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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