Question

I have a question.
Suppose I have matrix
A =

 1     2     3
 4     5     6
 7     8     9
10    11    12

I need to select n rolling rows from A and transpose elements in new matrix C in rows.
The loop that I use is:

n = 3;     %for instance every 3 rows of A

B = []; 

for i = 1:n 

    Btemp = transpose(A(i:i+size(A,1)-n,:)); 

    B = [B;Btemp]; 

end

C=B';

and that produces matrix C which is:

C =

 1     2     3     4     5     6     7     8     9
 4     5     6     7     8     9    10    11    12

This is what i want too do, but can I do the same job without the loop?

It takes 4 minutes to calculate for an A matrix of 3280×35 size.

Answer 1

I think you can make it work very fast if you make initialization. And one other trick is to take the transpose first, since MATLAB uses columns as first index instead of rows.

tic
A =  reshape(1:3280*35,[3280 35])'; %# Generate an example A
[nRows, nCols] = size(A);

n = 3; %for instance every 3 rows of A
B = zeros(nRows-n+1,nCols*n);
At = A';
for i = 1:size(B,1)
    B(i,:) = reshape(At(:,i:i+n-1), [1 nCols*n]); 
end
toc

The elapsed time is

Elapsed time is 0.004059 seconds.