C++ while Loops: Syntax, Examples, and Common Errors

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.

Suggested title: Types of while Loops: Counter-Controlled and Sentinel-Controlled

Here is the improved version of your student notes, using simple English and clear examples.

11 Types of while Loops

A while loop can be used in different ways depending on how the repetition is controlled. Two common types are counter-controlled loops and sentinel-controlled loops.

1. Counter-Controlled Loop

A counter-controlled loop repeats a specific number of times. A counter variable keeps track of the number of repetitions.

Example: Display numbers from 1 to 5

Code

int i = 1;
 
while (i <= 5)
{
    cout << i << endl;
    i++;
}

How It Works

• Initialization: int i = 1; sets the starting value of the counter.
• Condition: i <= 5 allows the loop to continue while i is less than or equal to 5.
• Update: i++ increases the counter by 1 after each repetition.
• Termination: The loop stops when i becomes 6 because the condition becomes false.

Output

1
2
3
4
5

Remember: A counter-controlled loop is used when the number of repetitions is known in advance.

2. Sentinel-Controlled Loop

A sentinel-controlled loop repeats until a special value, called a sentinel value, is entered or encountered. This value signals the loop to stop.

Example: Accept numbers until the user enters 0

Code

int num;
 
cout << "Enter a number (0 to stop): ";
cin >> num;
 
while (num != 0)
{
    cout << "You entered: " << num << endl;
 
    cout << "Enter a number (0 to stop): ";
    cin >> num;
}

How It Works

• Input: The user enters a number.
• Condition: num != 0 checks whether the entered number is not 0.
• Repetition: If the number is not 0, the loop displays it and asks for another number.
• Termination: When the user enters 0, the condition becomes false, and the loop stops.

Sample Output

Enter a number (0 to stop): 10
You entered: 10
Enter a number (0 to stop): 25
You entered: 25
Enter a number (0 to stop): 7
You entered: 7
Enter a number (0 to stop): 0

Remember: A sentinel-controlled loop is used when the number of repetitions is not known in advance. The loop continues until the sentinel value is entered.

3. Key Differences

12. 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++;
}

13. Practice Coding Exercises

Solve Coding Exercises 1–6 in the while loop section. Write, run, and test each C++ program to verify the output.

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.