[Numpy-discussion] Masked Arrays in NumPy 1.x

Pauli Virtanen pav@iki...
Tue Apr 10 14:18:23 CDT 2012


10.04.2012 06:52, Travis Oliphant kirjoitti:
[clip]
> 	4) I'm still not sure about whether the IGNORED
> concept is necessary or not. I really like the separation
> that was emphasized between implementation (masks versus
> bit-patterns) and operations (propagating versus non-propagating).
> Pauli even created another dimension which I don't totally grok
> and therefore can't remember.   Pauli?  Do you still feel that
> is a necessary construction?

I think the conclusion from that discussion subthread was only that
retaining commutativity of binary operations is probably impossible, if
unmasking of values is allowed. (I think in that discussion the big
difference between IGNORED and MISSING was that ignored values could be
unmasked while missing values could not.)

If I recall correctly, my suggestion was that you might be able to
rescue the situation by changing what assignment means, e.g. in
`x[:5] = y` what gets written to `x` at the points where values in `x`
and/or `y` are masked/ignored. But I think some counterexamples why this
will not work as intended came up.

`numpy.ma` operations are not commutative, which can be sometimes
surprising, but apparently one just has to be pragmatical and live with
this as there's no real way around it. I don't have very good
suggestions on how these features should be designed --- I use them too
seldom.

	Pauli



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