[Numpy-discussion] masked array with mask an instance of numpy.memmap

Pierre GM pgmdevlist@gmail....
Tue Mar 27 16:54:05 CDT 2007

On Tuesday 27 March 2007 17:42:18 Glen W. Mabey wrote:
> However, using a memmap for the mask value seems to load the entire
> object into memory, even if it is already of type bool.
> How hard would it be to change that behavior?

Oh, I have no idea about that...

> Also, it seems that median() is not implemented on masked arrays?

Great minds meet, I just uploaded some very basic stats functions for the new 
implementation of masked array in the scipy SVN.

> Is there some method for a masked array like ravel() but that only
> includes the non-masked values?

Yep, compressed(). Note that it will flatten your array first, so if you work 
with nd arrays, that's not the best solution. If you work with 2d arrays, you 
can simply loop on the other dimension, such as

(n,p) = data.shape
med = numpy.empty((n,),dtype=data.dtype)
for i in range(p):
	med[i] = numpy.median(data[:,i].compressed())

That works well if p < n/3.

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