The factorial of a number is the product of all the integers from 1 to that number. Mathematically, the formula for the factorial is as follows. If n is an integer greater than or equal to 1, then factorial of n is,

 (n!) = 1*2*3*4....*n

Also, the factorial value of 0 is equal to 1 and the factorial values for negative integers are not defined.

Problem Definition

Create a Python Program to find the factorial of the user-provided number.

Factorial of a number using For Loop

num = int(input("Enter the Number :"))
fact = 1
if num < 0:
    print("Factorial of negative number is not defined")
else:
    for i in range(1,num+1):
        fact *= i
    print("Factorial of {} is {}".format(num, fact))

Output

Enter the Number :
7
Factorial of 7 is 5040

In this program first, we are taking input from the keyboard using the input() function then we have a conditional statement for negative inputs since factorial of a negative number doesn't exist.

Then we have a for loop in range 1 to the number itself inside the loop we are simply iterating over loop variable i and multiplying it with the fact variable defined above. In the end, we are simply printing out the result using string formatting.

Factorial of a number using While Loop

The same program can be implemented using a while loop as follows,

num = int(input("Enter the Number :"))
fact = 1i = 1
if num < 0:
    print("Factorial of negative number is not defined")
else:
    while(i<=num):
        fact *= i
        i += 1
    print("Factorial of {} is {}".format(num, fact))

Output

Enter the Number :6
Factorial of 6 is 720

Factorial function

You can wrap the above code in a function as follows, 

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

# Example usage
number = int(input("Enter a number: "))
print(f"The factorial of {number} is {factorial(number)}")

Factorial of a number using recursion 

Alternatively, you can use recursion to calculate the factorial:

def factorial(n):
    if n == 0 or n == 1:
        return 1
    else:
        return n * factorial(n - 1)

# Example usage
number = int(input("Enter a number: "))
print(f"The factorial of {number} is {factorial(number)}")

Factorial of a number using math module

Since Python always has a better way of doing things, it comes with a built-in solution for computing factorial of a number.

import math

number = int(input("Enter a number: "))
factorial_result = math.factorial(number)

print(f"The factorial of {number} is {factorial_result}")

Output

Enter the Number :5
Factorial of 5 is 120

The math library which ships with Python has a factorial() method that returns the factorial of the provided number.