Before going to the program first let us see what is Factorial of a Number? and what is Recursion?

Factorial of a Number:

                  The factorial of a Number n, denoted by n! , is the product of all positive integers less than or equal to n.

The value of 0! is 1 according to the convention for an empty product.

For example,

5! = 5 * 4 * 3 * 2 * 1 = 120

Recursion:

                In C programming language, if a function calls itself over and over again then that function is known as Recursive Function.

The process of function calling itself repeatedly is known as Recursion.

Related: Factorial of a Number in C without using Recursion

Program code for Factorial of a Number using Recursion:

#include<stdio.h>
#include<conio.h>

void main()
{
    int num,f;
    clrscr();
    
    printf("\n  Enter the number: ");
    scanf("%d",&num);
    
    f=fact(num);
    printf("\n  The factorial of the number %d is %d",num,f);
    getch();
}

int fact(int n)
{
    if(n==0 || n==1)
        return 1;
    else
        return(n * fact(n-1));
}

Related: Factorial of a Number in C++ using Recursion

Working:

• First the computer reads the number to find the factorial of the number from the user.
• Then using recursive function the factorial value is calculated and returns the factorial value to main function.
• Finally the factorial value of the given number is printed.

Step by Step working of the above Program Code:

Let us assume that the number entered by the user is 4.

1. It assigns the value of n=4.
2. Then using a variable f the recursive function fact(n) is called.

2.1.   n==0 || n==1    (4==0 || 4==1)   if condition is false.

So it goes to the else part.

  else

return(n * fact(n-1))      So, (return(4 * fact(3)))      // function calls itself

2.2.  n==0 || n==1    (3==0 || 3==1)   if condition is false.

So it goes to the else part.

  else

return(n * fact(n-1))      So, (return(3 * fact(2)))      // function calls itself

2.3.  n==0 || n==1    (2==0 || 2==1)   if condition is false.

So it goes to the else part.

  else

return(n * fact(n-1))      So, (return(2 * fact(1)))      // function calls itself

2.4.  n==0 || n==1    (1==0 || 1==1)   if condition is true.

So it return the value 1 to fact(1) in step 2.3.

which further return the value 2 to fact(2) in step 2.2.

which further return the value 6 to fact(3) in step 2.1.

which further return the value 24 to f in main function.

3. Finally it prints as given below

The Factorial of the number 4 is 24

4. Thus the program execution is completed.

Output:

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