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Checking for existence of row in a matrix

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Stewart Tan
Stewart Tan on 20 Aug 2019
Commented: Stewart Tan on 20 Aug 2019
I have a sample matrix where:
my_mat = [1 2 4
5 3 1
6 9 7]
and what I'm trying to do is to check my_mat if there exist any rows where the first two element is equal to a new vector generated. If it does, then the third element will be incremented by 1. For example;
new_vec = [5 3]
Since [5 3] is an element in the second row as the first two element, then i would want to increment the third element of the row which is 1+1: hence i would get:
my_mat = [1 2 4
5 3 2
6 9 7]
Same goes, if i have another new vector generated:
new_vec = [6 9]
then the vector is found in the third row of my_mat, hence 7+1 for the third element:
my_mat = [1 2 4
5 3 2
6 9 8]
now, if the vec isn't found in any rows, then i would just add the new vec as another row in the matrix, with the third element initialized as 1
new_vec = [10 14]
my_mat = [my_mat;[new_vec,1]]
which would be:
my_mat = [1 2 4
5 3 2
6 9 8
10 14 1]
I would like to avoid using for loops since if the mat gets too large, searching each row with a for loop would result in slow performance. Is there any built in function i could use to perform this?

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Accepted Answer

Alex Mcaulley
Alex Mcaulley on 20 Aug 2019
Try with this:
my_mat = [1 2 4
5 3 1
6 9 7];
new = [6 9];
Lia = ismember(my_mat(:,1:2),new,'rows');
if nnz(Lia)
my_mat(Lia,3) = my_mat(Lia,3) + 1;
else
my_mat(end+1,:) = [new,1];
end

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Stewart Tan
Stewart Tan on 20 Aug 2019
@Alex Mcaulley thank you! It works well!

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