[Numpy-discussion] bug with with fill_values in masked arrays?
Tue Mar 25 10:12:00 CDT 2008
On Tuesday 25 March 2008 10:33:58 Chris Withers wrote:
> Pierre GM wrote:
> > Well, yeah, my bad, that depends on whether you use masked_invalid or
> > fix_invalid or just build a basic masked array.
> Yeah, well, if there were any docs I'd have a *clue* what you were
> talking about ;-)
My bad, I neglected an overall doc for the functions and their docstring. But
you know what ? As you're now at an intermediary level, you'll be able to
help: just write down the problems you encountered, and the solutions you
came up with, so that we could use your experience as the backbone for a
proper MaskedArray documentation
> > Oh, and NaNs will be transformed to 0 if you use ints...
> "use ints" in what context?
>>>x = numpy.ma.array([0,1,2,3,])
>>>x[-1] = numpy.nan
>>>[0 1 2 0]
See? No NaNs with an int array.
> > Nope, the idea is really is to make things as efficient as possible.
> For you, maybe. And for me, yes, except I wanted the NaNs to stick
Well, no problem, they should stick around. Note that if a NaN/Inf should
normally show up as the result of some operation (divide by zero for
example), it'll probably won't:
>>>x = numpy.ma.array([0,1,2,numpy.nan],dtype=float)
>>>[-- 1.0 0.5 nan]
>>>[ 1. 1. 0.5 NaN]
>>>[ Inf 1. 0.5 NaN]
> I'd argue that the masked singleton having a different fill value to the
> ma it comes from is a bug.
"It's not a bug, it's a feature"TM
> > And once again, it's not. numpy.ma.masked is a special value, like
> > numpy.nan or numpy.inf
> ...which is silly, since that forces it to have a fixed fill value,
> which it should not.
The fill_value for the mask singleton is meaningless, correct. However, having
numpy.ma.masked as a constant is really helpful to test whether a particular
value is masked, or to mask a particular value:
>>>x = numpy.ma.array([0,1,2,3])
>>>x[-1] = masked
>>>x[-1] is masked
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