[Numpy-discussion] maxima of masked arrays

George Nurser agn at noc.soton.ac.uk
Fri Feb 17 06:37:02 CST 2006


I am trying to get the n-1 dimensional array of maxima of an array  
taken over a given axis.

with ordinary arrays this works fine.

E.g.
In [49]: a = arange(1,13).reshape(3,4)

In [50]: a
Out[50]:
array([[ 1,  2,  3,  4],
        [ 5,  6,  7,  8],
        [ 9, 10, 11, 12]])

The maximum over all elements is:
In [51]: print a.max()
12

& the array of maxima over the 0-axis is OK too:
In [52]: print a.max(0)
[ 9 10 11 12]

But with a masked array there are problems.

In [54]: amask = ma.masked_where(a < 5,a)

In [55]: amask
Out[55]:
array(data =
[[999999 999999 999999 999999]
[     5      6      7      8]
[     9     10     11     12]],
       mask =
[[True True True True]
[False False False False]
[False False False False]],
       fill_value=999999)


The maximum over all elements is fine:
In [56]: amask.max()
Out[56]: 12

but trying to get an array of maxima over the 0-axis fails:

n [57]: amask.max(0)
Out[57]:
array(data =
[[999999 999999 999999 999999]
[     5      6      7      8]
[     9     10     11     12]],
       mask =
[[True True True True]
.......


Are there any workarounds for this?

-George.











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