Re: [Gnuplot-py-users] Gnuplot.py float precision (again)

[Benny M. <ben...@gm...>]

I suggest you look up things in numpy doc. If nothing there, use the python
way of determining things:

if m:
   if isinstance(m[0], numpy.float64):
        raise TypeError

Provide a patch to gnuplot when you make it work.

Benny

2009/3/3 Michael Haggerty <mh...@al...>

> Xaver Wurzenberger wrote:
> > Hi guys,
> >
> > I'm sorry to bring this up again, but I'm about to publish my code and
> I'd
> > really love to get this straight/fixed. In case you don't remember, I'm
> > trying to plot high precision numbers (float64, that is), but gnuplot.py
> > converts them to float32 arrays.
> > Michael Haggerty told me to 'workaround' by using numpy double arrays,
> but it
> > seems that's not working here. I think the problem might be here:
> > (/usr/share/pyshared/Gnuplot/utils.py, line 20ff)
> >
> >> def float_array(m):
> >>    """Return the argument as a numpy array of type at least 'Float32'.
> >>
> >>    Leave 'Float64' unchanged, but upcast all other types to
> >>    'Float32'.  Allow also for the possibility that the argument is a
> >>    python native type that can be converted to a numpy array using
> >>    'numpy.asarray()', but in that case don't worry about
> >>    downcasting to single-precision float.
> >>
> >>    """
> >>
> >>    try:
> >>        # Try Float32 (this will refuse to downcast)
> >>        return numpy.asarray(m, numpy.float32)
> >>    except TypeError:
> >>        # That failure might have been because the input array was
> >>        # of a wider data type than float32; try to convert to the
> >>        # largest floating-point type available:
> >>        # NOTE TBD: I'm not sure float_ is the best data-type for this...
> >>        try:
> >>            return numpy.asarray(m, numpy.float_)
> >>        except TypeError:
> >>            # TBD: Need better handling of this error!
> >>            print "Fatal: array dimensions not equal!"
> >>            return None
> >
> > If I understand this correctly, the line
> >> return numpy.asarray(m, numpy.float32)
> > is supposed to raise a TypeError if you give a numpy.float64 array.
> > However, my python shell doesn't:
> >
> >> (00:13:52)xaver@siduxbox:~$python
> >> Python 2.5.4 (r254:67916, Feb 18 2009, 03:00:47)
> >> [GCC 4.3.3] on linux2
> >> Type "help", "copyright", "credits" or "license" for more information.
> >>>>> import numpy
> >>>>> a = numpy.array( [2.000000001,3.0,4.0] )
> >>>>> a.dtype
> >> dtype('float64')
> >>>>> numpy.asarray(a, numpy.float32)
> >> array([ 2.,  3.,  4.], dtype=float32)
> >
> > Am I doing sth wrong? Can anyone confirm that?
>
> I don't see that you are doing anything wrong.  That array-conversion
> code was written long ago, originally for Numeric (not numpy).
> Apparently numpy behaves differently than Numeric in this situation,
> resulting in a Gnuplot.py bug.
>
> I suggest that you try to find the right incantation for Gnuplot.py to
> do this conversion without losing precision, and submit a patch to
> Gnuplot.py.
>
> Michael
>
>
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