Python – Factorial of a Number

Python Program to Find Factorial of a Number

Factorial of a Number can be calculated in many ways. Some of them are by using a for loop, or using a recursion function or a while loop.

In the following Python Factorial Examples, we will find factorial of a given whole number, using the above said procedures.

Problem Statement – Factorial of a Number

Factorial of a number n is given by,

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

For example,

0! = 1
1! = 1
2! = 2*1 = 2
3! = 3*2*1 = 6
4! = 4*3*2*1 = 24

Run this program

You have to read n from user through console input. And print the answer to the console.

Example 1: Factorial using For Loop and Range

In the following example, we shall use Python For Loop to find Factorial. For a range(1,n+1) of given n, multiply the element over each iteration and return the result after coming out of the loop.

Python Program

def factorial(n):
	result = 1
	for i in range(1,n+1):
		result = result*i
	return result

n = int(input('Enter a number: '))
result = factorial(n)
print(n,'! = ',result,sep="")

We have not validated if the number is negative. You may write the validation to check if the number is not negative and then proceed with finding the factorial.

Output

C:\python>python factorial.py
Enter a number: 4
4! = 24

C:\python>python factorial.py
Enter a number: 6
6! = 720

C:\python>python factorial.py
Enter a number: 12
12! = 479001600

Example 2: Factorial using Recursive Function

Recursive functions are popular among budding programmers. But when it comes to performance, recursion is not a preferable choice in Python.

Python Program

#recursion function
def factorial(n):
	return 1 if (n==1 or n==0) else n * factorial(n - 1);  

#read input from user
n = int(input('Enter a number: '))

#call recursion function
result = factorial(n)

#print result
print(result)

Example 3: Factorial using While Loop

We shall use Python While Loop in this solution of finding factorial of a number.

Python Program

#function computes factorial of a given number
def factorial(n):
	result = 1
	i=1
	while i<=n:
		result*=i
		i+=1
	return result

#read input from user
n = int(input('Enter a number: '))

#calculate factorial
result = factorial(n)

print(result)

Summary

In this tutorial of Python Examples, we learned different ways to write a Python Program to compute factorial of a given number.