How to copy a 2d matrix along higher dims e.g. 3rd up to 6th without using for loop?

In general how we can copy an N-dim matrix along higher dims (N+1, N+2, ..) without for loop?

I'm sure we can use repmat to copy a 2D matrix a 3 times along the 3rd dim e.g. (a,[1 1 3]) but I don't know can it work for higher dims.

[madhan ravi]

Why did you delete the previous question?

[nima]

So sorry. I think this question is more general that previous one to solve my problem.

Answer 1

[Matt J]

repmat can be used to copy along more than one dimension, e.g.,

>> repmat(3,4,5)

ans =

     3     3     3     3     3
     3     3     3     3     3
     3     3     3     3     3
     3     3     3     3     3

The real question is why you would want to do that. You would be consuming more memory without storing any new information. For most purposes, repmat can be avoided.

[nima]

Thank u so much.

It's needed to calculate .

if be a 4D matrix, then it is benificial to copy along 3rd and 4th dim and multiply it to and then sum it which is more time consuming than using for loop.

[Matt J]

repmat is not necessary for that. It is far more efficient to reshape this into a matrix-vector multiplication:

[L,M,N,P]=size(b);
B=reshape(b,L*M,N*P);
C=a(:).'*B;
c=reshape(C,L,M);

Note that this involves neither for-loops nor data copying (reshape does not copy any data).

[nima]

Oh. Thanks a lot Matt.