Python Calculate Factorial of a Number

In this tutorial, I will show you how to find the factorial of a number in Python using 3 different methods.

Actually, the algorithms is similar to another tutorial about Fibonacci sequence I wrote before, which uses recursion and for loop to find the Fibonacci sequence.

Contents

• What is Factorial of a Number?
• How to Find Factorial of a Number in Python?
  • Using math.factorial()
  • Using Recursion
  • Using For Loop
• Conclusion

What is Factorial of a Number?

Factorial of a number is the product of all integers from 1 to that number.

For example, the factorial of 5 is 1 x 2 x 3 x 4 x 5 = 120.

How to Find Factorial of a Number in Python?

To find the factorial of a number in Python, you can:

• Use the built-in function math.factorial()
• Use recursion
• Use a for loop

The first method is straightforward as Python already has a built-in function for this.

The second and third methods are for those who want to understand what it does behind the scene.

Using math.factorial()

The easiest way to calculate the factorial of a number is to use the math.factorial() function.

Create a file called main.py and add the following code:

import math
 
while True:
    if n <= 0:
        print("Stop")
        break
    n = int(input("Enter a number: "))
    print(math.factorial(n))

The above code will ask to enter a number until it reaches a negative integer.

Let's test it out by running the following command:

python main.py

You will get:

Enter a number: 5
120
Enter a number: 10
3628800
Enter a number: -1
Stop

Using Recursion

Recursion is an advanced technique that can be used to find the factorial of a number.

Remember that for recursion to work, we should have a base case and a recursive case.

From the definition of the factorial, we can see that:

• Base case: factorial(0) = 1
• Recursive case: factorial(n) = n * factorial(n - 1)

Let's implement this in Python:

def factorial(n):
    if n == 0:
        return 1
    return n * factorial(n - 1)
 
while True:
    n = int(input("Enter a number: "))
    if n < 0:
        print("Stop")
        break
    print(factorial(n))

Running the above code, we get the same result:

Enter a number: 20
2432902008176640000
Enter a number: 6
720
Enter a number: -1
Stop

This method is more complicated and less efficient as we should avoid using recursion as much as possible. So let's move to the next method.

Using For Loop

The last method is to use a for loop to calculate the factorial of a number in Python.

def factorial(n):
    result = 1
    for i in range(1, n + 1):
        result *= i
    return result
 
while True:
    n = int(input("Enter a number: "))
    if n < 0:
        print("Stop")
        break
    print(factorial(n))

In this case, we converted the recursion into a for loop.

Conclusion

I have shown you 3 ways to find the factorial of a number in Python.

• Using math.factorial()
• Using recursion
• Using a for loop

In practice, I recommend using the math.factorial() function as it's the easiest and most efficient way.