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Python Factorial Program (How to Find Factorial of a Number in Python?)

Learn how to find factorial of a number in Python with different types of programs. All programs and examples are covered in simple terms with code, output, as well as explanations. 

Factorial is a mathematical operation that finds the product of all positive integers up to a given number. It is commonly denoted by the exclamation mark (!). 

For example, the factorial of 5 is calculated as:

5! = 5 x 4 x 3 x 2 x 1 = 120.

In this tutorial on Python factorial program, we will walk you through the step-by-step process of creating a program that can calculate the factorial of any given number. We'll cover the fundamental concepts and provide you with easy-to-understand code examples.

By the end of this tutorial, you will have a solid understanding of how to use Python's programming constructs such as loops and conditional statements to implement a factorial calculation algorithm. This knowledge will serve as a strong foundation for your future Python programming endeavors.

So, let’s get started and understand the factorial code in Python. 

Python Factorial Program

Code

# Program to find factorial of a number in Python using function

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

# Taking input from the user
num = int(input("Enter a number: "))

# Checking if the number is negative
if num < 0:
    print("Factorial cannot be calculated for negative numbers.")
elif num == 0:
    print("Factorial of 0 is 1.")
else:
    result = factorial(num)
    print("Factorial of", num, "is", result)

Output

Enter a number: 10
Factorial of 10 is 3628800

Explanation

  • In this program, we define a recursive function called factorial that calculates the factorial of a given number n. 

  • The function checks if n is equal to 0, in which case it returns 1 (since the factorial of 0 is defined as 1). Otherwise, it recursively calls itself with n-1 and multiplies the current number n with the result.

  • We then take input from the user using the input function and convert it to an integer using int(). 

  • We check if the number is negative, in which case we display an appropriate error message. 

  • If the number is zero, we print that the factorial is 1. Otherwise, we call the factorial function with the given number and store the result in the result variable. Finally, we print the factorial of the number.

Python Course

Factorial of a Number in Python Using for loop

Code

# Factorial in Python using for loop

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

# Taking input from the user
num = int(input("Enter a number: "))

# Checking if the number is negative
if num < 0:
    print("Factorial cannot be calculated for negative numbers.")
elif num == 0:
    print("Factorial of 0 is 1.")
else:
    result = factorial(num)
    print("Factorial of", num, "is", result)

Output

Enter a number: 5
Factorial of 5 is 120

Explanation

  • Here, we define a function called factorial that takes a number n as input and uses a for loop to iterate from 1 to n. 

  • Within each iteration, we update the result variable by multiplying it with the current value of i. This way, we gradually calculate the factorial of the given number using for loop.

  • We take input from the user, check for negative numbers, and display the factorial accordingly.

  • By using a for loop, we avoid recursion and directly calculate the factorial by multiplying the numbers in the loop. 

This approach is especially useful when dealing with larger numbers, as it reduces the chances of exceeding recursion limits.

Python Factorial Program Using while loop

Code

# How to find factorial of a number in Python using while loop

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

# Taking input from the user
num = int(input("Enter a number: "))

# Checking if the number is negative
if num < 0:
    print("Factorial cannot be calculated for negative numbers.")
elif num == 0:
    print("Factorial of 0 is 1.")
else:
    result = factorial(num)
    print("Factorial of", num, "is", result)

Output

Enter a number: 0
Factorial of 0 is 1.

Explanation

  • In this Python factorial program, we define a function called factorial that takes a number n as input. Inside the function, we first check if n is equal to 0. If so, we return 1 since the factorial of 0 is defined as 1.

  • If n is not 0, we initialize a variable called result to 1. We then use a while loop to repeatedly execute the block of code as long as n is greater than 0. 

  • Within each iteration, we update the result variable by multiplying it with the current value of n, and then decrement n by 1. This way, we gradually calculate the factorial of the given number.

Using a while loop in this program provides an alternative approach to find the factorial of a number in Python, iterating until the number reaches 0. This can be especially useful in scenarios where you prefer a while loop over a for loop.

Factorial Program in Python Using Function Without Recursion

Code

# Python program for factorial of a number without recursion

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

# Taking input from the user
num = int(input("Enter a number: "))

# Checking if the number is negative
if num < 0:
    print("Factorial cannot be calculated for negative numbers.")
elif num == 0:
    print("Factorial of 0 is 1.")
else:
    result = factorial(num)
    print("Factorial of", num, "is", result)

Output

Enter a number: 12
Factorial of 12 is 479001600

Explanation

  • We define a function called factorial that takes a number n as input. Inside the function, we initialize a variable called result to 1. 

  • Then, we use a for loop to iterate from 1 to n, and in each iteration, we update the result variable by multiplying it with the current value of i. This way, we gradually calculate the factorial of the given number.

  • We follow the same steps as before to take input from the user, check for negative numbers, and display the factorial accordingly.

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Find the Factorial of a Number