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# Python Program to Find Factorial of a Number

In this article, I've created some programs in Python, that find and prints factorial of a given number by user at run-time. Here are the list of approaches used:

- Find Factorial of a Number using
**while**Loop - Using
**for**Loop - Using user-defined Function
- Using Recursion
- Using Class

Before creating these programs, let's remind about factorial formula, that is:

n! = (n)*(n-1)*(n-2)*...*3*2*1

Here **n** indicates a number of which, the factorial is going to find. The **!** (exclamation) indicates
factorial. So **n!** can be called as **n factorial**. For example, **5!** can be calculated as:

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

## Find Factorial using while Loop

To find factorial of any number in Python, you have to ask from user to enter the number, then find and print its factorial as per the formula given above, like shown in the program given below.

The question is, **write a Python program to find factorial of a given number using while loop**. Here is its answer:

print("Enter the Number: ") num = int(input()) fact = 1 i = 1 while i<=num: fact = fact*i i = i+1 print("\nFactorial =", fact)

Here is the initial output produced by this Python program:

Now supply the input say **5** as number and press `ENTER`

key to find its factorial like shown in the snapshot given below:

The dry run of above program with user input **5** goes like:

- Initial values,
**num=5**(entered by user),**fact=1**,**i=1** - Now the execution of
**while**loop begins - That is, the condition
**i<=num**or**1<=5**evaluates to be true, therefore program flow goes inside the loop (inside its body) - Inside the loop,
**fact*i**or**1*1**or**1**gets initialized to**fact**. And the value of**i**gets incremented by 1. So**i=2** - Again the condition
**i<=num**or**2<=5**evaluates to be true, therefore again**fact*i**or**1*2**or**2**gets initialized to**fact**. So**fact=2** - And
**i+1**or**2+1**or**3**gets initialized to**i**. So**i=3** - Again the condition,
**i<=num**or**3<=5**evaluates to be true, therefore program flow again goes inside the loop and executes its two statements. This process continues until the condition evaluates to be false - In this way, after exiting from the loop, the variable
**fact**holds the factorial value of a number stored in**num**variable - So just print the value of
**fact**as factorial of a given number

## Find Factorial using for Loop

This program does the same job as of previous program, but using **for** loop, instead of **while**. The **try-except** is
used in this program to handle with invalid inputs entered by user. And **end** is used to skip inserting an automatic newline.

print("Enter a Number: ", end="") try: num = int(input()) fact = 1 for i in range(1, num+1): fact = fact*i print("\nFactorial of", num, "=", fact) except ValueError: print("\nInvalid Input!")

Here is its sample run with same user input as of previous program:

In above program, the following code:

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

is applied to execute the following statement:

fact = fact*i

**num** number of times with value of **i** from 1 to **num**'s value.

## Find Factorial using Function

This program is created using a user-defined function named **findFact()**. This function receives a value as its
argument, then find and returns its factorial.

def findFact(n): f = 1 for i in range(1, n+1): f = f*i return f print("Enter a Number: ", end="") try: num = int(input()) fact = findFact(num) print("\nFactorial of", num, "=", fact) except ValueError: print("\nInvalid Input!")

This program produces same output as of previous program:

## Find Factorial using Recursion

The following program is created using a recursive function named **findFact()**. This function calls itself until
the value of its argument becomes equal to **1**.

def findFact(n): if n==1: return n else: return n*findFact(n-1) print("Enter a Number: ", end="") try: num = int(input()) fact = findFact(num) print("\nFactorial of", num, "=", fact) except ValueError: print("\nInvalid Input!")

## Find Factorial using Class

This is the last program to find and print factorial of a number entered by user, created using a class named **CodesCracker**. An
object **ob** is created of this class to access its member function (findFact()) using **dot (.)** operator:

class CodesCracker: def findFact(self, n): f = 1 for i in range(1, n + 1): f = f * i return f print("Enter a Number: ", end="") num = int(input()) ob = CodesCracker() print("\nFactorial of", num, "=", ob.findFact(num))

#### Same Program in Other Languages

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