Confusing math problem

Oscar Benjamin oscar.j.benjamin at gmail.com
Thu Feb 21 23:53:55 CET 2013


On 21 February 2013 22:39, Schizoid Man <schiz_man at 21stcentury.com> wrote:
> "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.

Then you want operator.pow:

>>> import operator
>>> operator.pow(3, 2)
9

math.pow is basically the (double)pow(double, double) function from
the underlying C library. operator.pow(a, b) is precisely the same as
a**b.

> 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.

There's no need to use strings if you're working with integers in
Python. The results with int (not float) will be exact and will not
overflow since Python's ints have unlimited range (unless your machine
runs out of memory but that only happens with *really* big integers).

If you want to do computations with non-integers and high precision,
take a look at the decimal and fractions modules.
http://docs.python.org/2/library/decimal.html
http://docs.python.org/2/library/fractions.html

There is also a good third party library, sympy, for more complicated
exact algebra:
http://sympy.org/en/index.html


Oscar



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