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MathGroup Archive 1995

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Re: Masked array operations

  • To: mathgroup at christensen.cybernetics.net
  • Subject: [mg1666] Re: [mg1613] Masked array operations
  • From: Lou Talman <me at talmanl.mscd.edu>
  • Date: Tue, 11 Jul 1995 04:27:14 -0400

Try:

In[1]:=
  Chi[0, x_] := {};
  Chi[1, x_] := {x};
  mask = {1, 0, 0, 1, 0, 1};
  data = {1, 2, 3, 4, 5, 6};

In[5]:=
  Inner[Chi, mask, data, Join]

Out[5]=
  {1, 4, 6}

Begin forwarded message:

>Date: Thu, 6 Jul 1995 22:12:55 -0400
>From: msreeve at netcom.com (Mark D. Reeve)
>Subject: [mg1613] Masked array operations 

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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