Confusing math problem
Schizoid Man
schiz_man at 21stcentury.com
Thu Feb 21 17:39:31 EST 2013
"Dave Angel" <davea at 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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