[Numpy-discussion] missing data discussion round 2

Matthew Brett matthew.brett@gmail....
Tue Jun 28 17:20:08 CDT 2011


On Tue, Jun 28, 2011 at 4:06 PM, Nathaniel Smith <njs@pobox.com> wrote:
> (You might think, what difference does it make if you *can* unmask an
> item? Us missing data folks could just ignore this feature. But:
> whatever we end up implementing is something that I will have to
> explain over and over to different people, most of them not
> particularly sophisticated programmers. And there's just no sensible
> way to explain this idea that if you store some particular value, then
> it replaces the old value, but if you store NA, then the old value is
> still there.

Ouch - yes.  No question, that is difficult to explain.   Well, I
think the explanation might go like this:

"Ah, yes, well, that's because in fact numpy records missing values by
using a 'mask'.   So when you say `a[3] = np.NA', what you mean is,
'a._mask = np.ones(a.shape, np.dtype(bool); a._mask[3] = False`"

Is that fair?

See you,


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