[Numpy-discussion] Masked arrays: Rationale for "False convention"

[Eric Firing]

On 2013/09/30 4:05 PM, josef.pktd at gmail.com wrote:
> On Mon, Sep 30, 2013 at 9:38 PM, Charles R Harris
> <charlesr.harris at gmail.com> wrote:
>>
>>
>>
>> On Mon, Sep 30, 2013 at 7:05 PM, Ondřej Čertík <ondrej.certik at gmail.com>
>> wrote:
>>>
>>> Hi,
>>>
>>> What is the rationale for using False in 'mask' for elements that
>>> should be included?
>>>
>>> http://docs.scipy.org/doc/numpy/reference/maskedarray.generic.html
>>>
>>> As opposed to using True for elements that should be included, which
>>> is what I was intuitively expecting when I started using the masked
>>> arrays. This "True convention" also happens to be the one used in
>>> Fortran, see e.g.:
>>>
>>> http://gcc.gnu.org/onlinedocs/gfortran/SUM.html
>>>
>>> So it's confusing why NumPy would chose a "False convention". Could it
>>> be, that NumPy views 'mask' as opacity? Then it would make sense to
>>> use True to make a value 'opaque'.
>>
>>
>> There was a lengthy discussion of this point back when the NA work was done.
>> You might be able to find the thread with a search.
>>
>> As to why it is as it is, I suspect it is historical consistency. Pierre
>> wrote the masked array package for numpy, but it may very well go back to
>> the masked array package implemented for Numeric.
>
> I don't know ancient history, but I thought it's "natural". (Actually,
> I never thought about it.)
>
> I always thought `mask` indicates the "masked" (invalid, hidden)
> values, and masked arrays mask the missing values.

Exactly.  It is also consistent with the C and Unix convention of 
returning 0 on success and 1, or a non-zero error code on failure.  In a 
similar vein, it works nicely with bit-mapped quality control flags, 
etc.  When nothing is flagged, the value is good, and consequently not 
masked out.

Eric

>
> http://en.wikipedia.org/wiki/Masking_tape
>
> Josef
>
>>
>> Chuck
>>
>>
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