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Obtaining all the indices of matached elements of one array in another

Asked by Mohammed Almatrafi on 1 Nov 2017
Latest activity Answered by Carl Witthoft on 27 Sep 2019
Given two vectors A and B, I'd like to find all the indices of matched elements of B in A. So if I have:
A=[1 2 1 2 2 3 1];
B=[1 2 3];
output=[1 3 7;2 4 5;6 0 0].
The first row corresponds to find(A==B(1)), and the second row corresponds to find(A==B(2)), etc;
I know this can be done easily with for loop, but looping over the vector B will be really slow as I have a really big vector of A and vector of B with 43200 elements. Moreover, I am doing a long process after this step, so I am looking for solution through vectorization. I tried different approaches, but non gave me all indices! I am not sure if there's solution through vectorization, but I thought I'd ask here.
Any ideas?

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3 Answers

Answer by Eric
on 1 Nov 2017
Edited by Eric
on 1 Nov 2017

You will run into problems if there are different amounts of A which equal B. If you care to know both indicies, use something like this:
[ia,ib] = find(bsxfun(@eq,shiftdim(A,1),B));
It will return all possible matches of all values of B to A, where the ib are the indicies of B and ia are the indicies in A. If you know for sure that there will always be 3 matches in all of A to every B, you can add something like
output = reshape(ia,[3 numel(ia)/3])';
to get your above output (confirmed with your provided test case of A and B).

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Thanks for your answer. there are different functions that can achieve the same ia and ib obtained by your function. the problem is that the number of repetitions of each element of B in A is different.
Thanks again.

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Answer by Jos (10584)
on 1 Nov 2017

As Eric said, you will run into trouble in most cases. I suggest you store the output in a cell array, like this:
A = [1 2 1 1 2 3]
B = [1 2 3 4]
output = arrayfun(@(x) find(A==x), B, 'un', 0)
% output{k} holds all the indices into A, where A equals B(k)

  2 Comments

Thank you for your suggested function. However, it seems that it takes more time than it takes with for loop to obtain the desired result.
Elapsed time is 0.001405 seconds. // Time taken with your function
Elapsed time is 0.000040 seconds. // with for loop
thanks a lot for you contribution.
well written for-loops with pre-allocation are nowadays very fast. As you asked for a vectorised solution, I just provided one :)

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Answer by Carl Witthoft on 27 Sep 2019

If your version of MATLAB isn't too old, look up help on intersect, to wit;
[dat, idxa, idxb] = intersect(a,b,'stable')

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