How To Find Factorial in Python [With Coding Examples]

Introduction

Everyone one of us must be familiar with the word factorial as we all got introduced to that in our primary school in Mathematics subject. Factorial is the product of all positive integers starting from one to the given number. Factorial is only calculated for positive values and cannot be calculated for Negative and Float types.

I wondered when I am learning factorial and other mathematical concepts that where I would be using them in my real life, thanks to Data Science as I was able to understand now the importance of all the mathematical components such as Linear Algebra, Probability, Statistics.

Let us see the importance of the Factorial, different ways to calculate it using python in this article.

Importance

Let us take an example that we have a race between 10 cars in a world race event, and we have a problem statement to find out how many ways that those 10 cars come first, second, third. As there are only 10 cars, we would like to just take a paper and write down the various combinations. But what if we have 100 cars or more events and we have the same or similar kind of problem statement in that?

In order to tackle these kinds of situations, we have something called Permutation. I guess you would be aware of this term as Permutations and Combinations in our primary school. These are very much needed if you want to ace your Data Analysis and statistical skills. If you are a beginner and interested to learn more about data science, check out our data science courses from top universities. This helps to solve the problem statement as stated below.

Solution

We have a total of 10 cars.

We need to find the possibility of 3 winners out of 10.

10! / (10-3)! = 10! / 7! = 720

So, we have a total of 720 possibilities for these 10 cars to come first, second, third in the race event. 

Python Implementation

Python is a high-level, interpreted and general-purpose programming language that focuses on code readability and the syntax used in Python Language helps the programmers to complete coding in fewer steps as compared to Java or C++ and it is built on top of C.

The language was founded in 1991 by the developer Guido Van Rossum. Python is widely used in bigger organizations because mainly in various Artificial Intelligence use cases such as Computer Vision, Natural Language Processing , Deep Learning , Speech Recognition, Face Recognition, Voice Recognition.

Python is a very powerful programming tool and can be used for a wide variety of use cases in real life. It offers a direct function that can compute the factorial of a given number without writing the code explicitly. But let us start with a naïve approach and at last get to know about that function.

Also Read: Why Python so popular with developers?

For Loop

We can calculate the factorial of a number by iterating from number 1 till the given number by multiplying at each step. Let us jump into the coding part of the above discussed approach.

Code

number = input (“Enter a Number:”) # Ideally you can use any print message
factorial = 1
if int (number) >=1: # To check whether the given number is positive or not.
for i in range (1, int(number)+1): # Loop from number 1
   factorial = factorial * I   # Multiplication with each number.
print ("Factorial of ", number, " is: ", factorial) # Print out the calculated factorial.

Output

Running the above code will give you the below output:

Enter a Number :5
Factorial of 5 is: 120

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Recursive Function

In this case we will be creating our own user defined function in python that will help us to calculate the factorial of a given number.

Code

number = input ("Enter a number: ")
def recursive_factorial(number): # User defined recursive function.
if number == 1: # Condition if the given number is equal to 1
   return number
elif number < 1: # Condition if given number is lesser than 1
   return ("The given number is lesser than one and the factorial cannot be calculated.")
else:
   return number*recursive_factorial(number - 1)
print (recursive_factorial(int(number)))

Output

Running the above code will give you the below output:

Enter a number: 5
120
Enter a number: -2

The given number is lesser than one and the factorial cannot be calculated

Enter a number: 1
1 

Find Factorial In Python Using Built-In Function

The built-in factorial() method is available in the math module. Let’s analyze the subsequent example to understand how to find factorial in Python using this method. 

Code:

# Python  program to find  
# factorial of given number  
import math  
def fact(n):  
    return(math.factorial(n))  
num = int(input("Enter the number:"))  
f = fact(num)  
print("Factorial of", num, "is", f)

Output:

Enter the number: 7
Factorial of 7 is 5040

The math module with the factorial() method has been imported. The factorial can only be calculated with an integer value. There is no need for reasoning here.

Find Factorial In Python Using Class

One can use Class to find and print a user-inputted number. The dot (.) operator is used to access this class’s member function (findFact()) by creating an object called ob: 

Code:

class CodesCracker:
    def findFact(self, n):
        f = 1
        for i in range(1, n + 1):
            f = f * i
        return f
print("Enter a Number: ", end="")
num = int(input())
ob = CodesCracker()
print("\nFactorial of", num, "=", ob.findFact(num))

How To Find Factorial In Python Using Function

FindFact(), a user-defined function, was used to build this factorial program in Python. This function takes, analyze, and interpret a value as an input. Then, it finds the factorial of that number in Pythin before making any return. 

Code: 

def findFact(n):
    f = 1
    for i in range(1, n+1):
        f = f*i
    return f
print("Enter a Number: ", end="")
try:
    num = int(input())
    fact = findFact(num)
    print("\nFactorial of", num, "=", fact)
except ValueError:
    print("\nInvalid Input!")

Find Factorial In Python Using Ternary Operator

Despite using a fact variable, this method leverages a conditional and a function expression. It does to determine whether the specified condition is true or false so it can derive the factorial of a positive integer. Typically, the syntax of any ternary operator is: 

[on_true] if [expression] else [on_false]

By eliminating the lengthy if-else lines and replacing them with a single-line condition test, the code is made more concise. 

Code:

#Define a function
def factorial(n):
    return 1 if (n==1 or n==0) else n * factorial(n-1);
#Enter input
n = int(input("Enter input number : "))
print("The factorial of",n,"is",factorial(n))

Output:

Enter input number 8
The factorial of 8 is 40320

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Factorial function in Math Package

Python is extensively known for its ease of use and user-friendly third party packages which will simplify many tasks. In the current scenario Python is the go-to language for Data Scientists.

Code

import math # Required package
number= input("Enter a number: ")
print("The factorial of ", number, " is : ")
print(math.factorial(int(number))) # Function to calculate the factorial

Output

Running the above code will give you the below output:

Enter a number: 5
The factorial of 5 is :
120
Enter a number: 5.6
Traceback (most recent call last):
The factorial of 5.6 is :
File "C:/Users....py", line 5, in
print(math.factorial(int(number)))
ValueError: invalid literal for int() with base 10: '5.6'

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 We are getting a value error because we cannot calculate the Factorial of float integer. When we are explicitly writing the python code we need to take care to check all the condition and output the relevant message, but in the factorial function of Math package in python it does everything for us which helps us to decrease our lines code when we have usage of Factorial in our Project or any problem statement.

Finding Factorial In Python: Exceptions in math.factorial()

When the input number is negative: 

Code:

# Python code to demonstrate math.factorial()
# Exceptions ( negative number )
import math
print("The factorial of -7 is : ", end="")
# raises exception
print(math.factorial(-7)

Output:

Traceback (most recent call last):
  File "/home/f29a45b132fac802d76b5817dfaeb137.py", line 9, in 
    print (math.factorial(-7))
ValueError: factorial() not defined for negative values

When the input  number is a Non–Integral value: 

Code:

# Python code to demonstrate math.factorial()
# Exceptions ( Non-Integral number )
import math
print("The factorial of 5.6 is : ", end="")
# raises exception
print(math.factorial(5.6))

Output:

Traceback (most recent call last):
  File "/home/3987966b8ca9cbde2904ad47dfdec124.py", line 9, in 
    print (math.factorial(5.6))
ValueError: factorial() only accepts integral values

Must Read: Python Tutorial

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Conclusion

In this article we got to know the importance and application of Factorial and other important mathematical concepts in real life. Went through the different types of code to calculate the Factorial of a given number. This article just covers the Factorial in Python but there are many other mathematical calculations available in the MATH package. Folks new to Python can have a deeper look into them and can even try a few.

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