A function that calls itself is known as a recursive function. And, this technique is known as recursion.

### How recursion works?

void recurse()
{
... .. ...
recurse();
... .. ...
}
int main()
{
... .. ...
recurse();
... .. ...
}

The recursion continues until some condition is met to prevent it.

To prevent infinite recursion, if...else statement (or similar approach) can be used where one branch makes the recursive call and other doesn't.

### Example: Sum of Natural Numbers Using Recursion

```
#include <stdio.h>
int sum(int n);
int main()
{
int number, result;
printf("Enter a positive integer: ");
scanf("%d", &number);
result = sum(number);
printf("sum=%d", result);
}
int sum(int num)
{
if (num!=0)
return num + sum(num-1); // sum() function calls itself
else
return num;
}
```

**Output**

`Enter a positive integer:
3
6`

Initially, the `sum()`

is called from the `main()`

function with `number` passed as an argument.

Suppose, the value of `num` is 3 initially. During next function call, 2 is passed to the `sum()`

function. This process continues until `num` is equal to 0.

When `num` is equal to 0, the if condition fails and the else part is executed returning the sum of integers to the `main()`

function.

### Advantages and Disadvantages of Recursion

Recursion makes program elegant and cleaner. All algorithms can be defined recursively which makes it easier to visualize and prove.

If the speed of the program is vital then, you should avoid using recursion. Recursions use more memory and are generally slow. Instead, you can use loop.