A subset of 3 dimensional matrix in Matlab

Question

Lets assume we have a 3 dimensional matrix A and X_IND = 4:8 and Y_IND = f(X_IND). f is a function like 2*x^2+1. How I can extract following vector out of A:

a = A(X_IND,Y_IND,3)

However, above equation in MATLAB leads to a matrix while the result should be an array since Y_IND is function of X_IND.

Answer 1

Use sub2ind to build a linear index. Here's an example:

>> A = randi(9,2,3,3)
A(:,:,1) =
     6     8     8
     5     2     7
A(:,:,2) =
     8     7     9
     8     7     2
A(:,:,3) =
     8     9     8
     2     4     8

>> X_IND = [1 2]; 
>> Y_IND = X_IND + 1;
>> Z_IND = 3;

>> Z_IND = repmat(3, size(X_IND)); %// all indices should have the same size

>> ind = sub2ind(size(A), X_IND, Y_IND, Z_IND); %// build linear index

>> A(ind)
ans =
     9     8