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Count number of rows that meet criteria based on elements in other rows

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Neuro
Neuro 2022-3-31
回答： Voss 2022-3-31
Hello. How can one count the number of rows based on the elements in the columns? For example, if we have the following matrix:
3 0 0 2
5 0 0 2
2 0 1 2
2 0 1 2
1 1 1 3
4 1 0 2
3 1 1 2
1 1 1 3
5 1 0 1
4 1 1 1
How can we count the number of rows that have number 2 in the first column and number 1 in the third column? I guess one should use 'length' right? Thanks!

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Voss
Voss 2022-3-31
nnz(A(:,1) == 2 & A(:,3) == 1)
% 1st column is 2 & 3rd column is 1
where A is your matrix

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