C Programming Lesson - Recursion in C language

We have learnt different types function C language and now I am going to explain recursive function in C. A function is called “recursive” if a statement within body of that function calls the same function for example look at below code:

void main()
{
printf(“recursive function called.\n”);
main();
}

When you will run this program it will print message “recursive function called.” indefinitely. If you are using Turbo C/C++ compiler then you need to press Ctrl + Break key to break this in definite loop.

Before we move to another example lets have attributes of “recursive function”:-

1. A recursive function is a function which calls itself.
2. The speed of a recursive program is slower because of stack overheads. (This attribute is evident if you run above C program.)
3. A recursive function must have recursive conditions, terminating conditions, and recursive expressions.

To understand how recursion works lets have another popular example of recursion. In this example we will calculate the factorial of n numbers. The factorial of n numbers is expressed as a series of repetitive multiplication as shown below:

Factorial of n = n(n-1)(n-2)……1.

Example :

          Factiorial of 5 = 5x4x3x2x1
                                 =120

So from line no. 6 – 14 is a user defined recursive function “factorial” that calculates factorial of any given number. This function accepts integer type argument/parameter and return integer value. If you have any problem to understand how function works then you can check my tutorials on C function (click here).

In line no. 9 we are checking that whether value of i is equal to 1 or not; i is an integer variable which contains value passed from main function i.e. value of integer variable x. If user enters 1 then the factorial of 1 will be 1. If user enters any value greater than 1 like 5 then it will execute statement in line no. 12 to calculate factorial of 5. This line is extremely important because in this line we implemented recursion logic.

Let’s see how line no. 12 exactly works. Suppose value of i=5, since i is not equal to 1, the statement:

f = i* factorial (i-1);

will be executed with i=5 i.e.

f = 5* factorial (5-1);

will be evaluated. As you can see this statement again calls factorial function with value i-1 which will return value:

4*factorial(4-1);

This recursive calling continues until value of i is equal to 1 and when i is equal to 1 it returns 1 and execution of this function stops. We can review the series of recursive call as follow:

f = 5* factorial (5-1);

f = 5*4* factorial (4-1);

f = 5*4*3* factorial (3-1);

f = 5*4*3*2* factorial (2-1);

f = 5*4*3*2*1;

f = 120;

I hope this will clear any confusion regarding recursive function.