[Numpy-discussion] MA - math operations do not preserve fill_value

Joe Van Andel vanandel at atd.ucar.edu
Tue Nov 6 13:15:04 CST 2001


Using Python 2.1 and Numeric 20.2.1 on Redhat Linux 7.1

Shouldn't masked arrays preserve the fill value of their operands, if
both operands have the same fill value?  Otherwise, if I want to
preserve the value of the fill_value, I have to write expressions like:


d=masked_values((a+b),a.fill_value()) 

Here's a demonstration of the problem:

>>> a = masked_values((1.0,2.0,3.0,4.0,-999.0), -999)
>>> b = masked_values((-999.0,1.0,2.0,3.0,4.0), -999)

>>> a
array(data =
 [   1.,   2.,   3.,   4.,-999.,],
      mask =
 [0,0,0,0,1,],
      fill_value=-999)
 
>>> b
array(data =
 [-999.,   1.,   2.,   3.,   4.,],
      mask =
 [1,0,0,0,0,],
      fill_value=-999)

>>> c=a+b
>>> c
array(data =
 [  1.00000002e+20,  3.00000000e+00,  5.00000000e+00,  7.00000000e+00,
        1.00000002e+20,],
      mask =
 [1,0,0,0,1,],
      fill_value=[  1.00000002e+20,])

>>> d=masked_values((a+b),a.fill_value())
>>> d
array(data =
 [-999.,   3.,   5.,   7.,-999.,],
      mask =
 [1,0,0,0,1,],
      fill_value=-999)
-- 
Joe VanAndel  	          
National Center for Atmospheric Research
http://www.atd.ucar.edu/~vanandel/
Internet: vanandel at ucar.edu




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