# C Programming Tutorial

## Recursion in C

When function is called within the same function, it is known as recursion in C. The function which calls the same function, is known as recursive function.

A function that calls itself, and doesn't perform any task after function call, is know as tail recursion. In tail recursion, we generally call the same function with return statement. An example of tail recursion is given below.

```
recursionfunction()
{
recursionfunction();//calling self function
}

```

#### 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.

#### Ex: recursion in C

Let's see an example to print factorial number using tail recursion in C language.

```
#include<stdio.h>
int factorial (int n)
{
if ( n < 0)
return -1; /*Wrong value*/
if (n == 0)
return 1; /*Terminating condition*/
return (n * factorial (n -1));
}
int main(){
int fact=0;
fact=factorial(5);
printf("\n factorial of 5 is %d",fact);
return 0;
}

```

Output

factorial of 5 is 120

We can understand the above program of recursive method call by the figure given below.

#### 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.