Question

I have a matrix sorted in ascending order.

S = 25;
RT = zeros(S,2);
  for i = 1:S;
    for j = 1:i;
    R = i *j;
    T = R + j;
    RT(j,:) = [R T];
    end

  end
sortRT = sortrows(RT, [1 2]);
disp(sortRT);

I want to find the sortRT elements which values is lower than 500 (for R) and 490 (for T) per column and place these values inside a matrix. Is it possible?

Answer 1

Just use find:

idx = find(sortRT(:,1)<500 & sortRT(:,2)<490)

idx' = 

 1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18

These are the rows where both R<500 and T<490. You can of course separate these two:

idxR500 = find(sortRT(:,1)<500);
idxT490 = find(sortRT(:,2)<490);

If you’re just going to copy elements or rows, then find isn’t even necessary and you can use logical indexing:

R500 = sortRT(find(sortRT(:,1)<500) , 1);

is the same as

R500 = sortRT(sortRT(:,1)<500 , 1);

This copies elements of the first column, if you want to copy the whole row, use the colon operator:

R500 = sortRT(sortRT(:,1)<500 , :);