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

[Michael H. <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