🔄 Lesson 6: Loops (Repetition)

⏱️ Estimated time: 25 minutes | Difficulty: Beginner

What Are Functions?

Functions are reusable blocks of code that perform a specific task. They help organize your program and avoid repeating code.

Function Syntax:

return_type function_name(parameters) {
    // function body
    return value;
}

Creating Functions

#include <stdio.h>

// Function that returns a value
int add(int a, int b) {
    return a + b;
}

// Function with no return value (void)
void greet(char name[]) {
    printf("Hello, %s!\n", name);
}

// Function with no parameters
void printLine() {
    printf("-------------------\n");
}

int main() {
    int result = add(5, 3);
    printf("5 + 3 = %d\n", result);  // 8

    greet("Alice");    // Hello, Alice!
    printLine();       // -------------------

    return 0;
}

Function Prototypes

If you define a function after main(), you need a prototype at the top:

#include <stdio.h>

// Prototype (declaration)
int multiply(int a, int b);

int main() {
    printf("%d\n", multiply(4, 5));  // 20
    return 0;
}

// Definition (after main)
int multiply(int a, int b) {
    return a * b;
}

Pass by Value vs Pass by Reference

// Pass by value (copy)
void doubleValue(int x) {
    x = x * 2;  // Only changes the local copy
}

// Pass by reference (pointer)
void doubleRef(int *x) {
    *x = *x * 2;  // Changes the original!
}

int main() {
    int num = 5;
    doubleValue(num);
    printf("%d\n", num);   // Still 5!

    doubleRef(&num);
    printf("%d\n", num);   // Now 10!
    return 0;
}

Recursion

int factorial(int n) {
    if (n <= 1) return 1;        // Base case
    return n * factorial(n - 1);  // Recursive call
}

printf("%d\n", factorial(5));  // 120