# Recursion

Recursion occurs when a function contains within it a call to itself. Recursion can result in very neat, elegant code that is intuitive to follow. It can also result in a very large amount of memory being used if the recursion gets too deep.

Common examples of where recursion is used :

• Walking recursive data structures such as linked lists, binary trees, etc.
• Exploring possible scenarios in games such as chess

Recursion always consists of two main parts. A terminating case that indicates when the recursion will finish and a call to itself that must make progress towards the terminating case.

For example, this function will perform multiplication by recursively adding :

``````#include <stdio.h>

unsigned int multiply(unsigned int x, unsigned int y)
{
if (x == 1)
{
/* Terminating case */
return y;
}
else if (x > 1)
{
/* Recursive step */
return y + multiply(x-1, y);
}

/* Catch scenario when x is zero */
return 0;
}

int main() {
printf("3 times 5 is %d", multiply(3, 5));
return 0;
}
``````

## Exercise

Define a new function called `factorial()` that will compute the factorial by recursive multiplication (5! = 5 x 4 x 3 x 2 x 1). Note that by convention, the factorial of 0 is equal to 1 (0! = 1).