interesting matrix indexing question without for loops

size(matrixA)=[b,n,m]
size(matrixB)=[n,m]

how can I create

matrixC(i,j)=matrixA(matrixB(i,j),i,j)

without using for-loops? What is this kind of indexing called? Thanks!

Answer 1

[Stephen23]

Use sub2ind to get the linear indices:

A = reshape(1:6*3*2,6,3,2);
B = [6,1;4,3,;5,2];
% Solution with loops:
for ii = 1:size(B,1)
    for jj = 1:size(B,2)
        C(ii,jj) = A(B(ii,jj),ii,jj);
    end
end
% Solution with SUB2IND:
S = size(B);
[I,J] = ndgrid(1:S(1),1:S(2));
X = sub2ind(size(A),B,I,J);
D = A(X)
% Compare:
isequal(C,D)

[Chris Hooper]

Stephen you're awesome thanks! I knew there was something like that... it's been a while!

Answer 2

[Bruno Luong]

 C = A(B+reshape(0:numel(B)-1,size(B))*size(A,1))

[Chris Hooper]

Awesome I'll check this one, cannot even wrap my head around it!!