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From: "yehudak ." <katye2007@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: Trailing zeros of 100!
Date: Sat, 2 Jan 2016 22:02:17 +0200
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Hello vbr,
That's EXACTLY what I needed. rstrip is new for me so I'm going to Dr.
Google to learn.

On my efforts I was struggling with .pop() but wasn't very successful...

Thank you so much,
Yehuda

On Sat, Jan 2, 2016 at 8:29 PM, Vlastimil Brom <vlastimil.brom@gmail.com>
wrote:

> 2016-01-02 18:34 GMT+01:00 yehudak . <katye2007@gmail.com>:
> [partly edited for bottom posting]
> > On Sat, Jan 2, 2016 at 5:24 PM, Vlastimil Brom <vlastimil.brom@gmail.com
> >
> > wrote:
> >>
> >> 2016-01-02 14:14 GMT+01:00 yehudak . <katye2007@gmail.com>:
> >> >
> [...]>> > On Sat, Jan 2, 2016 at 2:44 PM, Vlastimil Brom
> >> > <vlastimil.brom@gmail.com>
> >> > wrote:
> >> >>
> >> >> 2016-01-02 12:49 GMT+01:00  <katye2007@gmail.com>:
> >> >> > Hi, newbie here!
> >> >> > I'm trying to write a python program to find how many trailing
> zeros
> >> >> > are
> >> >> > in 100! (factorial of 100).
> >> >> > I used factorial from the math module, but my efforts to continue
> >> >> > failed. Please help.
> >> >> >
> >> >> > Thank you,
> >> >> > Yehuda
> >> >> > --
> >> >> > https://mail.python.org/mailman/listinfo/python-list
> >> >>
> >> > [...]
> >> >
> >> Hi,
> >> If you eventually have this as an assignment or other kind of
> >> (self)learning task, you would want to approach this with the methods
> >> you know, or are supposed to use.
> >> For the math context, you may find this explanations useful:
> >> http://www.purplemath.com/modules/factzero.htm
> >> a rather straightforward python implementation of this seems to be
> >> e.g. this recipe:
> >>
> >>
> http://code.activestate.com/recipes/577844-calculate-trailing-zeroes-in-a-factorial/
> >> Note, that you don't need to calculate the value of the factorial
> >> itself using this way.
> >> If you have problems with following or understanding the code, you may
> >> show your own attempts and tell what problems you encounter with your
> >> approach.
> >>
> >> My previous code sample is based on another - "brute-force" approach,
> >> the factorial is calculated (e.g. via the math module as you have
> >> found), then the integer is converted to a string, afterwards the part
> >> of the result consisting only of zeros - at the end of the string is
> >> matched with a regular expression and finally the length of it is
> >> determined.
> >>
> >> Regular expressions might be handy, but are not necesarilly elementary
> >> stuff for a newcomer in python programming.
> >> You can count the trailing zeros in other ways too - as was suggested
> >> - you can reverse the string and count from the beginning then,
> >> stopping before the first non-zero digit.
> >> The most straightforward way could be to loop (characterwise) through
> >> the (reversed) string, check each character whether it equals to "0"
> >> and stop as soon as there is another digit.
> >>
> >> hth,
> >>    vbr
> >
> >
> > vbr,
> > I tried using .pop() but could not get what I wanted .Also, I can't see
> an
> > advantage in reversing the number.
> > Would you care to write explicitly the program for me (and probably for
> > other too)?
> > Brute Force is the style I'm thinking about.
> >
> > Sorry, but I learn most from viewing the code.
> >
> > Appreciated,
> > Yehuda
> >
> Hi,
> reversing the string would be useful for directly looping over the
> string (the interesting zeros would be at the beginning of the
> reversed string.
> If you use pop() on a list of the digits, the items are taken from the
> end of the list by default, hence no reversing is needed.
> What problems do you have with this route? (you will need to convert
> from the integer result to string, then to list and use pop() and
> count the steps until you reach a non-zero digit)
>
> If you need this kind of soulution (computing the factorial,
> converting to string, counting the trailing zero digits), I believe,
> the most easily comprehensible version was posted by Tim Chase a bit
> earlier.
> In the interactive interpreter, with some intermediate steps added, it
> can look like this:
>
> >>> from math import factorial
> >>> fact100_int = factorial(100)
> >>> fact100_string = str(fact100_int)
> >>> fact100_string_without_trailing_zeros = fact100_string.rstrip("0")
> >>> len(fact100_string) - len(fact100_string_without_trailing_zeros)
> 24
> >>>
>
> [aditional info on the rstrip method of any string ("abcd" used for
> illustration here): ]
> >>> print("abcd".rstrip.__doc__)
> S.rstrip([chars]) -> str
>
> Return a copy of the string S with trailing whitespace removed.
> If chars is given and not None, remove characters in chars instead.
> >>>
>
> It should be noted that the approaches which involve computing of the
> factorial itself have much lower limits on the size compared to the
> algorithmic ones, but for the given case both are sufficient.
>
> hth,
>   vbr
>