[Numpy-discussion] Question about numpy.ma masking

[Gökhan Sever]

On Fri, May 7, 2010 at 3:28 PM, Pierre GM <pgmdevlist at gmail.com> wrote:

> On May 4, 2010, at 8:38 PM, Gökhan Sever wrote:
> > Hello,
> >
> > I have the following arrays read as masked array.
> >
> > I[10]: basic.data['Air_Temp'].mask
> > O[10]: array([ True, False, False, ..., False, False, False], dtype=bool)
> >
> > [12]: basic.data['Press_Alt'].mask
> > O[12]: False
> >
> > I[13]: len basic.data['Air_Temp']
> > -----> len(basic.data['Air_Temp'])
> > O[13]: 1758
> >
> >
> > The first item data['Air_Temp'] has only the first element masked and
> this result with mask attribute being created an equal data length bool
> array. On the other hand data['Press_Alt'] has no elements to mask yielding
> a 'False' scalar. Is this a documented behavior or intentionally designed
> this way? This is the only case out of 20 that breaks my code as following:
> :)
> >
> > IndexError                                Traceback (most recent call
> last)
> >
> >     130 for k in range(len(shorter)):
> >     131     if (serialh.data['dccnTempSF'][k] != 0) \
> > --> 132        and (basic.data['Air_Temp'].mask[k+diff] == False):
> >     133         dccnConAmb[k] = serialc.data['dccnConc'][k] * \
> >     134                         physical.data['STATIC_PR'][k+diff] * \
> >
> > IndexError: invalid index to scalar variable.
> >
> > since mask is a scalar in this case, nothing to loop terminating with an
> IndexError.
>
>
> Gokhan,
> Sorry for not getting back sooner, web connectivity was limited on my side.
> I must admit I can't really see what you're tring to do here, but I'll
> throw some random comments:
> * If you're using structured MaskedArrays, it's a really bad idea to call
> one of the fields "data", as it may interact in a non-obvious way with the
> actual "data" property (the one that outputs a view of the array as a pure
> ndarray).
>

Hello Pierre,

basic.data is a dictionary containing all masked array items. When I read
the original data into scripts, my main constructor-reader class
automatically converts data to masked arrays. basic.data['Air_Temp'] is a
masked array itself, little confusing for sure it also has 'data' attribute.


In the above example I check one condition looping in mask value. When mask
attribute isn't an bool-array (when there is no missing value in data) the
condition fails asserting an IndexError. I was wondering why it doesn't
yield a bool-array instead of giving me a scalar False.

-- 
Gökhan
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