How does this for loop work?

Q&A
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#1

Screen Link:
https://app.dataquest.io/m/380/permutations-and-combinations/4/permutations

My Code: 

def factorial(n):
    final_product = 1
    for i in range(n, 0, -1):
        final_product *= i
    return final_product

Hi. I was asked to write a function to calculate the factorial for any input. My function worked fine. However, when viewing the example answer, I was confused how their for loop worked. Specifically, I did not understand what range(n, 0, -1) was doing. Any explanations would be helpful.

Thanks.

#2

Hi @mjs139, the range () function takes 3 arguments: start, stop, and step. Essentially, it returns a sequence of numbers starting at a specified number (i.e., start argument), increment by a specified number(i.e., step argument), and stops before a specified number (i.e., stop argument).

So as applied to the factorial () function, it means that, for any n passed to range(), create a sequence of numbers starting from n, increment by -1 before you reach 0. Loop through the sequenence of numbers produced and multiply each number by the value of final_product at every looping. See here for more on the range() function.

Hopefully, this helps!

#3

That clears it up. Thanks.

#4

With respect to the following explanation. This is how the loop would iterate.

fgfgfgfgfgfg

Hope this solves your query. :smile:

1 Like
#5

@mjs139 glad you are clear now!. Could you then please mark the post as solved? If needed see How do I mark a post as solved?.

#6

Always good to add illustrations to enhance explanations. Thanks!

#7

So true buddy. :smile: