Factorial Program in Python

The factorial of a number n is denoted by n!. This is the product of all positive numbers less than or equal to n. It is calculated as -

n! = n X (n-1) X (n-3) X ……… X 3 X 2 X 1

The factorial operation generally come across many mathematical operations, like - combinatorics, algebra, analysis.

Example

Suppose, we want to get factorial of 6, then factorial will be -

6! = 6 X 5 X 4 X 3 X 2 X 1
720

Python gets factorial using For Loop

Here is the Python program to find the factorial of a number using for loop. When the user entered the number as input, it passes through the if else statement. If the number is greater than 0, then it passes through the for loop and finds the factorial of the number.

num = int(input("Enter a number : "))

factorial = 1

if num > 0:
   for x in range(1,num + 1):
       factorial = factorial*x
   print("The factorial of",num," : ",factorial)
elif num == 0:
   print("The factorial of 0 is 1")
else:
   print("Factorial does not exist for given input")

Output of the above code

(env) c:\python37\Scripts\projects>pythonfactorial.py
Enter a number: 6
The factorial of 6  :  720

(env) c:\python37\Scripts\projects>pythonfactorial.py
Enter a number: 5
The factorial of 5  :  120

(env) c:\python37\Scripts\projects>pythonfactorial.py
Enter a number: -1
Factorial does not exist for given input

(env) c:\python37\Scripts\projects>pythonfactorial.py
Enter a number: 0
The factorial of 0 is 1

Python gets factorial using Recursion function

Here is the Python program to find the factorial of a number using the recursion function. When user enters a number as input, it passes to the function. If the number equals to the 1, it returns 1. The factorial of a number less than equals to 0 does not exist. If the input is greater than 1, the function calls recursively and find the factorial of a number.

def factorial(num):
    if num == 1:
        return num
    elif num <= 0:
        return " Not Exist"
    else:
        return num * factorial(num - 1)


# Enter the input
num = int(input("Enter a Number: "))

print("Factorial of", num, " : ", factorial(num))

Output of the above code

(env) c:\python37\Scripts\projects>factorialpython.py
Enter a Number : 3
Factorial of 3 :  6

(env) c:\python37\Scripts\projects>factorialpython.py
Enter a Number : 1
Factorial of 1 :  1

(env) c:\python37\Scripts\projects>factorialpython.py
Enter a Number : 4
Factorial of 4  :  24

(env) c:\python37\Scripts\projects>factorialpython.py
Enter a Number : 0
Factorial of 0  :   Not Exist

(env) c:\python37\Scripts\projects>factorialpython.py
Enter a Number : 6
Factorial of 6  :  720

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