Skip to main content

How to calculate the factorial of a number in Python

How to calculate the factorial of a number in Python.

Here's a detailed step-by-step tutorial on how to calculate the factorial of a number in Python.

Step 1: Understanding Factorial​

Factorial is the product of an integer and all the integers below it. For example, the factorial of 5 (denoted as 5!) is calculated as: 5 x 4 x 3 x 2 x 1 = 120.

Step 2: Using a Loop to Calculate Factorial​

One common way to calculate factorial is by using a loop. Here's an example of how you can do it in Python:

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

In this code, we define a function called factorial that takes a single argument n, which represents the number for which we want to calculate the factorial.

We initialize a variable called result to 1, as the factorial of any number multiplied by 1 is equal to the number itself.

We then use a for loop to iterate through the range from 1 to n+1 (inclusive). Inside the loop, we multiply the current value of result by the loop variable i, and update the value of result accordingly.

Finally, we return the calculated factorial value stored in result.

Step 3: Testing the Function​

To test the factorial function, you can call it with different numbers and print the returned value. Here's an example:

print(factorial(5))  # Output: 120
print(factorial(10))  # Output: 3628800

Step 4: Using Recursion to Calculate Factorial​

Another approach to calculate factorial is by using recursion. Recursion is a technique where a function calls itself. Here's an example of how you can calculate factorial using recursion in Python:

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

In this code, the factorial function checks if the input n is equal to 0. If it is, it returns 1, as the factorial of 0 is defined as 1.

If n is not 0, the function calls itself with the argument n-1 and multiplies the result by n. This process continues until n becomes 0, and the function starts returning the calculated factorial values.

Step 5: Testing the Recursive Function​

To test the recursive factorial function, you can call it with different numbers and print the returned value. Here's an example:

print(factorial(5))  # Output: 120
print(factorial(10))  # Output: 3628800

That's it! You now know how to calculate the factorial of a number in Python using both a loop and recursion.