C++ Lecture Notes — Lesson 1: Introduction to Loops

C++ Lecture Notes — Lesson 1: Introduction to Loops

Learning Objectives

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

1. Explain what a loop is.
2. Understand why loops are used in programming.
3. Identify repetitive tasks in real-life situations.
4. Compare code written with and without loops.
5. Understand the basic idea of repetition, a condition, and stopping a loop.

1. What Is a Loop?

A loop is a programming structure that repeats a block of code multiple times.

In real life, we often repeat the same task. For example:

• A teacher takes attendance for 50 students.
• A student practises a question 10 times.
• A university system sends notifications to many students.
• A program displays a message 5 times.

Instead of writing the same instructions repeatedly, we can use a loop.

Simple definition: A loop allows us to execute the same instructions repeatedly without writing them again and again.

2. Why Do We Need Loops?

Suppose we want to display a message five times.

Without a loop:

#include <iostream>
using namespace std;

int main()
{
    cout << "Welcome to Ghazi University!" << endl;
    cout << "Welcome to Ghazi University!" << endl;
    cout << "Welcome to Ghazi University!" << endl;
    cout << "Welcome to Ghazi University!" << endl;
    cout << "Welcome to Ghazi University!" << endl;

    return 0;
}

Output:

Welcome to Ghazi University!
Welcome to Ghazi University!
Welcome to Ghazi University!
Welcome to Ghazi University!
Welcome to Ghazi University!

Problem: We have written the same instruction five times. What if we need to display it 100 times?

Writing 100 separate cout statements would be unnecessary and difficult to maintain.

We can solve this problem using a loop.

3. Our First Loop Program

We will use a while loop to display the same 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;
}

Output:

Welcome to Ghazi University!
Welcome to Ghazi University!
Welcome to Ghazi University!
Welcome to Ghazi University!
Welcome to Ghazi University!

The output is the same, but the code is shorter and easier to manage.

4. Understand the Code Step by Step

Let’s understand the program in simple language.

Step 1: Create a variable

int i = 1;

• int means the variable stores an integer.
• i is the variable name.
• 1 is its initial value.

Think of i as a counter that keeps track of how many times we have repeated the task.

Step 2: Check the condition

while (i <= 5)

The condition checks whether i is less than or equal to 5.

• If the condition is true, the loop body executes.
• If the condition is false, the loop stops.

Step 3: Execute the instruction

cout << "Welcome to Ghazi University!" << endl;

This statement displays the message.

Step 4: Update the counter

i++;

This increases i by 1.

For example:

i++;

is equivalent to:

i = i + 1;

Without this update, the condition would remain true indefinitely in this example, creating an infinite loop.

5. Trace Table: How Does the Loop Work?

A trace table helps us understand how a program executes step by step.

Notice that the loop checks the condition again after each update. When i becomes 6, the condition is false, so the program exits the loop.

6. Understand Loops Through a Simple Analogy

Imagine placing five books on a table, one by one.

Closed book on desk by blackboard

1. Initialization

Start counting from book number 1.

Books on the desk

2. Condition

Ask: Have we placed five books or fewer?

a diligent boy sits at a Desk and puts books on stacks.

3. Repetition and update

Place another book and increase the count.

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4. Stop

Stop when five books have been placed.

The same idea applies to programming: check, execute, update, and repeat until the condition becomes false.

7. Modify the Code

Start with the following program.

#include <iostream>
using namespace std;

int main()
{
    int i = 1;

    while (i <= 5)
    {
        cout << "Hello Students!" << endl;
        i++;
    }

    return 0;
}

Complete these activities in order.

Modification checklist

0 of 5

Change the repetition count

Change i <= 5 to i <= 3. How many times will the message appear?

Change the message

Replace Hello Students! with Welcome to the Programming Lab!

Change the starting value

Change int i = 1 to int i = 2. Keep the condition i <= 5. How many times will the message appear?

Change the update

Replace i++ with i = i + 2. Keep the starting value 1 and condition i <= 5. Predict the number of messages.

Display a counter

Replace the message with cout « i « endl;. Predict the output before running the program.

8. Practice Coding Exercises

Students should attempt these exercises independently.

Exercise 1 — Easy

Display a message

Write a C++ program that displays I am learning C++ five times using a while loop.

Exercise 2 — Easy

Print numbers

Write a program to display numbers from 1 to 10.

Exercise 3 — Easy

Countdown

Write a program to display numbers from 10 down to 1.

Exercise 4 — Intermediate

Even numbers

Write a program to display even numbers from 2 to 20.

Exercise 5 — Intermediate

User-controlled repetition

Ask the user to enter a number N. Display Welcome! exactly N times.

9. Common Mistakes

• Forgetting the update: The loop may never stop.
• Using the wrong condition: The program may repeat too many or too few times.
• Incorrect initialization: Starting from 0 instead of 1 can change the number of repetitions.
• Confusing i++ with i = i + 2: The first increases the value by 1; the second increases it by 2.
• Forgetting braces: Use braces to clearly group multiple statements in the loop body.

10. Quick Revision Questions

1. What is a loop in C++?
2. Why do we use loops instead of writing the same statement repeatedly?
3. What is the purpose of the variable i?
4. What does the condition i <= 5 mean?
5. What is the purpose of i++?
6. What is an infinite loop?
7. How many times will the following loop execute?

int i = 1;

while (i <= 4)
{
    cout << i << endl;
    i++;
}

Expected output:

1
2
3
4

Lesson Summary

Students should remember these four concepts:

• Initialization: Where repetition starts.
• Condition: Determines whether the loop continues.
• Loop body: The statements that repeat.
• Update: Changes the variable to move the loop toward termination.

The next lesson should cover the while loop in detail, including user input, number sequences, sums, trace tables, and exercises.