I need help preallocating an array of position vectors

Creating an array that stores the position vectors of circles detected in each frame of a video, how would i go about preallocating posvec?

posvec = [];
for frame_no = 1 : stack_size(4)
    framecropped = frame_stack(:,:,:,frame_no);
    imshow(framecropped);
    [pos,rad] = imfindcircles(framecropped,[20 100]);
    viscircles(pos,rad);
    drawnow
    posvec = [posvec; pos];
end

Answer 1

[Stephen23]

Use a cell array:

tmpout = cell(1,stack_size(4))
for frame_no = 1:stack_size(4)
    ..
    tmpout{frame_no} = pos;
end
matrix = vertcat(tmpout{:}); % optional

[Alexander Watkins]

Thank you! Although after my previous code, the circle y coordinates for each frame were also scattered on a graph and now this section breaks, why is this and what do I need to change?

s = size(posvec);
frames = (1:1:s(1));
figure(2)
scatter(frames,posvec(:,2));
line(frames,posvec(:,2));

[Stephen23]

Did you try plotting the data in matrix ?

[Alexander Watkins]

Ah I just forgot to change the vector name from posvec to the vertcat of posvec, thank you.