[Numpy-discussion] Questions about masked arrays

Gökhan Sever gokhansever@gmail....
Tue Oct 6 17:57:05 CDT 2009


On Tue, Oct 6, 2009 at 4:28 PM, Pierre GM <pgmdevlist@gmail.com> wrote:

>
> On Oct 6, 2009, at 4:42 PM, Gökhan Sever wrote:
>
> > Hello,
> >
> > I have a sample masked array data as shown below.
> >
> > 1-) When I list the whole array I see the fill value correctly.
> > However below that line, when I do access the 5th element,
> > fill_value flies upto 1e+20. What might be wrong here?
>
> Nothing. Your 5th element is the special constant numpy.ma.masked,
> which has its own filling_value by default. I'll check whether it's
> worth inheriting the fill_value from the original array. If you could
> give me a test case where you'd need that value to keep the original
> filling_value, that'd help me make up my mind.
>

Seeing a different filling value is causing confusion. Both for myself, and
when I try to demonstrate the usage of masked array to other people. Also
say, if I want to replace that one element back to its original state will
it use fill_value as 1e+20 or 999999.9999?


>
> > 2-) What is wrong with the arccos calculation? Should not that
> > result the same as with cos(d) result?
>

I first tested on 1.3.0, and later on my laptop using 1.4dev version which
is about an old month built.

Once again the results for each arc... function

I[31]: d
O[31]:
masked_array(data = --,
             mask = True,
       fill_value = 1e+20)


I[26]: arccos(d)
O[26]:
masked_array(data = 1.57079632679,
             mask = False,
       fill_value = 1e+20)


I[28]: arccosh(d)
O[28]:
masked_array(data = nan,
             mask = False,
       fill_value = 1e+20)


I[30]: arcsin(d)
O[30]:
masked_array(data = 0.0,
             mask = False,
       fill_value = 1e+20)


I[32]: arcsinh(d)
O[32]:
masked_array(data = --,
             mask = True,
       fill_value = 1e+20)


I[33]: arctan(d)
O[33]:
masked_array(data = --,
             mask = True,
       fill_value = 1e+20)


I[35]: arctanh(d)
O[35]:
masked_array(data = 0.0,
             mask = False,
       fill_value = 1e+20)


Only arcsinh and arctan results correctly.



>
> Mmh, what numpy are you using ? When I try with a recent one,
> np.arccos does output ma.masked...
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-- 
Gökhan
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