Question

Suppose a 3-D matrix :

>> a = rand(3,4,2)

a(:,:,1) =

    0.1067    0.7749    0.0844    0.8001
    0.9619    0.8173    0.3998    0.4314
    0.0046    0.8687    0.2599    0.9106


a(:,:,2) =

    0.1818    0.1361    0.5499    0.6221
    0.2638    0.8693    0.1450    0.3510
    0.1455    0.5797    0.8530    0.5132

I use linear indexing to have many element at a time:

>> index1 = [1 ; 2 ; 1 ; 3];
>> index2 = [1 ; 4 ; 2 ; 3];
>> index3 = [1 ; 1 ; 2 ; 1];

>> indices = sub2ind(size(a), index1, index2, index3)

>> a(indices)

ans =

    0.1067
    0.4314
    0.1361
    0.2599

I would like to do the same thing, put return all the values of the first dimensions. The size of this dimension may vary. The return should be, in that case:

>> indices = sub2ind(size(a), ??????, index2, index3);

>> a(indices)

ans =

    0.1067    0.9619    0.0046    % a(:,1,1)
    0.8001    0.4314    0.9106    % a(:,4,1)
    0.1361    0.8693    0.5797    % a(:,2,2)
    0.0844    0.3998    0.2599    % a(:,3,1)

Any way to do that in MatLab?

Answer 1

ind1 = repmat((1:size(a,1)),length(index2),1);
ind2 = repmat(index2,1,size(a,1));
ind3 = repmat(index3,1,size(a,1));

indices = sub2ind(size(a),ind1,ind2,ind3)

indices =

 1     2     3
10    11    12
16    17    18
 7     8     9

a(indices)


ans =

    0.1067    0.9619    0.0046
    0.8001    0.4314    0.9106
    0.1361    0.8693    0.5797
    0.0844    0.3998    0.2599