C Recursion

Recursion is when a function calls itself. Every recursive function needs a base case, which stops the calls. Without it the function calls itself forever, and the program crashes.

Factorial

Factorial of 5 is 5 x 4 x 3 x 2 x 1 = 120. We can say n! = n x (n-1)! and 0! = 1.

#include <stdio.h>

int factorial(int n) {
    if (n == 0) {              // base case
        return 1;
    }
    return n * factorial(n - 1);
}

int main(void) {
    printf("5! = %d\n", factorial(5));
    return 0;
}

Output:

The calls go like this: factorial(5) waits for factorial(4), which waits for factorial(3), and so on until factorial(0) returns 1. Then the answers come back in reverse order.

Fibonacci series

#include <stdio.h>

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

int main(void) {
    for (int i = 0; i < 8; i++) {
        printf("%d ", fib(i));
    }
    printf("\n");
    return 0;
}

Output:

Sum of digits

#include <stdio.h>

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

int main(void) {
    printf("%d\n", sumDigits(1234));
    return 0;
}

Output:

Recursion vs loop

• Recursion code is short and clean for problems like trees, but it uses more memory (stack).
• A loop is faster and safe for simple counting.
• The simple Fibonacci above repeats the same work many times, so it is slow for big numbers.

Next: C Scope and Storage Classes.

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