Help with vectorizing find command

Hi,

I'm working on some code for roulette wheel selection and have some problems vectorizing the last two lines of this section.

parents = zeros(1, 2);
indexes = [12; 32; 35; 36; 47];
cs = [0.21; 0.41; 0.58; 0.79; 1.00];
r = rand(1, 2);
parents(1) = indexes(find(r(1) < cs, 1));
parents(2) = indexes(find(r(2) < cs, 1));

I already tried the following, but it resulted in an error because the dimensions of r and cs don't match:

parents = indexes(find(r < cs, 1));

I know that vectorizing this particular example won't make much of a difference in terms of speed, but it will help me get better in terms of programming with Matlab. So any help would be appreciated..

Answer 1

[Azzi Abdelmalek]

parents =arrayfun(@(x) indexes(find(x < cs,1)),r);

But the for loop is 15 faster

Edit

parents=zeros(1,numel(r));  % Pre-allocate
for k=1:numel(r)
  parents(k)=indexes(find(r(k)< cs,1));
end

[dave]

Thanks for your help, Azzi. If the for loop is so much faster, I guess I'm gonna stick with it and forget about vectorizing these lines.

[Azzi Abdelmalek]

This does not mean there is no better way. In the above code I forget to Pre-allocate. Look at Edit

[Azzi Abdelmalek]

Speed test for a code in the loop (1000 times)

Elapsed time is 0.312394 seconds. Dpb's answer
Elapsed time is 0.009241 seconds.   Azzi for loop answer
Elapsed time is 0.181920 seconds.   Azzi's arrayfun answer

Answer 2

[dpb]

parents=accumarray([1:2]',r,[],@(x) indexes(find(x<cs,1)));

[dave]

Thanks dpb...nice one!

[dpb]

Didn't time it...that wasn't part of the request... :)