[Numpy-discussion] Operations on masked items

Ryan May rmay31@gmail....
Tue Feb 3 14:54:37 CST 2009


Pierre,

I know you did some preliminary work on helping to make sure that doing
operations on masked arrays doesn't change the underlying data.  I ran into the
following today.

import numpy as np
a = np.ma.array([1,2,3], mask=[False, True, False])
b = a * 10
c = 10 * a
print b.data # Prints [10 2 30] Good!
print c.data # Prints [10 10 30] Oops.

I tracked it down to __call__ on the _MaskedBinaryOperation class.  If there's a
mask on the data, you use:

	result = np.where(m, da, self.f(da, db, *args, **kwargs))

You can see that if a (and hence da) is a scalar, your masked values end up with
the value of the scalar.  If this is getting too hairy to handle not touching
data, I understand.  I just thought I should point out the inconsistency here.

Ryan

-- 
Ryan May
Graduate Research Assistant
School of Meteorology
University of Oklahoma


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