For the first part, see if this does what you want:
M = randi([-5 5], 10, 5); % Create Data
[R,C] = find(M > 0); % Positive Numbers
[Cu,ia] = unique(C); % First Instance In ‘ia’
FirstPos = [Cu R(ia)] % [Column Row] Meeting Criteria
Then you can subtract those numbers from whatever number you choose.
If you want the rows and columns to be linear indices instead (this can be easier to work with), use the sub2ind function to do the conversion from the ‘FirstPos’ array.
EDIT — Corrected typographical errors.
