How to Delete certain rows in a Matrix

I have a matrix that is 2 columns wide and 18,000 rows long. I wish to save the first 15 rows, then delete 90, save 15 more and remove 90 more rows. and so on until the matrix is completed. I am having trouble with this. should i use loops?

Answer 1

[dpb]

Use indexing expressions...

N1=15;                                        % number to save
N2=90;                                       % number to skip
i1=1:N1+N2:size(x,1);                         % locations to begin to save
i2=N1:N1+N2:size(x,1);                        % end locations to save
ix=cell2mat(arrayfun(@colon,i1,i2,'uni',0));  % the addressing vector of those to save
M=M(ix,:);                                    % select the desired rows only

One can do this without the intermediaries but this makes the logic more transparent.

[Grant Poggenpohl]

Thank you. very helpful

[dpb]

No problem. BTW, for large arrays, subset selection will be quite a bit quicker than deletion I suspect altho I didn't try timing...