Contents

**What is factorial?**

In Python, any other programming language or in common term the **factorial** of a number is the product of all the integers from *one* to that number. Mathematically, the formula for the factorial is as follows. If n is an integer greater than or equal to one, then factorial of n is,

```
(n!) = 1*2*3*4....*n
```

Also, the **factorial value** of * zero* is equal to

*and the factorial values for negative integers are not defined.*

**one**## Examples:

**4!**= 4 × 3 × 2 = 24**7!**= 7 × 6 × 5 × 4 × 3 × 2 = 5040- and factorial of
*one*is*one*

## Calculating From the Previous Value

We can easily calculate a **factorial** from the previous one:

**As a table:**

n | n! | ||
---|---|---|---|

1 | 1 | 1 | 1 |

2 | 2 × 1 | = 2 × 1! | = 2 |

3 | 3 × 2 × 1 | = 3 × 2! | = 6 |

4 | 4 × 3 × 2 × 1 | = 4 × 3! | = 24 |

5 | 5 × 4 × 3 × 2 × 1 | = 5 × 4! | = 120 |

6 | etc | etc |

- To work out 7! multiply 7 by 720 to get 5040
- And so on

### Example: 8! equals 40320. Try to calculate 9!

9! = 9 × 8!

9! = 9 × 40320 = **3,62880**

So the rule is:

n! = n × (n−1)!

Which says

“the **factorial** of any number is **that number** times the **factorial of (that number minus ***one***)**“

**Finding factorial of a number in Python** using Iteration

#### (i) Factorial of a Number using for Loop

**Program**

```
num = int(input("Enter the Number :"))
fact = 1
if num < 0:
print("Factorial of negative number is not defined")
else:
for i in range(1,num+1):
fact *= i
print("Factorial of {} is {}".format(num, fact))
```

**Output**

```
Enter the Number :7
Factorial of 7 is 5040
```

In this **python** program first, we are taking input from the keyboard using the `input() `

function then we have a conditional statement for negative inputs since **factorial** of a negative number doesn’t exist.

Then we have a for loop in range 1 to the number itself inside the loop we are simply iterating over loop variable `i`

and multiplying it with the `fact`

variable defined above. In the end, we are simply printing out the result using string formatting.

#### (ii) Factorial of a Number using While Loop

```
def factorialUsingWhileLoop(n):
fact = 1
while(n>1):
fact = fact*n
n = n - 1
print('Factorial is %d'%(fact))
if __name__== "__main__":
factorialUsingWhileLoop(4)
```

Similar to the above **program**, we can use one ‘*while*‘ loop to find out the **factorial.** The process is the same. The only difference is that we are using one ‘*while*‘ loop instead of a ‘*for loop*‘.

‘*factorialUsingWhileLoop*‘ method is used to find out the factorial using a *while* loop. Similar to the above program, the variable ‘*fact*‘ is used to hold the final **factorial value**. The while loop will run until the value of *‘n’* is greater than *‘one’*. On each iteration of the loop, we are decreasing the value of *‘n*‘ by ‘*one*‘. This loop will exit when the value of ‘*n*‘ will be ‘*0*‘. We are printing the factorial value when it ends.

Read more: What is Null in Python

**Finding factorial of a number in Python** using Recursion

**Recursion means** a method calling itself until some condition is met. A method which calls itself is called a recursive method. A recursive method should have a condition which must cause it to return else it will keep on calling itself infinitely resulting in memory overflow.

Calculating the **factorial** of a number using the **recursive** method should work on the following algorithm.

*. Create a method which accepts one argument.

*. Check if the value of the argument is *one*, then return *one* from this method. This will be the terminating condition of the recursive method.

*. Return the product of the argument and the return value of this method called (argument – *one*).

**Code follows**

```
# recursive method which returns the factorial of a number
def factorial_recursion(number):
# check if number is 1
if number == 1:
# return 1 from the method
return 1
return number * factorial_recursion(number-1)
# factorial = factorial_recursion(5)
# read number from keyboard
number = int(input("Enter a number\n"))
# find factorial
factorial = fact(number)
print("Factorial is "+str(factorial))
```

The method will keep on calling itself and return the product of the argument supplied to it with one less than the argument. When the argument value is *one*, it will return *one*. This way, the recursive method will **return** the product of all the numbers starting from the argument till *one*.

The output of the above program will be

Enter a number

6

Factorial is 720

## Python Program to find Factorial of a Number using Functions

This **python factorial** program is the same as the first example. However, we separated the logic using Functions

```
# Python Program to find Factorial of a Number
def factorial(num):
fact = 1
for i in range(1, num + 1):
fact = fact * i
return fact
number = int(input(" Please enter any Number to find factorial : "))
facto = factorial(number)
print("The factorial of %d = %d" %(number, facto))
```

**Output**:

```
Please enter any Number to find factorial : 6
The factorial of 6 = 720
```

## Built-in solution for computing factorial of a number in Python

```
from math import factorial
num = int(input("Enter the Number :"))
fact = factorial(num)
if num < 0:
print("Factorial of negative number is not defined")
else:
print("Factorial of {} is {}".format(num, fact))
```

**Output**:

```
Enter the Number :5
Factorial of 5 is 120
```

## Conclusion :

In this tutorial, we have learned how to find out the **factorial of a number in python**. We have learned how to calculate the factorial using four different methods. Try to run the examples shown above and drop one **comment below** if you have any **queries**.