[Numpy-discussion] numpy masked array oddity

Russell E. Owen rowen@cesmail....
Mon May 5 12:19:40 CDT 2008


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.

-- Russsell



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