[Numpy-discussion] missing data discussion round 2

Mark Wiebe mwwiebe@gmail....
Tue Jun 28 19:49:40 CDT 2011


On Tue, Jun 28, 2011 at 6:57 PM, Pierre GM <pgmdevlist@gmail.com> wrote:

>
> On Jun 29, 2011, at 1:39 AM, Mark Wiebe wrote:
>
> > On Tue, Jun 28, 2011 at 5:20 PM, Matthew Brett <matthew.brett@gmail.com>
> wrote:
> > Hi,
> >
> > 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?
> >
> > My favorite way of explaining it would be to have a grid of numbers
> written on paper, then have several cardboards with holes poked in them in
> different configurations. Placing these cardboard masks in front of the grid
> would show different sets of non-missing data, without affecting the values
> stored on the paper behind them.
>
> And when there's a hole (or just a blank) in your piece of paper ?


A hole means an unmasked element, no hole means a masked element.

-Mark


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