AI Coding & CSE Fundamentals

Print numbers from 1 to N using recursion

#include <stdio.h>

void printNumbers(int n) {
  if (n > 0) {
    printNumbers(n - 1);
    printf("%d ", n);
  }
}

int main() {
  int num;
  printf("Enter a positive integer: ");
  scanf("%d", &num);
  printNumbers(num);
  printf("\n");
  return 0;
}

Power of a Number Using Recursion

#include <stdio.h>

int power(int n, int k) {
  if (k == 0) {
    return 1;
  }
  return n * power(n, k - 1);
}

int main() {
  int base, exponent;
  printf("Enter base: ");
  scanf("%d", &base);
  printf("Enter exponent: ");
  scanf("%d", &exponent);
  printf("%d^%d = %d\n", base, exponent, power(base, exponent));
  return 0;
}

Reverse a Number Using Recursion

#include <stdio.h>

int reverse_num(int num, int reversed_num) {
    if (num == 0) {
        return reversed_num;
    }
    int remainder = num % 10;
    reversed_num = reversed_num * 10 + remainder;
    return reverse_num(num / 10, reversed_num);
}

int main() {
    int num;
    printf("Enter a number: ");
    scanf("%d", &num);
    int reversed = reverse_num(num, 0);
    printf("Reversed number: %d\n", reversed);
    return 0;
}

Sum of Digits using Recursion

#include <stdio.h>

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

int main() {
  int num;
  printf("Enter a number: ");
  scanf("%d", &num);
  int result = sumOfDigits(num);
  printf("Sum of digits: %d\n", result);
  return 0;
}

Sum of Natural Numbers (1 to N) using Recursion

#include <stdio.h>

int sum_recursive(int n) {
  if (n == 0) {
    return 0;
  }
  return n + sum_recursive(n - 1);
}

int main() {
  int num;
  printf("Enter a positive integer: ");
  scanf("%d", &num);
  if (num < 0) {
    printf("Please enter a non-negative number.\n");
    return 1;
  }
  int sum = sum_recursive(num);
  printf("Sum of natural numbers from 1 to %d is: %d\n", num, sum);
  return 0;
}

Fibonacci nth term using recursion

#include <stdio.h>

int fibonacci(int n) {
 if (n <= 1) {
 return n;
 }
 return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
 int n = 10;
 printf("Fibonacci(%d) = %d\n", n, fibonacci(n));
 return 0;
}

Factorial of a number using recursion

#include <stdio.h>

int factorial(int n) {
  if (n == 0) {
    return 1;
  } else {
    return n * factorial(n - 1);
  }
}

int main() {
  int num = 5;
  int result = factorial(num);
  printf("Factorial of %d = %d\n", num, result);
  return 0;
}

Square of a Number Using a Function

#include <stdio.h>

int square(int n) {
  return n * n;
}

int main() {
  int num = 5;
  int result = square(num);
  printf("Square of %d is %d\n", num, result);
  return 0;
}

Functions: Message Printing with a Helper Function

#include <stdio.h>

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

int main() {
  printMessage();
  return 0;
}

Even or Odd Checker Function

#include <stdio.h>

int isEven(int num) {
    if (num % 2 == 0) {
        return 1; 
    } else {
        return 0; 
    }
}

int main() {
    int number;
    printf("Enter an integer: ");
    scanf("%d", &number);
    
    if (isEven(number)) {
        printf("%d is even.\n", number);
    } else {
        printf("%d is odd.\n", number);
    }

    return 0;
}

Maximum of Two Numbers Using a Function

#include <stdio.h>

int findMax(int num1, int num2) {
 if (num1 > num2) {
 return num1;
 }
 return num2;
}

int main() {
 int a, b, max;
 scanf("%d %d", &a, &b);
 max = findMax(a, b);
 printf("%d", max);
 return 0;
}

Add Two Numbers with a Function

#include <stdio.h>

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

int main() {
 int num1 = 10;
 int num2 = 5;
 int sum = add(num1, num2);
 printf("Sum: %d\n", sum);
 return 0;
}

Get Current Year with a Function

#include <stdio.h>
#include <time.h>

int getCurrentYear() {
    time_t timer;
    struct tm* tm_info;
    time(&timer);
    tm_info = localtime(&timer);
    return tm_info->tm_year + 1900;
}

int main() {
    int year = getCurrentYear();
    printf("Current year: %d\n", year);
    return 0;
}

Functions: Returning a Fixed Value

#include <stdio.h>

int get_value() {
 return 100;
}

int main() {
 int result = get_value();
 printf("%d\n", result);
 return 0;
}

Positive or Negative Number Checker (Void Function)

#include <stdio.h>

void checkNumber(int num) {
  if (num > 0) {
    printf("Positive\n");
  } else if (num < 0) {
    printf("Negative\n");
  } else {
    printf("Zero\n");
  }
}

int main() {
  int number;
  scanf("%d", &number);
  checkNumber(number);
  return 0;
}

Print Name and Age using a Void Function

#include <stdio.h>

void printInfo(char name[], int age) {
    printf("Name: %s\n", name);
    printf("Age: %d\n", age);
}

int main() {
    char myName[] = "Alice";
    int myAge = 30;
    printInfo(myName, myAge);
    return 0;
}

Welcome Message with a Function

#include <stdio.h>

void welcomeMessage() {
    printf("Welcome to C programming!\n");
}

int main() {
    welcomeMessage();
    return 0;
}

Hello World with a Void Function

#include <stdio.h>

void printHelloWorld() {
 printf("Hello, World!\n");
}

int main() {
 printHelloWorld();
 return 0;
}

.

: `else { return n * factorial(n - 1); }`: If `n` is not 0, the function returns `n` multiplied by the factorial of `n-1`. This is where the function calls itself. Let's trace `factorial(3)`: 1. `factorial(3)` returns `3 * factorial(2)` 2. `factorial(2)` returns `2 * factorial(1)` 3. `factorial(1)` returns `1 * factorial(0)` 4. `factorial(0)` returns `1` (base case) So, the final result is `3 * 2 * 1 * 1 = 6`. **💡 Tips for Using Functions & Recursion:** - ✅ Keep functions small and focused on a single task. - ✅ Give functions descriptive names. - ✅ Comment your code to explain what each function does. - ⚠️ Be careful with recursion! Always have a base case to avoid infinite loops. - 🧠 Recursion can be elegant for some problems (like traversing trees), but iterative solutions (using loops) are often more efficient. Functions and recursion are essential tools for writing clean, organized, and reusable C code. Practice using them, and you'll become a much more proficient programmer! 👍