In this tutorial, we are going to learn a python program to find the factorial of the number given as input from the user.

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## Problem Statement

For any numbers that are input by the user, we have to calculate the factorial of that numbers.

**For example:**

**Case1:** If the user input the 5.

then the output should be 120 ( 1*2*3*4*5= 120).

**Case2:** If the user input the 6.

then the output should be 720 (1*2*3*4*5*6 =720).

## Factorial of a number

The factorial of a number is the multiplication of each and every number till the given number except 0.

**For example**:

**Case1:** factorial of number 4 is the multiplication of every number till number 4, i.e. 1*2*3*4

**Case 2**: factorial of number 3 is the multiplication of every number till number 3, i.e. 1*2*3

**NOTE:**

*The factorial of 0 is 1**The factorial of any negative number is not defined.**The factorial only exits for whole numbers.*

Let’s discuss some more about functions that we are going to use in our program.

## Python functions used in our program

### Range() function

It returns the sequence of a given number between a given range. Range() function is a Python built-in function that is used when the user needs to perform an action-specific number of times. It is commonly used with looping statements.

It takes 3 arguments

**1. start: **it is theinteger starting from which the series of integers is to be returned.

**2.** **stop: **it is the integer earlier than which the collection of integers is to be lower back.

**3.** **step: **it is the integer that determines the increment among each integer within the collection.

### ‘if –else’ conditional statement

‘if-else’ conditional statement follows the syntax as follow:

### ‘for’ loop Statement

The ‘for loop’ is a looping statement in python which is used to iterate over a sequence or any other iterable objects such as lists, tuples, dictionaries, strings, etc.

**Flow Chart for ‘for loop’**

- There can be any number of ‘elif’ block but there must be a condition assigned for each and every ‘elif’ statement.
- There must be a ‘if’ statement with a condition assigned and there must not be more than 1 ‘if’ statement.
- ‘else’ statement is optional.

## Our logic to calculate the factorial

- Our program will take an integer input from the user which should be a whole number.
- If the number input is negative then print ‘The factorial can’t be calculated’.
- Then use
**for**loop with**range**function to define the sequence of numbers which we will use to calculate the factorial. - After that, we will store the value of the factorial in a variable and finally print the value of the variable.

## Algorithm to calculate the factorial

Step 1: Start

Step 2: take input from the user for finding the factorial.

Step 3: Create a variable ‘factorial’ and assign the value 1.

Step 4: if(number<0):

print ‘cannot be calculated.

elif ( number == 1):

print 1

else:

for i in range(1, number+1):

factorial*=i

Step 5: print factorial

Step 6: Stop

## Python Code to calculate the factorial

**Output 1:**

**Explanation**:

The input number from the user to find the factorial is 5. The calculated value of the factorial is 1*2*3*4*5.

**Output 2:**

**Explanation**:

The input number from the user to find the factorial is 8. The calculated value of the factorial is 1*2*3*4*5*6*7*8.