Re:Log base 2 of large integers

From	Dave Angel <davea@davea.name>
Subject	Re:Log base 2 of large integers
Date	2014-08-13 10:10 -0400
Organization	news.gmane.org
References	<lsfnr6$pbc$1@news.albasani.net>
Newsgroups	comp.lang.python
Message-ID	<mailman.12926.1407938966.18130.python-list@python.org> (permalink)

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Mok-Kong Shen <mok-kong.shen@t-online.de> Wrote in message:
> 
> I like to compute log base 2 of a fairly large integer n but
> with math.log(n,2) I got:
> 
> OverflowError: long int too large to convert to float.
> 
> Is there any feasible work-around for that?
> 
> Thanks in advance.
> 
> M. K. Shen
> 

Easiest way to get the integer part is to convert to binary
 string,  and take the length of that string.
Call that m.

Then construct a string consisting of a 1 followed by m zeroes.
 
Convert that to a long,  and divide your original number by it.
 Now take the log base 2 of the quotient. Add that to m and you
 have your answer, plus or minus 1.

-- 
DaveA

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Thread

Log base 2 of large integers Mok-Kong Shen <mok-kong.shen@t-online.de> - 2014-08-13 15:05 +0200
  Re: Log base 2 of large integers Skip Montanaro <skip@pobox.com> - 2014-08-13 08:16 -0500
    Re: Log base 2 of large integers Mok-Kong Shen <mok-kong.shen@t-online.de> - 2014-08-13 15:50 +0200
  Re: Log base 2 of large integers Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-08-13 23:32 +1000
    Re: Log base 2 of large integers Mok-Kong Shen <mok-kong.shen@t-online.de> - 2014-08-13 15:46 +0200
      Re: Log base 2 of large integers Marko Rauhamaa <marko@pacujo.net> - 2014-08-13 17:00 +0300
      Re: Log base 2 of large integers Andrew Jaffe <a.h.jaffe@gmail.com> - 2014-08-13 15:06 +0100
    Re: Log base 2 of large integers Peter Otten <__peter__@web.de> - 2014-08-13 16:17 +0200
  Re: Log base 2 of large integers Peter Otten <__peter__@web.de> - 2014-08-13 15:58 +0200
    Re: Log base 2 of large integers Peter Pearson <ppearson@nowhere.invalid> - 2014-08-13 16:12 +0000
      Re: Log base 2 of large integers Marko Rauhamaa <marko@pacujo.net> - 2014-08-13 19:15 +0300
      Re: Log base 2 of large integers Chris Angelico <rosuav@gmail.com> - 2014-08-14 02:18 +1000
      Re: Log base 2 of large integers Ian Kelly <ian.g.kelly@gmail.com> - 2014-08-13 12:13 -0600
  Re:Log base 2 of large integers Dave Angel <davea@davea.name> - 2014-08-13 10:10 -0400

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