Python Program to Find the Factorial of a Number

num = int(input("Enter a number: "))    
factorial = 1    
if num < 0:    
   print(" Factorial does not exist for negative numbers")    
elif num == 0:    
   print("The factorial of 0 is 1")    
else:    
   for i in range(1,num + 1):    
       factorial = factorial*i    
   print("The factorial of",num,"is",factorial)    

Output:

Enter a number: 10
The factorial of 10 is 3628800

Explanation -

In the above example, we have declared a num variable that takes an integer as an input from the user. We declared a variable factorial and assigned 1. Then, we checked if the user enters the number less than one, then it returns the factorial does not exist for a negative number. If it returns false, then we check num is equal to zero, it returns false the control transfers to the else statement and prints the factorial of a given number.

Using Recursion

Python recursion is a method which calls itself. Let's understand the following example.

Example -

# Python 3 program to find  
# factorial of given number  
def fact(n):  
    return 1 if (n==1 or n==0) else n * fact(n - 1);  
  
num = 5  
print("Factorial of",num,"is",)  
fact(num))  

Output:

Factorial of 5 is 120

Explanation -

In the above code, we have used the recursion to find the factorial of a given number. We have defined the fact(num) function, which returns one if the entered value is 1 or 0 otherwise until we get the factorial of a given number.

Using built-in function

We will use the math module, which provides the built-in factorial() method. Let's understand the following example.

Example -

# Python  program to find  
# factorial of given number  
import math  
def fact(n):  
    return(math.factorial(n))  
  
num = int(input("Enter the number:"))  
f = fact(num)  
print("Factorial of", num, "is", f)  

Output:

Enter the number: 6
Factorial of 6 is 720

We have imported the math module that has factorial() function. It takes an integer number to calculate the factorial. We don't need to use logic.