I have a huuuge matrix storing information about X and Y coordinates of multiple particle trajectories , which in simplified version looks like that:

col 1- track number; col 2- frame number; col 2- coordinate X; col 3- coordinate Y

for example:

A = 
1   1   5.14832 3.36128
1   2   5.02768 3.60944
1   3   4.85856 3.81616
1   4   5.17424 4.08384
2   1   2.02928 18.47536
2   2   2.064   18.5464
3   1   8.19648 5.31056
3   2   8.04848 5.33568
3   3   7.82016 5.29088
3   4   7.80464 5.31632
3   5   7.68256 5.4624
3   6   7.62592 5.572

Now I want to filter out trajectories shorter than lets say 2 and keep remaining stuff like (note renumbering of trajectories):

B = 

1   1   5.14832 3.36128
1   2   5.02768 3.60944
1   3   4.85856 3.81616
1   4   5.17424 4.08384
2   1   8.19648 5.31056
2   2   8.04848 5.33568
2   3   7.82016 5.29088
2   4   7.80464 5.31632
2   5   7.68256 5.4624
2   6   7.62592 5.572

How to do it efficiently? I can think about some ideas using for loop and vertcat, but its the slowest solution ever :/

Thanks!