How to add elements to a vector in a loop

Question

Let's say I have a cell array raweeg each cell of which is a matrix with time points in the first column and some markers in the second. I want to create a vector to store the time points, when the marker is not equal to -1. I found a non-elegant (and not working) way to create a vector of zeros of size 1x1 and then to append the following values in a loop.

P.S.: There are exactly 96 non-"-1" values and corresponding time points.

startpoints = zeros(1,1);

for i = length(raweeg{1,1}(:,1))
   if raweeg{1,1}(i,2) ~= -1
   startpoints(end+1,1) = raweeg{1,1}(i,1); 
   end
end

Thank you

Answer 1

Vectorize it like this, for a given cell of raweeg:

startpoints = raweeg{1,1}(raweeg{1,1}(:,2) ~= -1, 1);

This is called logical indexing.

Just be sure your markers are not generated with floating point computations, or the comparisons will likely fail often.

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P.S. The problem in your code is the for loop statement, which should be:

for i = 1:length(raweeg{1,1}(:,1))

Or better, for i = 1:size(raweeg{1,1},1).

With out the "1:" part, it just has a single iteration, the last row.