C++ Lecture 2: The while Loop

C++ Lecture 2: The while Loop

1. Learning Objectives

By the end of this lecture, students will be able to:

• Understand the working of a while loop.
• Write a while loop using a condition.
• Print numbers in ascending and descending order.
• Calculate the sum of numbers using a loop.
• Trace a loop step by step.
• Identify and correct common loop errors.

2. What Is a while Loop?

A while loop repeats a block of code as long as a given condition is true.

Real-life example: Imagine filling a water bottle. You continue filling it while it is not full. When the bottle becomes full, you stop.

A while loop works in a similar way:

1. Check the condition.
2. If the condition is true, execute the loop body.
3. Update the variable.
4. Check the condition again.
5. Stop when the condition becomes false.

3. Syntax of the while Loop

while (condition)
{
    // Statements to repeat
}

Example:

int i = 1;

while (i <= 5)
{
    cout << i << endl;
    i++;
}

Understand the Code

• int i = 1; — starts the counter at 1.
• while (i <= 5) — repeats the loop while i is less than or equal to 5.
• cout << i << endl; — displays the current value of i.
• i++; — increases i by 1.

Output:

1
2
3
4
5

Remember: i++ means i = i + 1.

4. How Does the Loop Work?

Let’s trace the previous program.

When i becomes 6, the condition is false, so the loop ends.

5. Example 1: Print a Message Five Times

#include <iostream>
using namespace std;

int main()
{
    int i = 1;

    while (i <= 5)
    {
        cout << "Welcome to Ghazi University!" << endl;
        i++;
    }

    return 0;
}

Understand the code: The loop starts at 1 and repeats until i becomes greater than 5. The message is displayed five times.

Modify the code:

• Display the message 10 times.
• Change the message to "Welcome to C++ Programming!".
• Start i at 2 and repeat while i <= 6. How many times will the message appear?

6. Example 2: Print Numbers from 1 to 10

#include <iostream>
using namespace std;

int main()
{
    int i = 1;

    while (i <= 10)
    {
        cout << i << " ";
        i++;
    }

    return 0;
}

Output:

1 2 3 4 5 6 7 8 9 10

Understand the code: The counter begins at 1. Each repetition prints the current number and increases it by 1. The loop stops when the counter reaches 11.

Modify the code:

• Print numbers from 1 to 20.
• Print numbers from 5 to 15.
• Print numbers from 10 to 1.

7. Example 3: Print Even Numbers from 2 to 20

#include <iostream>
using namespace std;

int main()
{
    int i = 2;

    while (i <= 20)
    {
        cout << i << " ";
        i = i + 2;
    }

    return 0;
}

Output:

2 4 6 8 10 12 14 16 18 20

Understand the code: The counter starts at 2 and increases by 2 after each repetition. Therefore, only even numbers are displayed.

Modify the code:

• Print odd numbers from 1 to 19.
• Print even numbers from 10 to 30.
• Print multiples of 5 from 5 to 50.

8. Example 4: Countdown from 10 to 1

#include <iostream>
using namespace std;

int main()
{
    int i = 10;

    while (i >= 1)
    {
        cout << i << " ";
        i--;
    }

    return 0;
}

Output:

10 9 8 7 6 5 4 3 2 1

Understand the code:

• The counter starts at 10.
• The condition checks whether i >= 1.
• i-- decreases the counter by 1.
• The loop stops when i becomes 0.

Modify the code: Change the countdown to start from 20 and stop at 1.

9. Example 5: Sum of Numbers from 1 to 5

A loop can do more than display numbers. It can also calculate their sum.

#include <iostream>
using namespace std;

int main()
{
    int i = 1;
    int sum = 0;

    while (i <= 5)
    {
        sum = sum + i;
        i++;
    }

    cout << "Sum = " << sum;

    return 0;
}

Output:

Sum = 15

Understand the Code

• i controls how many times the loop repeats.
• sum stores the running total.
• sum = sum + i; adds the current value of i to sum.
• i++ moves to the next number.

Trace Table

Modify the code:

• Calculate the sum from 1 to 10.
• Calculate the sum from 1 to 100.
• Calculate the sum of even numbers from 2 to 20.

10. Example 6: Take Input from the User

Suppose the user enters a number, and the program displays all numbers from 1 up to that number.

#include <iostream>
using namespace std;

int main()
{
    int n;
    int i = 1;

    cout << "Enter a number: ";
    cin >> n;

    while (i <= n)
    {
        cout << i << " ";
        i++;
    }

    return 0;
}

Sample output:

Enter a number: 5
1 2 3 4 5

Understand the code: The value of n comes from the user. If the user enters 5, the loop displays numbers from 1 to 5. If the user enters 8, it displays numbers from 1 to 8.

Modify the code: Display numbers from n down to 1.

11. Common Mistakes in a while Loop

Mistake 1: Forgetting to update the counter

Incorrect:

int i = 1;

while (i <= 5)
{
    cout << i << " ";
}

Problem: The value of i never changes. The condition remains true, so the loop runs indefinitely.

Correct:

int i = 1;

while (i <= 5)
{
    cout << i << " ";
    i++;
}

Mistake 2: Using the wrong condition

int i = 1;

while (i >= 5)
{
    cout << i;
    i++;
}

Problem: The condition is false from the beginning because 1 is not greater than or equal to 5. The loop body never executes.

Mistake 3: Adding a semicolon after while

Incorrect:

while (i <= 5);
{
    cout << i;
    i++;
}

Problem: The semicolon creates an empty loop. It may cause an infinite loop if the condition remains true.

Correct:

while (i <= 5)
{
    cout << i;
    i++;
}

12. Classroom Practice

Student checklist

0 of 6

1. Repeat a message

Write a program to display your name five times.

2. Print a sequence

Display numbers from 1 to 50.

3. Print odd numbers

Display odd numbers from 1 to 25.

4. Countdown

Display numbers from 50 down to 1.

5. Calculate a sum

Calculate the sum of numbers from 1 to 20.

6. User input

Ask the user for a number and display its multiplication table from 1 to 10.

13. Quick Revision Quiz

Try to answer before checking the solutions.

1. What happens when a `while` condition is false at the beginning?

The loop executes once

The loop body does not execute

The program always crashes

2. What does `i++` do?

Adds 2 to i

Subtracts 1 from i

Adds 1 to i

3. Which statement is important to prevent an unintended infinite loop?

Update the counter when needed

Use `cout` twice

Always start at zero

Check Answers

14. Summary

• A while loop repeats statements while its condition is true.
• Initialize the counter before the loop.
• Write a suitable condition.
• Update the counter inside the loop when needed.
• Use trace tables to understand how values change.
• Always check that the loop will eventually stop.

Next lecture: The do-while loop — how it differs from the while loop and why it executes its body at least once.