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

Charles R Harris charlesr.harris at gmail.com
Mon Sep 30 21:38:58 EDT 2013

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.

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