Return inside loops in Go - Mini Project: Build & Apply

Return inside loops

📖 Scenario: Imagine you are checking a list of numbers to find the first even number. Once you find it, you want to stop checking and return that number immediately.

🎯 Goal: You will write a Go function that loops through a slice of integers and returns the first even number it finds. If no even number is found, it returns -1.

📋 What You'll Learn

Create a slice of integers named numbers with the values 3, 7, 10, 5, 8.

Create a function called firstEven that takes a slice of integers as input and returns an integer.

Inside firstEven, use a for loop with index i and value num to iterate over the slice.

Inside the loop, use an if statement to check if num is even (divisible by 2). If yes, return num immediately.

If the loop finishes without finding an even number, return -1.

In main, call firstEven(numbers) and print the returned value.

💡 Why This Matters

🌍 Real World

Finding the first item that meets a condition quickly is common in many programs, like searching for available seats, checking for errors, or validating inputs.

💼 Career

Understanding how to return early from loops helps write efficient code and is a common pattern in software development jobs.

1

Create the numbers slice

Create a slice of integers called numbers with the exact values 3, 7, 10, 5, 8.

Go

var numbers = []int{3, 7, 10, 5, 8} // Your code here

Hint

Use var numbers = []int{3, 7, 10, 5, 8} to create the slice.

2

Create the firstEven function

Create a function called firstEven that takes a parameter nums of type []int and returns an int.

Go

package main

var numbers = []int{3, 7, 10, 5, 8}

func firstEven(nums []int) int {
    // Your code here
}

Hint

Define the function with func firstEven(nums []int) int and return an integer.

3

Add loop and return inside firstEven

Inside the firstEven function, use a for loop with variables i and num to iterate over nums. Inside the loop, use an if statement to check if num % 2 == 0. If yes, return num immediately.

Go

package main

var numbers = []int{3, 7, 10, 5, 8}

func firstEven(nums []int) int {
    for i, num := range nums {
        // Your code here
    }
    return -1
}

Hint

Use for i, num := range nums and inside it check if num%2 == 0 then return num.

4

Call firstEven and print the result

In the main function, call firstEven(numbers) and print the returned value using fmt.Println. Remember to import fmt.

Go

package main

import "fmt"

var numbers = []int{3, 7, 10, 5, 8}

func firstEven(nums []int) int {
    for i, num := range nums {
        if num%2 == 0 {
            return num
        }
    }
    return -1
}

func main() {
    // Your code here
}

Hint

Use result := firstEven(numbers) and then fmt.Println(result) inside main.

Practice

1.

What happens when you use return inside a for loop in Go?

easy

A. The function stops immediately and returns the value.

B. Only the current loop iteration stops, and the loop continues.

C. The loop skips the current iteration and moves to the next.

D. The loop runs forever without stopping.

Solution

1. Step 1: Understand the role of return in functions

   In Go, return immediately ends the function and sends back a value.

2. Step 2: Effect of return inside a loop

   When return is inside a loop, it stops the whole function, not just the loop.

3. Final Answer:

   The function stops immediately and returns the value. -> Option A

4. Quick Check:

   Return ends function, not just loop [OK]

Hint: Return inside loop ends function immediately [OK]

Common Mistakes:

• Thinking return only stops the loop
• Confusing return with break
• Assuming loop continues after return

2.

Which of the following Go code snippets correctly uses return inside a for loop to return the first even number from a slice?

func firstEven(nums []int) int {
    for _, n := range nums {
        if n % 2 == 0 {
            // What goes here?
        }
    }
    return -1
}

easy

A. return n

B. break

C. continue

D. return nums

Solution

1. Step 1: Identify what to return inside the loop

   The function should return the first even number found, which is n.

2. Step 2: Use return to stop function and send back the number

   Using return n immediately ends the function and returns the even number.

3. Final Answer:

   return n -> Option A

4. Quick Check:

   Return value inside loop to stop function [OK]

Hint: Use return with value to exit function inside loop [OK]

Common Mistakes:

• Using break instead of return
• Using continue which skips iteration
• Returning the whole slice instead of a number

3.

What is the output of this Go program?

package main
import "fmt"
func findFirstPositive(nums []int) int {
    for _, n := range nums {
        if n > 0 {
            return n
        }
    }
    return 0
}
func main() {
    fmt.Println(findFirstPositive([]int{-3, -2, 0, 5, 10}))
}

medium

A. 10

B. -3

C. 0

D. 5

Solution

1. Step 1: Trace the loop over the slice

   The loop checks each number: -3 (not >0), -2 (not >0), 0 (not >0), 5 (>0).

2. Step 2: Return the first positive number found

   When it reaches 5, it returns 5 immediately, ending the function.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Return stops at first positive number 5 [OK]

Hint: Return inside loop returns first matching value [OK]

Common Mistakes:

• Assuming loop continues after return
• Returning 0 instead of first positive
• Confusing index with value

4.

Find the error in this Go function that tries to return the first negative number from a slice:

func firstNegative(nums []int) int {
    for _, n := range nums {
        if n < 0 {
            break
        }
    }
    return 0
}

medium

A. Returning 0 instead of -1 when no negative found

B. Missing else statement after if

C. Using break instead of return inside the loop

D. Loop variable should be index, not value

Solution

1. Step 1: Understand break vs return inside loop

   Using break only stops the loop, but the function continues and returns 0.

2. Step 2: Correct usage is return to stop function and return value

   To return the first negative number, use return n inside the if block.

3. Final Answer:

   Using break instead of return inside the loop -> Option C

4. Quick Check:

   Return ends function; break only stops loop [OK]

Hint: Use return, not break, to exit function inside loop [OK]

Common Mistakes:

• Using break instead of return
• Expecting break to return value
• Not returning value inside loop

5.

You want to write a Go function that returns the first string in a slice that starts with the letter 'A'. If none is found, return "Not found". Which code correctly uses return inside the loop?

func findA(words []string) string {
    for _, w := range words {
        if len(w) > 0 && w[0] == 'A' {
            // What goes here?
        }
    }
    return "Not found"
}

hard

A. break

B. return w

C. continue

D. return words[0]

Solution

1. Step 1: Identify the correct return value inside the loop

   The function should return the first word starting with 'A', which is w.

2. Step 2: Use return to immediately end the function and return the word

   Using return w stops the function and returns the found word.

3. Final Answer:

   return w -> Option B

4. Quick Check:

   Return found word inside loop to stop function [OK]

Hint: Return matching item inside loop to stop function early [OK]

Common Mistakes:

• Using break which only stops loop
• Using continue which skips iteration
• Returning wrong element like words[0]