value in concurrent Go program

What it should do

What happens instead

More async and race conditions to practise

Unexpected zero value in concurrent Go program

What it should do

What happens instead

More async and race conditions to practise