Filter cell array of objects

I have a .net array object where each object is called Centroid. It has the following structure.

ans = 
  Centroid with properties:
             x: 112.5769
             y: 29.5762
         count: 1250
      strength: 12.3399
  ans = 
    Centroid with properties:
             x: 21.5000
             y: 18.0690
         count: 58
      strength: 12.3400

I would like to create a cell array that only a cell for each of these objects. But I only want the ones that are greater than zero. Since there are 5 million of these. The following code works, but is too slow (stepper is the .net object that returns the Centroids).

              ctroids = this.stepper.centers;
              out = {};
              j = 0;
              for i = 1:ctroids.Length;
                  if (ctroids(i).count >0)
                      j = j+1;
                      out{j} = ctroids(i);
                  end
              end

Answer 1

[Guillaume]

Wouldn't this work:

allctroids = cell(this.stepper.centers); %convert .Net Array into cell array
filteredctroids = allctroids(cellfun(@(c) c.count > 0, allctroids));

[Doctor G]

allctroids = cell(this.stepper.centers);
Error using cell
Conversion to cell from PipeLine.Centroid[] is not possible.

Here is the defenition of centers:

centers: [1x1 PipeLine.Centroid[]]

Answer 2

[Jan]

Try to pre-allocate the output:

ctroids = this.stepper.centers;
out = cell(1, ctroids.Length);
j = 0;
for i = 1:ctroids.Length;
  if (ctroids(i).count >0)
     j = j+1;
     out{j} = ctroids(i);
  end
end
out = out(1:j);

Please explain, what "too slow" exactly means. It matters if it takes days and you need minutes, of if you talk about a real-time processing.

[Doctor G]

It was taking about 10 minutes, on a 64 bit, 4 core, alienware A51. (Windows 7). I will try this out and get back to you. I don't need real time, but under a minute would be much better. Best is under 10seconds.

[Doctor G]

I implemented your code as part of my clean() method. At the end is some work to get just the (x,y) pairs out of the cells (which I need for the plot shown afterwards, though I think this might also be simplified). But the bad news is that the issue is not about pre-allocation. The tic/toc timeing came out at 588.5401 which is 9.8 minutes.

        function clean(this)
            tic;
            ctroids = this.centers;
            out = cell(1, ctroids.Length);
            j = 0;
            for i = 1:ctroids.Length;
                if (ctroids(i).count >0)
                    j = j+1;
                    out{j} = ctroids(i);
                end
            end
            this.cleanTime = toc;
            out = out(1:j);
            for i = 1:j
                this.cx(i) = out{i}.x+1;
                this.cy(i) = out{i}.y+1;
            end
        end
        function plot(this)
            hold on
            this.show(this.mask);
            plot(this.cx, this.cy, 'r.');
            hold off
        end