is it possible to use "find" to process every element of an array without loop

Question

zhang 2012-5-11

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Hi guys,

There is an array, saying a1, a2, ..., an.

For every element in the array, I want to do something like find(X<ai), where "X" is another array.

Is it possible to do it without loop and "cellfun"?

It seems cellfun is very slow.

Or maybe another way even not using find?

Thanks,

Zhong

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Daniel Shub 2012-5-11

So for every element of array a you want a list of the elements of array X that are less than the current element of a and you want to do this without a loop? How big are your a and X? I would bet a loop working on a sorted X and a would be super fast.

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Answer 1

Andrei Bobrov 2012-5-11

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https://ww2.mathworks.cn/matlabcentral/answers/38097-is-it-possible-to-use-find-to-process-every-element-of-an-array-without-loop#answer_47560

在 MATLAB Online 中打开

eg:

a = randi(9,7,1);
X = randi(9,12,1);

solution:

[I,J] = find(bsxfun(@lt,X,a.'));
out = accumarray(J,I,[],@(x){x});

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Jonathan Sullivan 2012-5-11

I must say, that is a very slick solution!

I think you might have missed a close parenthesis at the end of your last line. It should read:

out = accumarray(J,I,[],@(x){x});

Andrei Bobrov 2012-5-12

Thank you, yes!

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Answer 2

Daniel Shub 2012-5-11

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/38097-is-it-possible-to-use-find-to-process-every-element-of-an-array-without-loop#answer_47530

在 MATLAB Online 中打开

I know it has a loop, but what about something like this:

a = randperm(1e5);
X = 2e3*randn(1, 1e5);
[b, ai] = sort(a);
[Y, Xi] = sort(X);
z = cell(size(a));
for iz = 1:length(z)
  ii = find(Y<b(iz));
  z{ai(iz)} = Xi(ii);
  jj = 0;
  if ~isempty(ii)
    jj = max(ii);
  end
  Y = Y((jj+1):end);
  Xi = Xi+jj;
end

Even with two "big" vectors, the code completes pretty fast. Given I take the time to sort the data, I bet there is a way to optimize the find and get even faster performance.