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Re: Confusing math problem

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"Dave Angel" <davea@davea.name> wrote in message
> On 02/21/2013 02:33 PM, Schizoid Man wrote:
> However, there is an important inaccuracy in math.pow, because it uses 
> floats to do the work.  If you have very large integers, that means some 
> of them won't be correct.  The following are some examples for 2.7.3 on 
> Linux:
>
>  a  b  math.pow(a,b)       a**b
>  3 34 1.66771816997e+16 16677181699666569
>  3 35 5.0031545099e+16 50031545098999707
> ...
>  5 23 1.19209289551e+16 11920928955078125
>
> The built-in pow, on the other hand, seems to get identical answers for 
> all these cases.  So use pow() instead of math.pow()

I see. I thought using the ** was shorthand for math.pow() and didn't think 
that one would be integer operations and the other floats. I'm performing 
some large integer arithmetic operations. I would normally do this my 
writing my own multiplication class and storing results as strings, but a 
friend suggested that I look at Python.

I ran this one example and was quite surprised at the difference, since 9183 
is the correct answer.

> One other test:
>
> diff = set(map(int, result1)).symmetric_difference(set(result2))
> if diff:
>     print diff
>     print len(diff)
>
> shows me a diff set of 15656 members.  One such member:
>
> 13552527156068805425093160010874271392822265625000000000000000000000000000000000000000000000000000000000000000000L
>
> Notice how using floats truncated lots of the digits in the value?

I'm running this test now, but the Mac's fan has kicked in (it's a slightly 
older machine) so might it let run through the night.

I appreciate the help.

> -- 
> DaveA 

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Thread

Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 19:33 +0000
  Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 15:25 -0500
    Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 22:39 +0000
      Re: Confusing math problem Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-21 22:53 +0000
        Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 23:41 +0000
          Re: Confusing math problem Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-22 00:04 +0000
          Re: Confusing math problem Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-21 17:19 -0700
    Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 23:39 +0000
  Re: Confusing math problem Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-21 13:42 -0700
  Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 07:46 +1100
    Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 22:44 +0000
      Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 11:29 +1100
      Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 21:19 -0500
  Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 15:49 -0500
  Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 08:23 +1100
    Re: Confusing math problem Peter Pearson <ppearson@nowhere.invalid> - 2013-02-21 21:59 +0000
      Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 09:11 +1100
      Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 17:33 -0500
      Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 10:15 +1100
    Re: Confusing math problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 09:16 +0000
      Re: Confusing math problem Serhiy Storchaka <storchaka@gmail.com> - 2013-02-22 13:48 +0200
  Re: Confusing math problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 09:27 +0000

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