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From: "yehudak ." <katye2007@gmail.com>
Newsgroups: comp.lang.python
Subject: Re: Trailing zeros of 100!
Date: Sat, 2 Jan 2016 22:09:04 +0200
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Hi again,
I looked a little deeper at your code. Smart solution and kudos.

Yehuda

On Sat, Jan 2, 2016 at 10:02 PM, yehudak . <katye2007@gmail.com> wrote:

> 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
>>
>
>