Number of times the rows of a small matrix appear amongst the rows of a larger matrix, vectorization

Question

Adri 2020-6-13

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/547716-number-of-times-the-rows-of-a-small-matrix-appear-amongst-the-rows-of-a-larger-matrix-vectorization

编辑： madhan ravi 2020-6-13

Hi!

I need to find the number of times the elements in the rows of a small (Sx2) matrix appear in a larger (Lx4) matrix. I would like to do this in a faster way than doing it with a loop if possible. Some properties the matrices have:

both matrices have unique elements on their rows and the order of appeareance doesn't matter
an element from the small matrix can always be found at least once in the large matrix

For example:

L = [1     2     3     4
     2     5     7     4
     2     6     8     3
     3     1     2     8
     8     6     4     2];
S = [2 1
     2 4
     5 3];
for kk = 1:length(S(:,1))
times_each_row_appears(kk,1) = nnz(sum(ismember(L,S(kk,:)),2)==2); % sum(_,2) acts along the rows, sum==2 when both elements are found
end
times_each_row_appears

returns

times_each_row_appears =
     2
     3
     0

Does anyone have any suggestion on how to do this faster without a loop, preferrably on matrices with thousands of rows? Thank you!

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madhan ravi 2020-6-13

You forgot to post the expected result.

Adri 2020-6-13

在 MATLAB Online 中打开

For the matrices in the example, the result should be

result =
     2
     3
     0

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Answer 1

madhan ravi 2020-6-13

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/547716-number-of-times-the-rows-of-a-small-matrix-appear-amongst-the-rows-of-a-larger-matrix-vectorization#answer_450792

编辑：madhan ravi 2020-6-13

在 MATLAB Online 中打开

[m, n] = size(S);
times_each_row_appears = zeros(m,1); %preallocate for speed!
for kk = 1:m
    times_each_row_appears(kk,1) = nnz(sum(ismember(L,S(kk,:)),2)==n);
end

Alternative is to use arrayfun(...) but loop is the fastest anyhow!