Extend a matrix by extending its vectors

I have an d*m*n matrix A (interpretation: for each n, there are m vectors of dimension d). I would like to extend each d-dimensional vector as follows:

Consider a vector of dimension d: (1,2,...,d) (it's (x_1,x_2,...,x_n) but I removed the 'x_' for simplicity). The goal is to extend v to obtain a d*d vector of the form:

(1,1,...,1,2,...,2,...,d,d,...,d)

There are several ways to extend a single vector, as already suggested here. Suppose that a function called 'extend' does that job. Then, to extend every vector in the matrix A, we can do:

B = zeros(d*d,m,n);

for  i = 1:n
    for j = 1:m
        B(:,j,i) = extend(A(:,j,i);
    end
end

However, this is quite slow because of the loop. Could anybody please suggest me a fast way to do that?

Thank you in advance for your help.

[José-Luis]

What makes you thing it's slow because of the loop? Not all loops are evil. It would depend on what your extend() function looks like.

[f10w]

Hmm, because using a loop is slow in general (but I may be wrong :P).

Answer 1

[Andrei Bobrov]

B = A(kron((1:d)',ones(d,1)),:,:);

[f10w]

Thanks, Andrei. That answers my question.