Help with vectorizing find command

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dave
dave 2013-7-29
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..

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回答（2 个）

Azzi Abdelmalek
Azzi Abdelmalek 2013-7-29
编辑：Azzi Abdelmalek 2013-7-29
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
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Azzi Abdelmalek
Azzi Abdelmalek 2013-7-29
编辑：Azzi Abdelmalek 2013-7-29
This does not mean there is no better way. In the above code I forget to Pre-allocate. Look at Edit
Azzi Abdelmalek
Azzi Abdelmalek 2013-7-29
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

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dpb
dpb 2013-7-29
parents=accumarray([1:2]',r,[],@(x) indexes(find(x<cs,1)));
  2 个评论
dpb
dpb 2013-7-29
Didn't time it...that wasn't part of the request... :)

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