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I recently started teaching a course on C programming and as part of my preparation for class, I usually write out lesson notes that I will be teaching from. I have therefore decided to make these into blog posts so anyone else learning C can also benefit from them. Therefore, this lesson is part of the series I am calling: C Programming for Absolute Beginners. I hope you enjoy it. Ask any questions or leave comments if you need further clarification or want to make a suggestion.

So far, your programs can:

• Store data

• Make decisions

• Respond to user input

Now we teach them something powerful:

Repetition.

Because in real programming, we often want to do something more than once. You can access the previous lesson here.

Print numbers from 1 to 10.
Ask the user for input multiple times.
Repeat a calculation.

That’s where loops come in.

What Is a Loop?

A loop allows a block of code to run multiple times.

Instead of writing:

printf("Hello\n");
printf("Hello\n");
printf("Hello\n");

We can write a loop that prints it 3 times automatically.

C has three main loops:

• while

• do-while

• for

Let’s go step by step.

1. The while Loop

The while loop runs as long as the condition is true.

Syntax

while (condition) {
    // code runs repeatedly
}

Example: Print Numbers 1 to 5

#include <stdio.h>

int main() {
    int i = 1;

    while (i <= 5) {
        printf("%d\n", i);
        i++;
    }

    return 0;
}

How This Works

• i starts at 1

• The loop runs while i <= 5

• After printing, i++ increases the value

• When i becomes 6, the loop stops

Important:
If you forget i++, the loop will never stop.
This is called an infinite loop.

2. The do-while Loop

The do-while loop is similar to while.

The difference:

• It runs at least once, even if the condition is false.

Syntax

do {
    // code
} while (condition);

Notice the semicolon at the end.

Example

#include <stdio.h>

int main() {
    int i = 1;

    do {
        printf("%d\n", i);
        i++;
    } while (i <= 5);

    return 0;
}

The loop:

• Runs first

• Then checks the condition

Even if i started at 10, it would still print once.

3. The for Loop

The for loop is commonly used when you know how many times you want to repeat something.

Syntax

for (initialization; condition; update) {
    // code
}

Example: Print 1 to 5

#include <stdio.h>

int main() {

    for (int i = 1; i <= 5; i++) {
        printf("%d\n", i);
    }

    return 0;
}

This does the same thing as the while loop, but in a shorter form.

Let’s break it down:

• int i = 1 → starting point

• i <= 5 → condition

• i++ → increase after each loop

Everything is in one line.

When Should You Use Each Loop?

• Use while when you don’t know exactly how many times it will run.

• Use do-while when the code must run at least once.

• Use for when you know the number of repetitions.

For beginners, the for loop is usually easier to manage.

Common Beginner Mistakes

1. Forgetting to update the counter (i++)

2. Writing wrong conditions

3. Creating infinite loops

4. Forgetting semicolon in do-while

Example of infinite loop:

while (i <= 5) {
    printf("%d\n", i);
}

Here, i never changes.
The program will run forever.

Mini Project: Print Even Numbers

Write a program that prints even numbers from 2 to 20.

Example solution:

#include <stdio.h>

int main() {

    for (int i = 2; i <= 20; i += 2) {
        printf("%d\n", i);
    }

    return 0;
}

Here, i += 2 increases by 2 each time.

Practice Exercises

1. Print numbers from 10 down to 1.

2. Ask the user to enter a number and print its multiplication table.

3. Use a loop to calculate the sum of numbers from 1 to 100.

Final Thoughts

With loops, your programs can now:

• Repeat tasks

• Count automatically

• Handle multiple inputs

• Perform repeated calculations

You now understand:

• Variables

• Conditions

• Input

• Loops

That is the foundation of C programming.

Next, we’ll combine everything to build small programs that feel real.