I need a little bit of help with functions

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1.44/5 (2 votes)

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Hello, I can't seem to understand this code, it's about factor a number but I don't understand how works the function, I would be glad if someone can help with this.

def factor(x):
if x==0:
return 1
return x * factor(x - 1)

x=int(input())
print (factor(x))

What I have tried:

Just understanding....................................................................

Posted 5-Jan-18 15:20pm

Member 13609237

Updated 6-Jan-18 7:59am

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User 7407470 · Accepted Answer · 2018-01-05T17:00:00

Solution 1

This function calculates the factorial of the number x.
Please see: https://en.wikipedia.org/wiki/Factorial[^]

The function calls itself: it's a recursive function.
Please see: https://www.programiz.com/python-programming/recursion[^]

Posted 5-Jan-18 17:00pm

User 7407470

Comments

Member 13609237 6-Jan-18 10:38am

Hello, thanks for your answer, I have some little questions. How does the function knows how to stop calling itself? And, Why is necessary the "if statement" "If x == 0 then return 1" If I delete those 2 lines and let only the "return x*factor(x-1)", the program doesn't even work! Why is that?

User 7407470 · Accepted Answer · 2018-01-06T05:07:00

Solution 2

Eventually the value of x reaches 0 (because of the x-1 instruction) and stops calling itself (the factorial of 0 is defined as 1).
The whole function is also explained in the second link in my first answer. You can also write the function as:

Python

def factor(x):
    result = 1
    while x > 0: # x > 1 will do
       result = x * result
       x = x - 1
    return result

Posted 6-Jan-18 5:07am

User 7407470

Comments

Member 13609237 6-Jan-18 14:01pm

Hi, I think now I see why are necessary the lines "if x == 0:
return 1"
It's because the function would not have a limit, and it could turn into a loop, right?
Well, thank you very much.

[no name] 6-Jan-18 14:17pm

You're welcome and yes, you're right about the function limit. A function must end sometime... :)
A recursive function is usually not easy to follow and understand. Well done!
Thanks for your kind replies and votes!