Example to find the sum of natural numbers by using a recursive function.

To understand this example, you should have the knowledge of following C programming topics:

The positive numbers 1, 2, 3... are known as natural numbers. The program below takes a positive integer from the user and calculates the sum up to the given number.

You can find the sum of natural numbers using loop as well. However, you will learn to solve this problem using recursion here.

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

**Output**

Enter a positive integer: 20 Sum = 210

Suppose the user entered 20.

Initially, the `addNumbers()`

is called from the `main()`

function with 20 passed as an argument.

The number (20) is added to the result of `addNumbers(19)`

.

In the next function call from `addNumbers()`

to `addNumbers()`

, 19 is passed which is added to the result of `addNumbers(18)`

. This process continues until `n` is equal to 0.

When `n` is equal to 0, there is no recursive call and this returns the sum of integers to the `main()`

function.