Return inside loops in C++ - 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 function that goes through a list of numbers and returns the first even number it finds using a return statement inside a loop.

📋 What You'll Learn

Create a vector called numbers with the exact values: 3, 7, 10, 5, 8

Create an integer variable called firstEven and initialize it to -1

Write a for loop using int i = 0; i < numbers.size(); i++ to check each number

Inside the loop, use an if statement to check if the current number is even

If the number is even, use return to immediately return that number

Outside the loop, return firstEven if no even number is found

Print the returned value from the function

💡 Why This Matters

🌍 Real World

Finding the first item that meets a condition quickly is common in searching tasks, like finding the first available seat or the first error in a log.

💼 Career

Understanding how to return early from loops helps write efficient code in many programming jobs, especially when processing data or searching.

1

Create the data vector

Create a std::vector<int> called numbers with these exact values: 3, 7, 10, 5, 8

C++

#include 
#include 

int findFirstEven(const std::vector& numbers) {
    // Your code here
}

int main() {
    // Create the vector numbers here
    // Your code here
    return 0;
}

Hint

Use std::vector<int> numbers = {3, 7, 10, 5, 8}; to create the vector.

2

Add a variable to hold the default return value

Inside the function findFirstEven, create an integer variable called firstEven and set it to -1

C++

#include 
#include 

int findFirstEven(const std::vector& numbers) {
    // Create firstEven variable here
    // Your code here
}

int main() {
    std::vector numbers = {3, 7, 10, 5, 8};
    return 0;
}

Hint

Write int firstEven = -1; inside the function.

3

Write the loop and return inside it

Inside the function findFirstEven, write a for loop using int i = 0; i < numbers.size(); i++. Inside the loop, use an if statement to check if numbers[i] % 2 == 0. If true, immediately return numbers[i]. After the loop, return firstEven.

C++

#include 
#include 

int findFirstEven(const std::vector& numbers) {
    int firstEven = -1;
    // Add for loop and return inside it here
    // Your code here
}

int main() {
    std::vector numbers = {3, 7, 10, 5, 8};
    return 0;
}

Hint

Use a for loop and inside it an if to check even numbers. Use return inside the if.

4

Call the function and print the result

In main(), call findFirstEven(numbers) and store the result in an integer variable called result. Then print result using std::cout.

C++

#include 
#include 

int findFirstEven(const std::vector& numbers) {
    int firstEven = -1;
    for (int i = 0; i  numbers = {3, 7, 10, 5, 8};
    // Call findFirstEven and print the result here
    // Your code here
    return 0;
}

Hint

Call the function, save the return value in result, then print it with std::cout << result << std::endl;.

Practice

1.

What happens when a return statement is executed inside a loop in a C++ function?

easy

A. The loop runs twice before the function stops.

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

C. The function immediately stops and returns the value.

D. The loop skips to the next iteration without stopping the function.

Solution

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

   The return statement immediately ends the function and sends back a value.

2. Step 2: Analyze return inside a loop

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

3. Final Answer:

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

4. Quick Check:

   Return inside loop = function stops [OK]

Hint: Return inside loop ends function immediately [OK]

Common Mistakes:

• Thinking return only stops the loop
• Confusing return with continue or break
• Assuming code after return runs

2.

Which of the following is the correct syntax to return the first even number found in a vector nums using a loop?

for (int i = 0; i < nums.size(); i++) {
    if (nums[i] % 2 == 0) {
        // return here
    }
}

easy

A. return nums[i];

B. return i;

C. return nums;

D. return i + 1;

Solution

1. Step 1: Identify what to return

   The question asks to return the first even number, so we return the value at index i.

2. Step 2: Match the correct return statement

   return nums[i]; returns the actual even number found.

3. Final Answer:

   return nums[i]; -> Option A

4. Quick Check:

   Return value, not index [OK]

Hint: Return the value, not the index, when asked for element [OK]

Common Mistakes:

• Returning the index instead of the value
• Returning the whole vector
• Returning incorrect arithmetic on index

3.

What is the output of the following C++ function when called with findFirstPositive({-3, -1, 0, 4, 5})?

int findFirstPositive(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] > 0) {
            return nums[i];
        }
    }
    return -1;
}

medium

A. -1

B. 4

C. 0

D. 5

Solution

1. Step 1: Trace the loop to find first positive

   The loop checks each element: -3 (no), -1 (no), 0 (no), 4 (yes, positive).

2. Step 2: Return the first positive number found

   At 4, the function returns immediately with 4.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   First positive = 4 [OK]

Hint: Return stops at first positive number found [OK]

Common Mistakes:

• Returning -1 instead of first positive
• Confusing 0 as positive
• Returning last positive instead of first

4.

Identify the error in this function that tries to return the first negative number from a vector nums:

int findNegative(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] < 0) {
            return nums[i];
        } else {
            return 0;
        }
    }
    return -1;
}

medium

A. The loop condition is incorrect.

B. The function never returns any value.

C. The return type should be void.

D. The function returns 0 too early inside the loop.

Solution

1. Step 1: Analyze the else return inside the loop

   The else block returns 0 immediately if the first element is not negative, stopping the function early.

2. Step 2: Understand why this is wrong

   This prevents checking all elements; function should only return 0 after full loop if no negative found.

3. Final Answer:

   The function returns 0 too early inside the loop. -> Option D

4. Quick Check:

   Return in else inside loop causes early stop [OK]

Hint: Avoid return in else inside loop; return after loop instead [OK]

Common Mistakes:

• Returning inside else causes premature exit
• Thinking loop runs fully before return
• Misunderstanding return type requirements

5.

Write a function that returns the index of the first zero in a vector nums. If no zero is found, return -1. Which of the following implementations is correct?

Option A:
int findZero(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            return i;
        }
    }
    return -1;
}

Option B:
int findZero(std::vector<int> nums) {
    int index = -1;
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            index = i;
        }
    }
    return index;
}

Option C:
int findZero(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            break;
        }
    }
    return i;
}

Option D:
int findZero(std::vector<int> nums) {
    for (int i = 0; i < nums.size(); i++) {
        if (nums[i] == 0) {
            return nums[i];
        }
    }
    return -1;
}

hard

A. Option A

B. Option B

C. Option C

D. Option D

Solution

1. Step 1: Verify the correct logic for finding first zero index

   The implementation that returns i immediately when nums[i] == 0 and -1 after the loop is correct.

2. Step 2: Why others fail

   The one setting a variable continues the loop, returning the last zero's index. The break version doesn't return -1 if no zero found and i goes out of scope. The last returns the value 0 instead of index.

3. Final Answer:

   Option A -> Option A

4. Quick Check:

   Return index immediately on first zero [OK]

Hint: Return index immediately when found, else after loop return -1 [OK]

Common Mistakes:

• Returning value instead of index
• Returning last zero index instead of first
• Using break without returning causes scope errors