Find the factorial of a number in python using for loop

num=int[input["enter a number: "]]

fac=1

foriinrange[1,num+ 1]:

fac=fac*i

print["factorial of ",num," is ",fac]

num=int[input["enter a number: "]]

fac=1

i=1

whilei= 1: num = num * n n = n - 1 return num

Let’s test that it works:

f = factorial[5]
print[f]

Output:

120

There you have it!

To learn more about factorials and some alternative ways to calculate the factorial, please read along.

What Is the Factorial of a Number in Mathematics

In mathematics, the factorial is the product of all positive whole numbers less than or equal to a target number.

The factorial is denoted with an exclamation mark.

For example, let’s calculate the factorial of 5:

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

In layman’s terms, the factorial answers the question “In how many ways can you form a queue of n persons?”.

Recursion and Factorial in Python

In programming, recursion refers to a function that calls itself.

Usually, calculating the factorial acts as an introduction to recursion in programming courses.

This is because the factorial can be expressed recursively as:

n! = n x [n – 1]!

In this expression, there is a factorial operator [!] on both sides.

Essentially, this expression says “Factorial of n equals to n times the factorial of n – 1”.

Let’s use this as a basis to formulate a recursive factorial function in Python:

def factorial[n]: 
    if [n==1 or n==0]:
        return 1
    else:
        # n! = n x [n - 1]!
        return n * factorial[n - 1]

In this function:

• If the target number is 0 or 1, we are going to return 1, because the factorial of 0 or 1 is 1.
• If the target number is not 0 or 1, we are going to multiply it by the factorial of 1 less than the target.

Now you can test this code to make sure it works:

print[factorial[5]]

Output:

120

Alternatives to Calculating the Factorial in Python

You now know what is the factorial of a number and how it is commonly calculated using recursion.

But there are some alternatives, that is:

1. A built-in function
2. While loop
3. For loop

Let’s go through each of these approaches next.

1. Built-In Factorial Function in Python

Unless you are practicing your Python skills, you should use the built-in factorial function from the math module.

For example:

from math import factorial

print[factorial[5]]

Output:

120

2. While Loop Factorial in Python

Any recursive function can be written as an iterative one.

In other words, you can use a regular loop instead of recursion.

Although, the feasibility to convert a recursive function into an iterative function varies based on the complexity of the function.

When talking about recursive factorial, it is trivial to convert it into an iterative one using a while loop.

All you need to do is:

1. Create a loop where you start a result from 1
2. Subtract 1 from the target number
3. Multiply the result by it.
4. Do this until the target reaches 1 and you have successfully computed the factorial.

Here is how it looks in code:

def factorial[n]:
    num = 1
    while n >= 1:
        num = num * n
        n = n - 1
    return num

3. For Loop Factorial in Python

You can also convert a recursive factorial function to one that uses a for loop in Python.

To do this:

1. Specify a target number.
2. Set the result at 1.
3. Start a for loop from 1 to the target number + 1.
4. Multiply the result by each number in this range.
5. Return the result.

Here is how it looks in code:

def factorial[n]: 
    num = 1
    
    for i in range[1, n + 1]:
        num = num * i
        
    return num

Example run:

print[factorial[5]]

Output:

120

Conclusion

Today you learned how to calculate the factorial of a number using a while loop in Python.

To recap, the factorial of a number is a whole number multiplied by all positive whole numbers less than it.

Commonly, you see factorial being calculated with a recursion. But it is just as fine to calculate the factorial using an iterative approach, that is, a while or a for loop.

If you do not need to implement the factorial yourself, use the built-in math.factorial function in Python.

Thanks for reading.

Happy coding!

Further Reading

50 Python Interview Questions

How do you find the factorial of a for loop?