Efficient way to restrict matrix within specified values

Question

I am working on a big Matlab testbench with thousands of lines of code, and I am trying to optimize the most time-consuming routines, determined via the profiler in Matlab. I noticed that one of those most time-consuming operations is the following:

list = list((list(:,1) >= condxMin) & (list(:,1) <= condxMax) & (list(:,2) >= condyMin) & (list(:,2) <= condyMax),:);

Concretely, I have a big list of coordinates (50000 x 2 at least) and I want to restrict the values of this list so as to keep only the points that verify both of these conditions : list(:,1) must be within [condxMin, condxMax] and list(:2) within [condyMin condyMax].

I was wondering if there was a more efficient way to do it, considering that this line of code is already vectorized. Also, I am wondering if Matlab does a short-circuiting or not. If it doesn't, then I don't think there is a way to do it without breaking the vectorization and do it with a while loop, where I would write instead something like this:

j=1;
for i=1:size(list,1)
   if(cond1 && cond2 && cond3 && cond4)
      newlist(j,1:2) = list(i,1:2);
      j=j+1;
   end
end

Thank you in advance for your answer :)

Answer 1

Looks like the original vectorized version is the fastest way I can find, barring any really clever ideas. Matlab does do short circuiting, but not for matrices. The loop implementation you you showed would be very slow, since you're not pre-allocating (nor are you able to pre-allocate the full matrix).

I tried a couple of variations on this, including a for loop which used a short circuited && to determine whether the index was bad or not, but no such luck. On the plus side, the vectorized version you've got runs at 0.21s for a 5 million element coordinate list.