While loop in C++ - Mini Project: Build & Apply

Counting Down with a While Loop

📖 Scenario: Imagine you have a rocket that needs to count down before launch. We will write a simple program that counts down from a number to zero using a while loop.

🎯 Goal: Build a C++ program that uses a while loop to count down from 5 to 0 and print each number.

📋 What You'll Learn

Create an integer variable called count and set it to 5

Use a while loop that runs while count is greater than or equal to 0

Inside the loop, print the current value of count

Decrease count by 1 in each loop iteration

Print the countdown numbers on separate lines

💡 Why This Matters

🌍 Real World

Counting down is common in timers, games, and events where you wait before starting something important.

💼 Career

Understanding loops is essential for programming tasks like automation, data processing, and controlling program flow.

1

Set up the countdown variable

Create an integer variable called count and set it to 5.

C++

#include 

int main() {
    // Create an integer variable called count and set it to 5
    // Your code here

    return 0;
}

Hint

Use int count = 5; inside the main function.

2

Write the while loop condition

Add a while loop that runs while count is greater than or equal to 0.

C++

#include 

int main() {
    int count = 5;

    // Add a while loop that runs while count is >= 0
    // Your code here

    return 0;
}

Hint

Use while (count >= 0) { } to start the loop.

3

Print the count and decrease it

Inside the while loop, print the current value of count using std::cout, then decrease count by 1 using count--.

C++

#include 

int main() {
    int count = 5;

    while (count >= 0) {
        // Print count
        // Decrease count by 1
        // Your code here
    }

    return 0;
}

Hint

Use std::cout << count << std::endl; to print and count--; to decrease.

4

Run the program to see the countdown

Add a return 0; at the end of main and run the program to print the countdown from 5 to 0.

C++

#include 

int main() {
    int count = 5;

    while (count >= 0) {
        std::cout << count << std::endl;
        count--;
    }

    // Run the program to see the output
    // Your code here
}

Hint

Run the program and check the output shows numbers from 5 down to 0, each on its own line.

Practice

1.

What does a while loop do in C++?

easy

A. Executes code only once

B. Repeats code a fixed number of times

C. Stops the program immediately

D. Repeats code while a condition is true

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs the code inside it repeatedly as long as the condition is true.

2. Step 2: Compare options with definition

   Only Repeats code while a condition is true correctly states this behavior.

3. Final Answer:

   Repeats code while a condition is true -> Option D

4. Quick Check:

   while loop = repeat while true [OK]

Hint: While loops run as long as condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Assuming while loops always run once

2.

Which of the following is the correct syntax for a while loop in C++?

???
{
    // code
}

easy

A. while condition

B. while {condition}

C. while (condition)

D. while: (condition)

Solution

1. Step 1: Recall correct while loop syntax

   The condition must be inside parentheses after the keyword while.

2. Step 2: Check each option

   while (condition) uses while (condition), which is correct syntax in C++.

3. Final Answer:

   while (condition) -> Option C

4. Quick Check:

   while loop syntax = while (condition) [OK]

Hint: Condition in while must be inside parentheses [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using curly braces instead of parentheses
• Adding colon after while

3.

What is the output of this C++ code?

int i = 1;
while (i <= 3) {
    std::cout << i << " ";
    i++;
}

medium

A. 1 2 3 4

B. 1 2 3

C. 0 1 2

D. 3 2 1

Solution

1. Step 1: Analyze loop condition and increments

   Variable i starts at 1 and increases by 1 each loop until i > 3.

2. Step 2: Trace output values

   i = 1 prints "1 ", then 2 prints "2 ", then 3 prints "3 ". Loop stops after i becomes 4.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Prints 1 to 3 with spaces [OK]

Hint: Count from start to condition limit inclusive [OK]

Common Mistakes:

• Printing extra number after loop ends
• Starting count from 0 instead of 1
• Printing numbers in reverse

4.

Find the error in this code snippet:

int count = 5;
while (count > 0)
    std::cout << count << " ";

medium

A. Infinite loop because count is never changed

B. Variable count is not declared

C. Syntax error in while condition

D. Missing semicolon after cout statement

Solution

1. Step 1: Check loop body and variable changes

   The loop prints count but does not change it inside the loop.

2. Step 2: Understand loop behavior

   Since count stays 5, condition is always true, causing an infinite loop.

3. Final Answer:

   Infinite loop because count is never changed -> Option A

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Update loop variable inside while to avoid infinite loop [OK]

Common Mistakes:

• Forgetting to change loop variable
• Assuming loop ends automatically
• Misreading syntax as error

5.

Write a while loop in C++ that prints all even numbers from 2 to 10 inclusive.

Which code snippet correctly does this?

???

hard

A. int n = 2;
while (n <= 10) {
std::cout << n << " ";
n += 2;
}

B. int n = 1;
while (n < 10) {
if (n % 2 == 0) std::cout << n << " ";
n++;
}

C. int n = 2;
while (n < 10) {
std::cout << n << " ";
n += 2;
}

D. int n = 0;
while (n <= 10) {
std::cout << n << " ";
n += 2;
}

Solution

1. Step 1: Understand the requirement

   We need to print even numbers from 2 to 10 inclusive, so numbers: 2,4,6,8,10.

2. Step 2: Check each option

   int n = 2; while (n <= 10) { std::cout << n << " "; n += 2; } starts at 2, increments by 2, and stops at 10 inclusive, printing correct numbers.

3. Final Answer:

   2 4 6 8 10 -> Option A

4. Quick Check:

   Start at 2, add 2 each time, stop at 10 [OK]

Hint: Start at 2, add 2, stop at 10 inclusive [OK]

Common Mistakes:

• Stopping before 10 (using < instead of <=)
• Starting at 1 and missing even numbers
• Including 0 which is not in range