Python Program to find Factorial of a Number

Factorial is denoted by the exclamation mark (!). We can define it as the product of all positive integers less than or equal to that number & greater than 0. For example, the factorial of 4 (4!) means 4 x 3 x 2 x 1, which equals 24. This blog post will explore how to write a Python program to find or calculate the factorial of a number efficiently.

We begin with a basic understanding of the factorial algorithm and how to calculate it in mathematics. Next, we show you different approaches to writing Python program to find factorial of a number using For Loop, While Loop, Functions, and Recursion.

Python Program to find factorial of a Number using math function

The math or mathematical module in this programming language has a factorial() function that directly calculates and returns the results for any positive number. You can use this function by importing the math library.

The Mathematical Representation of the Factorial of n is represented as n!. Where n! = n * (n-1) * (n -2) * …….* 1. For instance, consider n = 5.

• 5! = 5 * (5 – 1) * (5 – 2) * (5 – 3) * (5 – 1)
• 5! = 5 x 4 x 3 x 2 x 1 = 120

It is denoted with the exclamation mark (!), and in this program code, we use the built-in math module factorial function on the number to find it.

import math 

a = int(input(" Please enter any Integer : "))

ft = math.factorial(a)
print("The Result of %d  = %d" %(a, ft))

In this example, we use the math prod() function that accepts the iteration value, finds the product of each value, and returns the factorial of a number.

import math

num = int(input("Enter any Number : "))

if num < 0:
    print("Please Enter Positive Integer Only.")
elif num == 0:
    print("The Fact of 0 = 1.")
else:
    fact = math.prod(range(1, num + 1))
    print(f"The Fact of {num} is {fact}.")

Python Program to find Factorial of a Number using For Loop

This code allows the user to enter any integer. Using this given value, this program finds the Factorial of a number using For Loop. The for loop will start from 1 and multiply the current value by the next integer until it reaches the user given number. For more examples, refer to the Python Basic Programs.

number = int(input(" Please enter any Number : "))
fact = 1

for i in range(1, number + 1):
    fact = fact * i
print("The factorial of %d  = %d" %(number, fact))

The user entered integer in the above program example is 4. Please refer to the Python math functions and Python While Loop articles in Python.

First Iteration
i = 1, Fact = 1 and number = 5

Fact = Fact * i;
Fact = 1 * 1 = 1

2nd Iteration: i = 2, Fact = 1 and Number = 5
Fact = 1 * 2 = 2

3rd Iteration: i = 3, Fact = 2 and Number = 5
Fact = 2 * 3 = 6

4th Iteration: i = 4, Fact = 6 and Number = 5
Fact = 6 * 4 = 24

Next, i become 5. So, the Python For Loop Terminated.

The For Loop Time Complexity is O(n), and Space Complexity is O(1)

The above code for the factorial program returns 1 for the negative values. So, we need the Python If else statement to print a message for negative input.

num = int(input("Enter any Number : "))
fact = 1

if num < 0:
    print("Please Enter Positive Integer Only.")
else:
    for i in range(1, num + 1):
        fact = fact * i
    print("The factorial of %d  = %d" %(num, fact))

Enter any Number : -9
Please Enter Positive Integer Only.

Enter any Number : 6
The factorial of 6  = 720

Python Program to find Factorial of a Number using while Loop

In this program, we just replaced the for loop with the while Loop to find the factorial of a number.

value = int(input(" Please enter any Value : "))
fact = 1
i = 1

while(i <= value):
    fact = fact * i
    i = i + 1

print("The Result of %d  = %d" %(value, fact))

Python Program to find the Factorial of a Number using Functions

This code is the same as the first example. However, we separated the factorial logic using the Functions. Check out the Functions in Python.

def calculating(num):
    faco = 1

    for i in range(1, num + 1):
        faco = faco * i

    return faco


val = int(input(" Please enter any Value : "))

faco = calculating(val)
print("The Result of %d  = %d" %(val, faco))

Python Program for Factorial of a Number using Recursion

This code passes the user entered value to the Function. Within this recursive function, this example program finds the factorial of a number using the recursive function or recursively.

Within the user-defined function of this program, the Python If Else Statement checks whether the integer is Equal to 0 or 1. If the condition is TRUE, then the function returns 1. If the condition is False, the function recursively returns Num * (Num -1).

def factFind(num):
    if num < 0:
        print("Please Enter Positive Integer Only.")
    elif num == 0:
        return 1
    else:
        return num * factFind(num - 1)

num = int(input("Enter any Num : "))
print(f"Fact of {num} is {factFind(num)}.")

User Entered Value = 6.

Fac = num * factFind (num -1);
= 6 * factFind (5)
= 6 * 5 * factFind (4)
= 6 * 5 * 4 * factFind (3)
= 6 * 5 * 4 * 3 * factFind (2)
= 6 * 5 * 4 * 3 * 2 * factFind(1)
= 6 * 5 * 4 * 3 * 2
= 720

The Recursive function Time and Space Complexity are O(n).

Program to find Factorial of a number using List comprehension

This Program finds the Factorial of a Number using list comprehension. Check out the list comprehensions in Python.

num = int(input("Enter any Number : "))

if num < 0:
    print("Please Enter Positive Integer Only.")
elif num == 0:
    print("The Fact of 0 = 1.")
else:
    fact = 1
    [fact := fact * i for i in range(1, num + 1)]
    print(f"The Fact of {num} is {fact}.")

Enter any Number : 5
The Fact of 5 is 120.

Factorial of a number using the lambda reduce function

This Python Program for the Factorial of a Number example uses the Python lambda and reduce functions.

from functools import reduce

num = int(input("Enter any Number : "))

if num < 0:
    print("Please Enter Positive Integer Only.")
elif num == 0:
    print("The Fact of 0 = 1.")
else:
    fact = reduce(lambda a, b: a * b, range(1, num + 1), 1)
    print(f"The Fact of {num} is {fact}.")

This example is the same as the above. However, it uses the lambda recursive function.

from functools import reduce

def factorial(num):
    return reduce(lambda a, b: a * b, range(1, num + 1), 1)

num = int(input("Enter any Number : "))

if num < 0:
    print("Please Enter Positive Integer Only.")
elif num == 0:
    print("The Fact of 0 = 1.")
else:
    print(f"The Fact of {num} is {factorial(num)}.")

Enter any Number : 0
The Fact of 0 = 1.

Using the ternary operator in a Python factorial program

This program is the same as the recursive function logic. However, here, we use the ternary operator to put everything in one line. If the given value is 0 or 1, it returns 1. Otherwise, the program calls the function (fact) recursively to calculate the factorial of a given number.

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

n = int(input("Enter the Number = "))
print(fact(n))

Enter the Number = 7
5040

Use NumPy’s prod() to find the factorial of a number

In the program below, we use the NumPy module’s built-in prod() function to find the product of numbers in a given range. Here, the range starts at 1 and ends at the user-entered number. Within this range, the NumPy prod() multiplies all numbers.

import numpy as np
n = int(input("Enter the Number = "))

if n >= 0:
    print(np.prod(range(1, n + 1)))
else:
    print("Enter Positive Number!")

Enter the Number = 5
120

Enter the Number = -10
Enter Positive Number!

Also Read

• Python program to print the First Digit of a Number
• Python program to print Last Digit in a Number