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MathGroup Archive 1995

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Masked array operations

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg1613] Masked array operations
  • From: msreeve at netcom.com (Mark D. Reeve)
  • Date: Thu, 6 Jul 1995 22:12:55 -0400
  • Organization: NETCOM On-line Communication Services (408 261-4700 guest)

Suppose I have a data array, say 1-D, and a boolean (1/0 or True/False) array
of the same length.  How I can I reduce the data array by the boolean array?

e.g. data={1,2,3,4,5,6};
     mask={1,0,0,1,0,1};

I want something like the APL operator "/" such that mask/data yields 
{1,4,6}.  Ideally this operator should work over any arbitrary dimension 
of the array.

You can do it with Select by first pairing the data and the mask, but 
this seems slow.  The best I have come up with is:

ReduceX[arr_,mask_,dim_:1]:=
	Module[{p},
		p=Flatten[Position[mask,1],1];
		Return[Map[#[[p]]&,arr,{dim-1}]];
	];

where dim is the dimension (first is 1) over which to perform the 
reduction.  This seems rather unwieldy for such an outwardly simple 
operation.  Can anyone improve it?

Mark Reeve
msreeve at netcom.com  OR  mreeve at xti.com


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