# Interview Preparation topic: Recursive Function

**RECURSION**

Recursion is the process of repeating items in a self-similar way.

If a program allows you to call a function inside the same function, then it is called a recursive call of the function.

```
void recursion()
{
recursion(); /* function calls itself */
}
int main()
{
recursion();
}
```

While using recursion, programmers need to be careful to define an exit condition from the function, otherwise it will go into an infinite loop.

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.

Recursive functions are very useful to solve many mathematical problems, such as calculating sum of natural numbers, calculating the factorial of a number, generating Fibonacci series, etc.

**Example 1: SUM OF NATURAL NUMBERS**

The following example calculates the sum of natural numbers using a recursive function:

```
#include <stdio.h>
int sum(int n);
int main()
{
int result;
int number = 5;
result = sum(number);
printf("Sum of %d Natural Numbers Using Recursion is %d\n",number, result);
}
int sum(int num)
{
if (num != 0)
{
return num + sum(num - 1); // sum() function calls itself
}
else
{
return num;
}
}
```

When the above code is compiled and executed, it produces the following result:

Sum of 5 Natural Numbers Using Recursion is 15

Initially, the sum() is called from the main() function with number passed as an argument.

Suppose, the value of num is 5 initially. During next function call, 4 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.

**Example 2: FACTORIAL OF A NUMBER**

The following example calculates the factorial of a given number using a recursive function:

```
#include <stdio.h>
int factorial(unsigned int i);
int main()
{
int number = 5;
printf("Factorial of %d is %d\n", number, factorial(number));
return 0;
}
int factorial(unsigned int i)
{
if(i <= 1)
{
return 1;
}
return i * factorial(i - 1);
}
```

When the above code is compiled and executed, it produces the following result:

Factorial of 5 is 120

**Example 3: FIBONACCI SERIES**

The following example generates the Fibonacci series for a given number using a recursive function:

```
#include <stdio.h>
int fibonacci(int i);
int main()
{
int number=8;
for (i = 0; i < number-1; i++)
{
printf("%d\t\n", fibonacci(i));
}
return 0;
}
int fibonacci(int i)
{
if(i == 0)
{
return 0;
}
if(i == 1)
{
return 1;
}
return fibonacci(i-1) + fibonacci(i-2);
}
```

When the above code is compiled and executed, it produces the following result:

0 1 1 2 3 5 8

**Discussions**