[Numpy-discussion] numpy masked array oddity
Robert Kern
robert.kern@gmail....
Mon May 5 12:33:20 CDT 2008
On Mon, May 5, 2008 at 12:19 PM, Russell E. Owen <rowen@cesmail.net> wrote:
> The object returned by maskedArray.compressed() appears to be a normal
> numpy array (based on repr output), but in reality it has some
> surprising differences:
>
> import numpy
> a = numpy.arange(10, dtype=int)
> b = numpy.zeros(10)
> b[1] = 1
> b[3] = 1
> ma = numpy.core.ma.array(a, mask=b, dtype=float)
> print ma
> # [0.0 -- 2.0 -- 4.0 5.0 6.0 7.0 8.0 9.0]
> c = ma.compressed()
> print repr(c)
> # array([ 0. 2. 4. 5. 6. 7. 8. 9.])
> c.sort()
> #Traceback (most recent call last):
> # File "<stdin>", line 1, in <module>
> # File
> "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-pac
> kages/#numpy/core/ma.py", line 2132, in not_implemented
> # raise NotImplementedError, "not yet implemented for numpy.ma arrays"
> #NotImplementedError: not yet implemented for numpy.ma arrays
> d = numpy.array(c)
> d.sort()
> # this works fine, as expected
>
> Why is "c" in the example above not just a regular numpy array? It is
> not a "live" view (based on a quick test), which seems sensible to me.
> I've worked around the problem by making a copy (d in the example
> above), but it seems most unfortunate to have to copy the data twice.
I don't know the reason why it's not an ndarray, but you don't have to
copy the data again to get one:
c = ma.compressed().view(numpy.ndarray)
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
-- Umberto Eco
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