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