Array doesn't get filled - for loop

hello, the arrays idx_u, idx_o, X wont get filled by the for loop, why?

idx_u = zeros(m,1);
idx_o = zeros(m,1);
X = zeros(m,1);
for r = 1:m
  if nnz(gradMag(:,r)) > 0
    idx_o = find(gradMag(:,r),1,'first');
    idx_u = find(gradMag(:,r),1,'last');
    X = r;
  else
    idx_o = NaN;
    idx_u = NaN;
    X = r;
  end
end

Answer 1

[dpb]

Because you didn't write the array indices inside the loop; you filled each array with a constant each pass thru the loop; the last value found will be the value remaining for all elements.

idx_u = nan(m,1); idx_o = idx_u;  % fill w/ NaN initially; will overwrite finite locations
for r = 1:m
  if any(gradMag(:,r))
    ix=find(gradMag(:,r));   % get all locations in one call
    idx_o(i)=min(ix);        % I'm guessing but you can test that is as
    idx_u(i)=max(ix);        % fast or faster than two calls to FIND()
  end
end
X=1:m;  % is just the index vector; no need for inside loop

[Payjay]

of course! i got it later in the day, sorry for the obvious question! thanks, anyways!