Loops

Imagine you needed to print “Hello!” five times. You could write: printf(“Hello!\n”); printf(“Hello!\n”); printf(“Hello!\n”); printf(“Hello!\n”); printf(“Hello!\n”);

This is tedious and not scalable. What if you needed to do it 500 times? This is where LOOPS come in. A LOOP is a programming structure that repeats a sequence of instructions until a specific condition is met. Each single execution of the loop’s body is called an ITERATION.

C has three main types of loops:

1. The for loop
2. The while loop
3. The do-while loop

The for loop is ideal when you know exactly how many times you want to repeat a block of code. It has a very specific structure with three parts inside its parentheses, separated by semicolons.

for (INITIALIZATION; CONDITION; POST-ITERATION) { // Code to be repeated }

1. INITIALIZATION: A statement that runs only ONCE before the loop starts. It’s typically used to declare and initialize a counter variable.
2. CONDITION: An expression that is checked BEFORE each iteration. If the condition is true, the loop body runs. If it’s false, the loop terminates.
3. POST-ITERATION: A statement that runs AFTER each iteration. It’s typically used to increment or decrement the counter variable. i++ is shorthand for i = i + 1.

The while loop is simpler. It repeats a block of code AS LONG AS its condition remains true. It’s useful when you don’t know the exact number of iterations beforehand.

while (condition) { // Code to be repeated }

The condition is checked BEFORE each iteration.

CRITICAL: You must ensure that something inside the loop’s body will eventually make the condition false. Otherwise, you will create an INFINITE LOOP, and your program will never end!

The do-while loop is a variation of the while loop. The key difference is that the condition is checked AT THE END of the loop’s body.

This means a do-while loop is GUARANTEED to execute at least once.

This makes it perfect for situations like input validation, where you need to get the input first before you can check if it’s valid.

Full Source

/**
 * @file 6_loops.c
 * @brief Part 1, Lesson 6: Loops
 * @author dunamismax
 * @date 06-15-2025
 *
 * This lesson introduces loops, which are control structures that allow
 * a block of code to be executed repeatedly.
 */

/*
 * =====================================================================================
 * |                                   - LESSON START -                                  |
 * =====================================================================================
 *
 * Imagine you needed to print "Hello!" five times. You could write:
 * printf("Hello!\n");
 * printf("Hello!\n");
 * printf("Hello!\n");
 * printf("Hello!\n");
 * printf("Hello!\n");
 *
 * This is tedious and not scalable. What if you needed to do it 500 times?
 * This is where LOOPS come in. A LOOP is a programming structure that repeats a
 * sequence of instructions until a specific condition is met. Each single
 * execution of the loop's body is called an ITERATION.
 *
 * C has three main types of loops:
 * 1. The `for` loop
 * 2. The `while` loop
 * 3. The `do-while` loop
 */

#include <stdio.h>

int main(void)
{
    // --- Part 1: The `for` Loop ---

    /*
     * The `for` loop is ideal when you know exactly how many times you want to
     * repeat a block of code. It has a very specific structure with three parts
     * inside its parentheses, separated by semicolons.
     *
     * for (INITIALIZATION; CONDITION; POST-ITERATION) {
     *     // Code to be repeated
     * }
     *
     * 1. INITIALIZATION: A statement that runs only ONCE before the loop starts.
     *    It's typically used to declare and initialize a counter variable.
     * 2. CONDITION: An expression that is checked BEFORE each iteration. If the
     *    condition is true, the loop body runs. If it's false, the loop terminates.
     * 3. POST-ITERATION: A statement that runs AFTER each iteration. It's
     *    typically used to increment or decrement the counter variable. `i++` is
     *    shorthand for `i = i + 1`.
     */
    printf("--- Part 1: The 'for' Loop (Countdown) ---\n");

    // Let's create a countdown from 5 down to 1.
    // Initialization: `int i = 5`
    // Condition: `i > 0` (loop as long as i is greater than 0)
    // Post-iteration: `i--` (decrement i by 1 after each loop; shorthand for i = i - 1)
    for (int i = 5; i > 0; i--)
    {
        printf("%d...\n", i);
    }
    printf("Blast off!\n\n");

    // --- Part 2: The `while` Loop ---

    /*
     * The `while` loop is simpler. It repeats a block of code AS LONG AS its
     * condition remains true. It's useful when you don't know the exact number
     * of iterations beforehand.
     *
     * while (condition) {
     *     // Code to be repeated
     * }
     *
     * The condition is checked BEFORE each iteration.
     *
     * CRITICAL: You must ensure that something inside the loop's body will
     * eventually make the condition false. Otherwise, you will create an
     * INFINITE LOOP, and your program will never end!
     */
    printf("--- Part 2: The 'while' Loop (Simple Menu) ---\n");

    int menu_choice = 0;

    // Loop as long as the user has not chosen option 4 to quit.
    while (menu_choice != 4)
    {
        printf("Menu:\n");
        printf("1. Start New Game\n");
        printf("2. Load Game\n");
        printf("3. Options\n");
        printf("4. Quit\n");
        printf("Enter your choice: ");

        scanf("%d", &menu_choice); // Get user input. This updates the loop variable.

        printf("You chose: %d\n\n", menu_choice);
    }
    printf("Thanks for playing!\n\n");

    // --- Part 3: The `do-while` Loop ---

    /*
     * The `do-while` loop is a variation of the `while` loop. The key difference
     * is that the condition is checked AT THE END of the loop's body.
     *
     * This means a `do-while` loop is GUARANTEED to execute at least once.
     *
     * This makes it perfect for situations like input validation, where you need
     * to get the input first *before* you can check if it's valid.
     */
    printf("--- Part 3: The 'do-while' Loop (Input Validation) ---\n");
    int secret_number;

    do
    {
        printf("Please enter a number between 1 and 10: ");
        scanf("%d", &secret_number);

        // This check happens AFTER the user has already entered a number.
        if (secret_number < 1 || secret_number > 10)
        {
            printf("Invalid input. Please try again.\n");
        }

    } while (secret_number < 1 || secret_number > 10); // The condition is checked here.

    printf("You successfully entered the number %d.\n", secret_number);

    return 0;
}

/*
 * =====================================================================================
 * |                                    - LESSON END -                                   |
 * =====================================================================================
 *
 * In this lesson, you learned how to make your program perform repetitive tasks.
 *
 * Key Takeaways:
 * - Loops are used to execute a block of code multiple times.
 * - The `for` loop is great for when you know the number of iterations in advance.
 *   It consists of an initializer, a condition, and a post-iteration step.
 * - The `while` loop is great for when you want to loop as long as a condition is
 *   true, and the number of iterations isn't fixed.
 * - The `do-while` loop is similar to `while`, but it always executes its body at
 *   least once because the condition is checked at the end.
 *
 * Mastering loops is a massive step towards writing complex and useful programs.
 *
 * HOW TO COMPILE AND RUN THIS CODE:
 *
 * 1. Open a terminal or command prompt.
 * 2. Navigate to the directory where you saved this file.
 * 3. Use the GCC compiler to create an executable file:
 *    `gcc -Wall -Wextra -std=c11 -o 6_loops 6_loops.c`
 * 4. Run the executable:
 *    - On Linux/macOS:   `./6_loops`
 *    - On Windows:       `6_loops.exe`
 */

How to Compile and Run

cc -Wall -Wextra -std=c11 -o 6_loops 6_loops.c
./6_loops