C++ Lecture 4: The for Loop

C++ Lecture 4: The for Loop

1. Learning Objectives

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

• Understand the purpose and syntax of a for loop.
• Use a for loop to repeat statements.
• Print ascending and descending number sequences.
• Calculate sums and multiplication tables.
• Modify existing programs and solve coding exercises.

2. What Is a for Loop?

A for loop repeats a block of code a specified number of times or while a condition remains true.

Real-life example: A teacher wants to mark attendance for 30 students. Instead of writing the same instructions 30 times, a loop can repeat the task.

A for loop is especially useful when the repetition follows a clear counting pattern.

3. Syntax of the for Loop

for (initialization; condition; update)
{
    // Statements to repeat
}

There are three main parts:

Remember the order: Initialize → Check condition → Execute body → Update → Check again.

4. Example 1: Print Numbers from 1 to 5

#include <iostream>
using namespace std;

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

    return 0;
}

Output:

1
2
3
4
5

Understand the Code

• int i = 1 starts the counter at 1.
• i <= 5 allows the loop to run while i is 5 or less.
• i++ increases i by 1 after each repetition.
• When i becomes 6, the condition is false and the loop stops.

Modify the Code

1. Print numbers from 1 to 10.
2. Print numbers from 1 to 20.
3. Start from 5 and print up to 15.
4. Display each number on the same line instead of separate lines.

5. Example 2: Print Numbers in Descending Order

#include <iostream>
using namespace std;

int main()
{
    for (int i = 10; i >= 1; i--)
    {
        cout << i << " ";
    }

    return 0;
}

Output:

10 9 8 7 6 5 4 3 2 1

Understand the Code

• int i = 10 starts from 10.
• i >= 1 continues the loop until the counter becomes less than 1.
• i-- decreases the counter by 1.

Modify the Code

1. Print numbers from 20 to 1.
2. Print numbers from 50 to 10.
3. Print numbers from 100 to 90.

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

#include <iostream>
using namespace std;

int main()
{
    for (int i = 2; i <= 20; i = i + 2)
    {
        cout << i << " ";
    }

    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 each time. Therefore, only even numbers are displayed.

i = i + 2 means the new value of i is its previous value plus 2.

Modify the Code

1. Print odd numbers from 1 to 19.
2. Print multiples of 5 from 5 to 50.
3. Print even numbers from 20 down to 2.

7. Example 4: Calculate the Sum from 1 to 10

#include <iostream>
using namespace std;

int main()
{
    int sum = 0;

    for (int i = 1; i <= 10; i++)
    {
        sum = sum + i;
    }

    cout << "Sum = " << sum;

    return 0;
}

Output:

Sum = 55

Understand the Code

• int sum = 0 initializes the total.
• The loop starts at 1 and continues up to 10.
• sum = sum + i adds each number to the running total.
• After the loop finishes, the program displays the final sum.

For example:

Modify the Code

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

8. Example 5: Multiplication Table

#include <iostream>
using namespace std;

int main()
{
    int n;

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

    for (int i = 1; i <= 10; i++)
    {
        cout << n << " x " << i
             << " = " << n * i << endl;
    }

    return 0;
}

Sample output:

Enter a number: 5
5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
5 x 4 = 20
5 x 5 = 25
5 x 6 = 30
5 x 7 = 35
5 x 8 = 40
5 x 9 = 45
5 x 10 = 50

Understand the Code

• cin >> n receives a number from the user.
• The loop repeats ten times.
• n * i calculates each multiplication result.
• endl moves the output to a new line.

Modify the Code

1. Display the table up to 20 instead of 10.
2. Display the multiplication table in reverse order, from 10 to 1.
3. Ask the user for both the number and the table’s ending value.

9. Example 6: Take User Input and Print a Sequence

#include <iostream>
using namespace std;

int main()
{
    int n;

    cout << "Enter the ending number: ";
    cin >> n;

    for (int i = 1; i <= n; i++)
    {
        cout << i << " ";
    }

    return 0;
}

Sample output:

Enter the ending number: 7
1 2 3 4 5 6 7

Understand the Code

The user decides how many numbers to print. If the user enters 7, the loop prints 1 through 7.

Modify the Code

1. Ask the user for a starting number and an ending number.
2. Ask the user for a number and print its countdown to 1.
3. Ask the user for a number and print all even numbers up to that number.

10. Example 7: Nested for Loops — Star Pattern

A nested loop is a loop placed inside another loop.

#include <iostream>
using namespace std;

int main()
{
    for (int i = 1; i <= 4; i++)
    {
        for (int j = 1; j <= 5; j++)
        {
            cout << "* ";
        }

        cout << endl;
    }

    return 0;
}

Output:

* * * * *
* * * * *
* * * * *
* * * * *

Understand the Code

• The outer loop controls the rows. It runs 4 times.
• The inner loop controls the stars in each row. It prints 5 stars.
• After the inner loop finishes, cout << endl; starts a new row.
• The inner loop completes all five repetitions for every single outer-loop repetition.

Modify the Code

Try changing the program to produce these patterns.

Pattern A — Right-angle triangle

*
* *
* * *
* * * *
* * * * *

Pattern B — Increasing numbers

1
1 2
1 2 3
1 2 3 4
1 2 3 4 5

Pattern C — Rectangle of numbers

1 2 3 4
1 2 3 4
1 2 3 4

11. Difference Between while, do-while, and for

All three loops can solve many of the same problems. Choose the loop that makes the program easiest to understand.

12. Common Mistakes

Mistake 1: Using the wrong condition

for (int i = 1; i >= 5; i++)
{
    cout << i;
}

The condition is false at the beginning, so the body does not execute.

Mistake 2: Forgetting the update

for (int i = 1; i <= 5; )
{
    cout << i;
}

The counter never changes, so the condition stays true and the loop does not end normally.

Mistake 3: Adding an unnecessary semicolon

for (int i = 1; i <= 5; i++);
{
    cout << "Hello";
}

The semicolon creates an empty loop. The block after it is not the loop body.

13. Coding Exercises

Solve these tasks independently, without copying the examples.

Level 1 — Basic

1. Print your name 10 times.
2. Print numbers from 1 to 100.
3. Print numbers from 100 down to 1.
4. Print all odd numbers from 1 to 49.

Level 2 — Intermediate

1. Calculate the sum of numbers from 1 to n, where n is entered by the user.
2. Display the multiplication table of a user-entered number.
3. Count how many numbers between 1 and 100 are divisible by 5.
4. Calculate the sum of all even numbers from 2 to 100.

Level 3 — Problem Solving

1. Ask the user for five numbers and calculate their total.
2. Ask the user for a number and calculate its factorial. For example, \(5! = 5 \times 4 \times 3 \times 2 \times 1 = 120\).
3. Print a right-angle triangle of stars with the number of rows entered by the user.
4. Print a multiplication table for every number from 1 to 5.

14. Quick Revision Quiz

1. Which part of a `for` loop changes the counter?

Initialization

Condition

Update

2. How many times does this loop run: `for (int i = 1; i <= 5; i++)`?

4 times

5 times

6 times

3. Which loop executes its body at least once?

while

do-while

for

4. In a nested loop, which loop controls the rows in our star-pattern example?

Outer loop

Inner loop

Both always control the same thing

Check Answers

15. Lesson Summary

• A for loop is useful for repeating statements, especially when counting.
• Initialization sets the starting value.
• The condition determines whether the loop continues.
• The update changes the counter.
• Nested loops are useful for patterns, tables, and two-dimensional repetition.