goAsync race conditionMedium

Unexpected output in concurrent range loop processing

Time: 0:00

BugFind the bug and fix the code

What it should do

Process a slice of integers concurrently and return their squares. Example: processItems([1, 2, 3]) -> [1, 4, 9]

What happens instead

The output slice contains incorrect or duplicate values.

package main

import (
	"fmt"
	"sync"
)

func processItems(items []int) []int {
	var wg sync.WaitGroup
	output := make([]int, len(items))
	for _, item := range items {
		wg.Add(1)
		go func() {
			defer wg.Done()
			output[item] = item * item
		}()
	}
	wg.Wait()
	return output
}

func main() {
	items := []int{1, 2, 3}
	fmt.Println(processItems(items))
}

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

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This bug belongs to the async and race conditions pattern

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

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package main import ( "fmt" "sync" ) func processItems(items []int) []int { var wg sync.WaitGroup output := make([]int, len(items)) for _, item := range items { wg.Add(1) go func() { defer wg.Done() output[item] = item * item }() } wg.Wait() return output } func main() { items := []int{1, 2, 3} fmt.Println(processItems(items)) }

Unexpected output in concurrent range loop processing

What it should do

What happens instead

More async and race conditions to practise

Unexpected output in concurrent range loop processing

What it should do

What happens instead

More async and race conditions to practise