## Introduction

In this program, you will learn to calculate the factorial of a number.

A particular value will be entered by the user, and then through this program, we will compute its factorial.

For example, **Factorial of 5** is (**!** is the symbol for factorial):

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

Similarly, **Factorial of 7 **is

7! =7*6*5*4*3*2*1=5040

Factorial of negative numbers doesn’t exist.

Factorial of 0 and 1 is 1.

What is the use of this program?

It is easy to calculate the factorial up to certain values, but it is difficult to calculate factorial like **100! **,** 500!** etc. A small code written in C will make this task quite easy.

To understand this example, you should know about the following C programming topics:

1. C data types

2. Operators in C

3. if-else statement

4. for loop

## Program

#include<stdio.h> int main() { int x; printf("Enter an integer:\n"); scanf("%d",&x); int fact=1; if(x>0) // x is a positive integer { for (int i=1; i<=x; i++) { fact=fact*i; // fact is updated when value of i is multiplied in it } printf("Factorial of %d is %d",x,fact); } else if(x==0) // x is 0 { printf("Factorial of %d is 1",x); } else // x is a negative integer { printf("Factorial does not exist"); } return 0; }

## Output

## Explanation

In the above program, we have to calculate the factorial of ‘x’.

Now we have three conditions, either ‘x’ is a positive integer or ‘x’ is 0 or ‘x’ is a negative integer. if-else statements are used to check all these conditions.

If the value of ‘x’ is greater than 0 then the ‘if’ block will be executed.

**for (int i=1; i<=x; i++)**

‘i’ will start from 1 and iterate until it reaches ‘x’.

‘fact’ variable is initialized with 1. When the value of ‘i’ is 1, the statement

** fact=fact*i;** will return 1*1=1

Thus, fact=1

‘i’ is incremented and now its value is 2

**fact=fact*i;** will return 1*2=2

Thus, fact=2

i’ is incremented and now its value is 3

**fact=fact*i;** will return 2*3=6

Thus, fact=6

In the end when the value of ‘i’ reaches 10 then

**fact=fact*i;** will return 362880*10=3628800

thus fact=3628800

now ‘i’ is updated to 11 which will terminate the loop.

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