[Numpy-discussion] Unexpected MaskedArray behavior
Wed Dec 17 10:57:03 CST 2008
Pierre GM wrote:
> On Dec 16, 2008, at 1:57 PM, Ryan May wrote:
>> I just noticed the following and I was kind of surprised:
>>>>> a = ma.MaskedArray([1,2,3,4,5], mask=[False,True,True,False,False])
>>>>> b = a*5
>> masked_array(data = [5 -- -- 20 25],
>> mask = [False True True False False],
>> array([ 5, 10, 15, 20, 25])
>> I was expecting that the underlying data wouldn't get modified while
>> masked. Is
>> this actual behavior expected?
> Meh. Masked data shouldn't be trusted anyway, so I guess it doesn't
> really matter one way or the other.
> But I tend to agree, it'd make more sense leave masked data untouched
> (or at least, reset them to their original value after the operation),
> which would mimic the behavior of gimp/photoshop.
> Looks like there's a relatively easy fix. I need time to check whether
> it doesn't break anything elsewhere, nor that it slows things down too
> much. I won't have time to test all that before next week, though. In
> any case, that would be for 1.3.x, not for 1.2.x.
> In the meantime, if you need the functionality, use something like
I agree that masked values probably shouldn't be trusted, I was just surprised to
see the behavior. I just assumed that no operations were taking place on masked
Just to clarify what I was doing here: I had a masked array of data, where the
mask was set by a variety of different masked values. Later on in the code,
after doing some unit conversions, I went back to look at the raw data to find
points that had one particular masked value set. Instead, I was surprised to see
all of the masked values had changed and I could no longer find any of the
special values in the data.
Graduate Research Assistant
School of Meteorology
University of Oklahoma
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