Hi,

On Wed, Jun 29, 2011 at 6:22 PM, Mark Wiebe <mwwiebe@gmail.com> wrote:
> On Wed, Jun 29, 2011 at 8:20 AM, Lluís <xscript@gmx.net> wrote:
>>
>> Matthew Brett writes:
>>
>> >> Maybe instead of np.NA, we could say np.IGNORE, which sort of conveys
>> >> the idea that the entry is still there, but we're just ignoring it.  Of
>> >> course, that goes against common convention, but it might be easier to
>> >> explain.
>>
>> > I think Nathaniel's point is that np.IGNORE is a different idea than
>> > np.NA, and that is why joining the implementations can lead to
>> > conceptual confusion.
>>
>> This is how I see it:
>>
>> >>> a = np.array([0, 1, 2], dtype=int)
>> >>> a[0] = np.NA
>> ValueError
>> >>> e = np.array([np.NA, 1, 2], dtype=int)
>> ValueError
>> >>> b  = np.array([np.NA, 1, 2], dtype=np.maybe(int))
>> >>> m  = np.array([np.NA, 1, 2], dtype=int, masked=True)
>> >>> bm = np.array([np.NA, 1, 2], dtype=np.maybe(int), masked=True)
>> >>> b[1] = np.NA
>> >>> np.sum(b)
>> np.NA
>> >>> np.sum(b, skipna=True)
>> 2
>> >>> b.mask
>> None
>> >>> m[1] = np.NA
>> >>> np.sum(m)
>> 2
>> >>> np.sum(m, skipna=True)
>> 2
>> >>> m.mask
>> [False, False, True]
>> >>> bm[1] = np.NA
>> >>> np.sum(bm)
>> 2
>> >>> np.sum(bm, skipna=True)
>> 2
>> >>> bm.mask
>> [False, False, True]
>>
>> So:
>>
>> * Mask takes precedence over bit pattern on element assignment. There's
>>  still the question of how to assign a bit pattern NA when the mask is
>>  active.
>>
>> * When using mask, elements are automagically skipped.
>>
>> * "m[1] = np.NA" is equivalent to "m.mask[1] = False"
>>
>> * When using bit pattern + mask, it might make sense to have the initial
>>  values as bit-pattern NAs, instead of masked (i.e., "bm.mask == [True,
>>  False, True]" and "np.sum(bm) == np.NA")

>
> There seems to be a general idea that masks and NA bit patterns imply
> particular differing semantics, something which I think is simply false.

Well - first - it's helpful surely to separate the concepts and the
implementation.

Concepts / use patterns (as delineated by Nathaniel):
A) missing values == 'np.NA' in my emails.  Can we call that CMV
(concept missing values)?
B) masks == np.IGNORE in my emails . CMSK (concept masks)?

Implementations
1) bit-pattern == na-dtype - how about we call that IBP
(implementation bit patten)?
2) array.mask.  IM (implementation mask)?