MATLAB: Can I get rid of this for loop

marginal meansvectorization

I have a matrix that has 3 columns that consist of -1, 0, and 1 (let's call them the "level columns"), and a fourth column that contains real numbers (call it the "data column"). For the level columns, I want to find the average of the "-1", "0", and "1" terms in the data column. So for the following matrix,
    -1    -1     0   837
    -1     0    -1   880
    -1     0     1   879
    -1     1     0   864
     0    -1    -1   834
     0    -1     1   833
     0     0     0   874
     0     0     0   875
     0     0     0   876
     0     1    -1   860
     0     1     1   859
     1    -1     0   829
     1     0    -1   872
     1     0     1   874
     1     1     0   856
the -1 average for column 1 would be 865. Is there a way that I can do that all at once, for each level column, and each level column value? Right now, I loop over the level columns and do this:
[mean(testMat(testMat(:,col) == -1,4)) mean(testMat(testMat(:,col) == 0,4)) mean(testMat(testMat(:,col) == 1,4))]
Is there a better way?
Thanks!

Best Answer

You cannot get rid of the for loop, but you can hide it inside a cellfun call. Note that my answer works for any number of columns, unlike the other solutions.
M = [...
-1    -1     0   837
-1     0    -1   880
-1     0     1   879
-1     1     0   864
 0    -1    -1   834
 0    -1     1   833
 0     0     0   874
 0     0     0   875
 0     0     0   876
 0     1    -1   860
 0     1     1   859
 1    -1     0   829
 1     0    -1   872
 1     0     1   874
 1     1     0   856 ];
%
[~,~,X] = cellfun(@unique,num2cell(M(:,1:end-1),1),'Uni',0);
N = cellfun(@(s)accumarray(s,M(:,end),[],@mean),X, 'Uni',0);
out = [N{:}]
creates this output matrix, where the rows correspond to [-1;0;1]:
>> out
out =
        865       833.25        861.5
     858.71       875.71       858.71
     857.75       859.75       861.25
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