Checking for existence of row in a matrix

Question

Stewart Tan 2019-8-20

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/476798-checking-for-existence-of-row-in-a-matrix

评论： Stewart Tan 2019-8-20

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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KSSV 2019-8-20

Read about ismember

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Answer 1

Alex Mcaulley 2019-8-20

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https://ww2.mathworks.cn/matlabcentral/answers/476798-checking-for-existence-of-row-in-a-matrix#answer_388285

在 MATLAB Online 中打开

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