Functions in C

#include <stdio.h>

// Function declaration (prototype)
return_type function_name(parameter_list);

// Function definition
return_type function_name(parameter_list) {
    // function body
    return value;
}

// Example
int add(int a, int b) {      // Function header
    int sum = a + b;          // Function body
    return sum;               // Return statement
}

#include <stdio.h>

// Function declaration (prototype)
int add(int a, int b);
int subtract(int a, int b);
void printMessage(void);

int main() {
    int x = 10, y = 5;
    
    // Function calls
    int sum = add(x, y);
    int diff = subtract(x, y);
    
    printf("Sum: %d\n", sum);
    printf("Difference: %d\n", diff);
    
    printMessage();
    
    return 0;
}

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

int subtract(int a, int b) {
    return a - b;
}

void printMessage(void) {
    printf("Hello from function!\n");
}

void greet(void) {
    printf("Hello!\n");
}

// Call: greet();

void greetPerson(char name[]) {
    printf("Hello, %s!\n", name);
}

// Call: greetPerson("Alice");

int getNumber(void) {
    return 42;
}

// Call: int num = getNumber();

int multiply(int a, int b) {
    return a * b;
}

// Call: int result = multiply(5, 3);

#include <stdio.h>
#include <math.h>

int main() {
    double num = 16.0;
    
    // Square root
    printf("sqrt(%.0f) = %.2f\n", num, sqrt(num));
    
    // Power
    printf("pow(2, 3) = %.2f\n", pow(2, 3));
    
    // Absolute value
    printf("abs(-10) = %d\n", abs(-10));
    printf("fabs(-10.5) = %.2f\n", fabs(-10.5));
    
    // Rounding
    printf("ceil(3.2) = %.2f\n", ceil(3.2));
    printf("floor(3.8) = %.2f\n", floor(3.8));
    printf("round(3.5) = %.2f\n", round(3.5));
    
    // Trigonometric
    double angle = 45.0 * M_PI / 180.0;  // Convert to radians
    printf("sin(45°) = %.4f\n", sin(angle));
    printf("cos(45°) = %.4f\n", cos(angle));
    
    // Logarithm
    printf("log(10) = %.4f\n", log(10));
    printf("log10(100) = %.2f\n", log10(100));
    
    return 0;
}

#include <stdio.h>

// Factorial: n! = n * (n-1)!
// Base case: 0! = 1, 1! = 1
int factorial(int n) {
    if (n <= 1)
        return 1;  // Base case
    return n * factorial(n - 1);  // Recursive case
}

// Fibonacci: F(n) = F(n-1) + F(n-2)
// Base case: F(0) = 0, F(1) = 1
int fibonacci(int n) {
    if (n <= 0)
        return 0;
    if (n == 1)
        return 1;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

// Sum of digits
int sumOfDigits(int n) {
    if (n == 0)
        return 0;
    return (n % 10) + sumOfDigits(n / 10);
}

// Power: x^n
int power(int base, int exp) {
    if (exp == 0)
        return 1;
    return base * power(base, exp - 1);
}

int main() {
    printf("Factorial of 5: %d\n", factorial(5));
    
    printf("Fibonacci sequence: ");
    for (int i = 0; i < 10; i++)
        printf("%d ", fibonacci(i));
    printf("\n");
    
    printf("Sum of digits of 12345: %d\n", sumOfDigits(12345));
    printf("2^8 = %d\n", power(2, 8));
    
    return 0;
}

#include <stdio.h>

// Global variable
int globalVar = 10;

void demoLocal() {
    int localVar = 20;  // Local variable
    printf("Local: %d\n", localVar);
    printf("Global: %d\n", globalVar);
}

void demoStatic() {
    static int count = 0;  // Retains value between calls
    count++;
    printf("Static count: %d\n", count);
}

int main() {
    printf("Global in main: %d\n", globalVar);
    
    demoLocal();
    
    // Static demonstration
    demoStatic();  // Prints 1
    demoStatic();  // Prints 2
    demoStatic();  // Prints 3
    
    return 0;
}

#include <stdio.h>

// Function prototypes
void showMenu(void);
int add(int a, int b);
int subtract(int a, int b);
int multiply(int a, int b);
float divide(int a, int b);

int main() {
    int choice, num1, num2;
    
    do {
        showMenu();
        printf("Enter choice: ");
        scanf("%d", &choice);
        
        if (choice >= 1 && choice <= 4) {
            printf("Enter two numbers: ");
            scanf("%d %d", &num1, &num2);
        }
        
        switch(choice) {
            case 1:
                printf("Result: %d\n", add(num1, num2));
                break;
            case 2:
                printf("Result: %d\n", subtract(num1, num2));
                break;
            case 3:
                printf("Result: %d\n", multiply(num1, num2));
                break;
            case 4:
                if (num2 != 0)
                    printf("Result: %.2f\n", divide(num1, num2));
                else
                    printf("Error: Division by zero!\n");
                break;
            case 5:
                printf("Goodbye!\n");
                break;
            default:
                printf("Invalid choice!\n");
        }
        printf("\n");
    } while (choice != 5);
    
    return 0;
}

void showMenu(void) {
    printf("===== CALCULATOR =====\n");
    printf("1. Add\n");
    printf("2. Subtract\n");
    printf("3. Multiply\n");
    printf("4. Divide\n");
    printf("5. Exit\n");
}

int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }
int multiply(int a, int b) { return a * b; }
float divide(int a, int b) { return (float)a / b; }