Hello,
I am trying to work with a structured array and a mask, and am
encountering some problems.
For example:
>>> xtype = numpy.dtype([("n", numpy.int32), ("x", numpy.float32)])
>>> a = numpy.zeros((4), dtype=xtype)
>>> b = numpy.arange(0,4)
>>> a2 = numpy.zeros((4), dtype=xtype)
>>> mask = numpy.where(b%2 == 0)
>>> a2[:]["n"] += b ! this changes the values of a2
>>> a[mask]["n"] += b[mask] ! this does not change the values of a
>>> a2
array([(0, 0.0), (1, 0.0), (2, 0.0), (3, 0.0)],
dtype=[('n', '<i4'), ('x', '<f4')])
>>> a
array([(0, 0.0), (0, 0.0), (0, 0.0), (0, 0.0)],
dtype=[('n', '<i4'), ('x', '<f4')])
Why do the values of a2 get updated, and yet the values of a do not?
How do I update a's contents using a mask?
If "a" is not a record array, but instead simply an array of ints, then
the a[mask] += b[mask] statement does alter the values of a.
>>> a = numpy.zeros((4))
>>> a[mask] += b[mask]
>>> a
array([ 0., 0., 2., 0.])
What is it about a numpy record array that prevents the mask statement
from working, and how do I get around this?
Thanks,
Catherine
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