On 5/25/11 3:27 PM, Catherine Moroney wrote:
Hello,
I am trying to work with a structured array and a mask, and am encountering some
problems.
You will want to ask numpy questions on the numpy mailing list:
http://www.scipy.org/Mailing_Lists
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?
Only the final [] on the left-hand side of the assignment actually turns into a
.__setitem__() call to the object that is the result of the expression to its
left. a[mask] makes a copy while a2[:] makes a view.
You could do
a["n"][mask] += b[mask]
since a["n"] will also make a view and the .__setitem__() on it will propagate
back to the original memory.
--
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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