Python – Factorial of a Number

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Contents

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

Examples

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 = 4
result = factorial(n)
print(n,'! = ',result,sep="")

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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

4! = 24

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 to find factorial of a number n
def factorial(n):
    if n < 0:
        print('Invalid number to find factorial.')
    elif n==1 or n==0 :
        return 1
    else :
        return n * factorial(n - 1);  

# Take a number
n = 10

# Call recursion function
result = factorial(n)

# Print result
print(result)

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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

# Take a number
n = 5

# Calculate factorial
result = factorial(n)

print(result)

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