Extract values from 3'rd dimmenision with 2D matrix indices

Hi,

Say I have 3D matrix (A), and 2D matrix (B) which each element is the third dimmesnions index I want to extract from A:

A = rand(3,3,3);

B = [1 1 1;

2 2 2;

1 2 3];

So if A is:

A(:,:,1) =

0.8147 0.9134 0.2785

0.9058 0.6324 0.5469

0.1270 0.0975 0.9575

A(:,:,2) =

0.9649 0.9572 0.1419

0.1576 0.4854 0.4218

0.9706 0.8003 0.9157

A(:,:,3) =

0.7922 0.0357 0.6787

0.9595 0.8491 0.7577

0.6557 0.9340 0.7431

Then the result C would be

C = [0.8147 0.9134 0.2785;

0.1576 0.4854 0.4218;

0.1270 0.8003 0.7431];

How can I find C in matlab?

Would very appreciate any help with that

Thanks in Advence!

Answer 1

[Daniel]

Hi omri,

hope this helps:

mult = zeros(3,3,3);
mult(:,:,:) = reshape([B==1,B==2,B==3],[3,3,3]);
A_1 = A .* mult;
C = sum(A_1,3)

Cheers,

Daniel

[omri r]

Thank you very much Daniel!

This works.

How would I do that in more general way when I have big matrices?

Lets say that, A is 768x1024X72 elements.

and B is 768X1024 elemnts (the values of B can be 1 to 72)

What would be the best approch in this case?

Omri

[Daniel]

In this case I would use a for-loop:

A_1 = zeros(768,1024,72);
for i=1:72
    A_1(:,:,i) = A(:,:,i) .* double(B==i);
end
C = sum(A_1,3);

[omri r]

Works like a charm.

Thank you very much!