[Numpy-discussion] Some minor issues with numpy and fractions

Charles R Harris charlesr.harris@gmail....
Mon Apr 27 17:03:45 CDT 2009


On Mon, Apr 27, 2009 at 3:25 PM, Michael S. Gilbert <
michael.s.gilbert@gmail.com> wrote:

> On Mon, 27 Apr 2009 17:04:17 -0400, Michael S. Gilbert wrote:
> > I had mentioned recently some interest in using fractions in the numpy
> > polynomial class. Suprisingly, it actually works for the most part out
> > of the box, which is great. However, there are some minor issues.  For
> > example:
> >
> > >>>> numpy.poly1d( [ fractions.Fraction(1,2) , fractions.Fraction(1,8) ]
> )/fractions.Fraction(3,4)
> >
> > fails, so I have to move the division into the inner list, which leads
> > to  a lot of duplicate typing and general messiness:
> >
> > >>>> numpy.poly1d( [ fractions.Fraction(1,2)/fractions.Fraction(3,4) ,
> fractions.Fraction(1,8)/fractions.Fraction(3,4) ] )
> >
> > Another item of general interest for numpy is that there is no simple
> > way to allocate memory for fraction array.
> >
> > >>>> def zeros( nsamples ):
> > >>>>    list = []
> > >>>>    for n in range( nsamples ):
> > >>>>        list.append( fractions.Fraction( 0 ) )
> > >>>>    return numpy.array( list )
> >
> > is a solution, but it would be nice to have this built-in, e.g.:
> >
> > >>>> numpy.zeros( n , dtype=numpy.fraction )
> >
> > Perhaps developing a fraction dtype would solve the above poly1d issue
> > as well?  Anyway, just some thoughts to ponder.
>
> It also seems that numpy's basic math operations do not work with
> fractions:
>
> >>> math.sin(fractions.Fraction(1,2))
> 0.47942553860420301
> >>> numpy.sin(fractions.Fraction(1,2))
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> AttributeError: sin
> >>> numpy.exp(fractions.Fraction(1,2))
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
> AttributeError: exp
>

Yep. When objects are involved the ufuncs expect the functions to be object
specific and defined as methods.

Chuck
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