Interview Preparation topic: Recursive Function

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.

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

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) 
   {
   if(i <= 1) 
      {
      return 1;
      }
   return i * factorial(i - 1);
   }

int  main() 
   {
   int i = 5;
   printf("Factorial of %d is %d\n", i, factorial(i));
   return 0;
   }

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

Factorial of 5 is 120

FIBONACCI SERIES

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

#include <stdio.h>
int fibonacci(int i) 
   {
   if(i == 0) 
      {
      return 0;
      }
	
   if(i == 1) 
      {
      return 1;
      }
   return fibonacci(i-1) + fibonacci(i-2);
   }

int  main() 
   {
   int i;
	
   for (i = 0; i < 7; i++) 
      {
      printf("%d\t\n", fibonacci(i));
      }
	
   return 0;
   }

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

0	1	1	2	3	5	8	

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 number, result;
   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

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