# Matlab and manipulating matrices [closed]

Suppose I have a matrix $A$ of size

$n_1 \times n_2 \times n_3$

Now, I have another matrix $B$ of size $n_1 \times n_2 \times N$ where $N<n_3$

I'd like to create the following matrix $C = [A(m,l,B(m,l,:))]_{1 \le m \le n_1, 1 \le l \le n_2}$, whch has the same size as $B$. What is the most efficient way of doing this without having to create loops?

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## closed as off-topic by Shobhit, azimut, Dennis Gulko, amWhy, NorbertOct 29 '13 at 18:40

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "This question is not about mathematics, within the scope defined in the help center." – Shobhit, azimut, Dennis Gulko, amWhy, Norbert
If this question can be reworded to fit the rules in the help center, please edit the question.

If you have three dimensions, then it isn't really a matrix anymore; it's a rank-3 tensor. –  Ｊ. Ｍ. Jul 9 '12 at 17:32
what does $B(m,l)$ mean if $B$ is a 3D array? –  chaohuang Jul 9 '12 at 18:00
This is for mathworks.com/matlabcentral/answers. –  Dirk Jul 9 '12 at 18:14
sorry i should be $B(m,l,:)$, It's basically a vector of size $N$. Thanks –  Stuck_pls_help Jul 9 '12 at 18:48
When it comes to issues like efficiency of numerical algorithms, you may consider asking instead at scicomp.stackexchange.com –  Willie Wong Jul 10 '12 at 6:34
C = A(repmat(reshape(1:n1*n2,n1,n2),[1,1,N])+(n1*n2)*(B-1));

The formula assumes that the elements of B are in the range $[1,n_3]$ not $[0,n_3-1]$. –  p.s. Jul 25 '12 at 23:22
Do you have any suggestions if I wanted to create another matrix of the following form: $C = [A(m,l,B(m,l,n,:),n)]_{1 \le m \le n_1, 1 \le l \le n_2, 1 n\le n_3}$ and $B$ is an $n_1 \times n_2 \times n_3 \times N<n_3$ –  Stuck_pls_help Jul 26 '12 at 3:29