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Python Program to Find Factorial of a Number



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In this article, I've created some programs in Python, that find and prints factorial of a given number by user at run-time. Here are the list of approaches used:

Before creating these programs, let's remind about factorial formula, that is:

n! = (n)*(n-1)*(n-2)*...*3*2*1

Here n indicates a number of which, the factorial is going to find. The ! (exclamation) indicates factorial. So n! can be called as n factorial. For example, 5! can be calculated as:

5! = 5*4*3*2*1
   = 120

Find Factorial using while Loop

To find factorial of any number in Python, you have to ask from user to enter the number, then find and print its factorial as per the formula given above, like shown in the program given below.

The question is, write a Python program to find factorial of a given number using while loop. Here is its answer:

print("Enter the Number: ")
num = int(input())

fact = 1
i = 1
while i<=num:
    fact = fact*i
    i = i+1

print("\nFactorial =", fact)

Here is the initial output produced by this Python program:

find factorial of number python

Now supply the input say 5 as number and press ENTER key to find its factorial like shown in the snapshot given below:

factorial of number python

The dry run of above program with user input 5 goes like:

Find Factorial using for Loop

This program does the same job as of previous program, but using for loop, instead of while. The try-except is used in this program to handle with invalid inputs entered by user. And end is used to skip inserting an automatic newline.

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

Here is its sample run with same user input as of previous program:

python find factorial of number

In above program, the following code:

for i in range(1, num+1):

is applied to execute the following statement:

fact = fact*i

num number of times with value of i from 1 to num's value.

Find Factorial using Function

This program is created using a user-defined function named findFact(). This function receives a value as its argument, then find and returns its factorial.

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!")

This program produces same output as of previous program:

Find Factorial using Recursion

The following program is created using a recursive function named findFact(). This function calls itself until the value of its argument becomes equal to 1.

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

print("Enter a Number: ", end="")
try:
    num = int(input())
    fact = findFact(num)
    print("\nFactorial of", num, "=", fact)
except ValueError:
    print("\nInvalid Input!")

Find Factorial using Class

This is the last program to find and print factorial of a number entered by user, created using a class named CodesCracker. An object ob is created of this class to access its member function (findFact()) using dot (.) operator:

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))

Same Program in Other Languages

Python Online Test


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