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Re: Log base 2 of large integers

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Am 13.08.2014 15:32, schrieb Steven D'Aprano:
> Mok-Kong Shen wrote:
>
>>
>> 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?
>
> If you want the integer log2, that is, the floor of log2, the simplest way
> is calculate it like this:
>
> def log2(n):
>      """Return the floor of log2(n)."""
>      if n <= 0: raise ValueError
>      i = -1
>      while n:
>          n //= 2
>          i += 1
>      return i
>
> log2(511)
> => returns 8
> log2(512)
> => returns 9
> log2(513)
> => returns 9
>
>
> Does that help?

That is too inaccurate (e.g. for 513 above) for me, I would like
to get accuracy around 0.01 and that for very large n.

M. K. Shen

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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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