Functions in C

#include <stdio.h>

// Function declaration (prototype)
int add(int a, int b);
void swap(int *a, int *b);  // Pointers for pass-by-reference

// Function definition
int add(int a, int b) {
    return a + b;
}

// Pass by value (copy — original unchanged)
void doubleValue(int x) {
    x = x * 2;  // Only modifies local copy!
}

// Pass by reference (pointer — modifies original)
void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

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

// Function pointer
int multiply(int a, int b) { return a * b; }
int subtract(int a, int b) { return a - b; }

// Function that accepts a function pointer
int operate(int a, int b, int (*operation)(int, int)) {
    return operation(a, b);
}

int main() {
    // Basic function calls
    printf("3 + 4 = %d\n", add(3, 4));
    printf("5! = %d\n", factorial(5));

    // Pass by reference with swap
    int x = 10, y = 20;
    swap(&x, &y);
    printf("x=%d, y=%d\n", x, y);  // x=20, y=10

    // Function pointers
    int result = operate(10, 3, multiply);  // 30
    printf("10 * 3 = %d\n", result);
    result = operate(10, 3, subtract);      // 7
    printf("10 - 3 = %d\n", result);

    return 0;
}