# Python Tutorial to Calculate the Factorial of a Number

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A factorial is the product of an integer and all the integers below it; for example, the factorial of 4 (written as “4!”) is 4 * 3 * 2 * 1 = 24. Here is a simple Python program that calculates the factorial of a given number:

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

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

print(factorial(number))
``````

## Here is how the program works:

1. We define a function `factorial()` that takes an integer `n` as an argument.
2. The function contains a base case, which is `if n == 0: return 1`. This is important because the factorial of 0 is 1, and we want to make sure our program terminates.
3. If `n` is not 0, the function returns `n * factorial(n-1)`. This is known as recursion, and it means that the function will call itself with a smaller value `n` until the base case is reached.
4. Outside the function, we ask the user to enter a number, which is stored in the variable `number`.
5. We call the `factorial()` function and pass `number` it as an argument. The result of the function is printed on the screen.

## Here is a more detailed explanation of the Python program for calculating the factorial of a number:

1) First, we define a function called `factorial()` that takes an integer `n` as an argument. The function definition is as follows:

``````
def factorial(n):
``````

2) Next, we add a conditional statement to check if `n` is equal to 0. If it is, the function returns 1. This is the base case for our function, which means that it will stop calling itself once the value of `n` reaches 0. The base case is important because it allows our program to terminate and avoid an infinite loop.

``````
if n == 0:
return 1
``````

3) If `n` is not 0, the function returns `n * factorial(n-1)`. This line of code uses recursion, which means that the function calls itself with a smaller value of `n` until the base case is reached. In this case, the function will multiply `n` by the result of `factorial(n-1)`, which is the factorial of `n-1`.

``````
else:
return n * factorial(n-1)
``````

4) Outside the function, we ask the user to enter a number, which is stored in the variable `number`. We use the `int()` function to convert the user input from a string to an integer.

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

5) Finally, we call the `factorial()` function and pass `number` as an argument. The result of the function is printed to the screen using the `print()` function.

``````
print(factorial(number))
``````

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