Trailing zeros of 100!
Vlastimil Brom
vlastimil.brom at gmail.com
Sat Jan 2 13:29:37 EST 2016
2016-01-02 18:34 GMT+01:00 yehudak . <katye2007 at gmail.com>:
[partly edited for bottom posting]
> On Sat, Jan 2, 2016 at 5:24 PM, Vlastimil Brom <vlastimil.brom at gmail.com>
> wrote:
>>
>> 2016-01-02 14:14 GMT+01:00 yehudak . <katye2007 at gmail.com>:
>> >
[...]>> > On Sat, Jan 2, 2016 at 2:44 PM, Vlastimil Brom
>> > <vlastimil.brom at gmail.com>
>> > wrote:
>> >>
>> >> 2016-01-02 12:49 GMT+01:00 <katye2007 at 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
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