How to add a matrix A to another matrix B with a different size and it should maintain rows of B which do not have the first two elements equal to the first two elements of a row in matrix A?
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I got the following problem. if I have the following two matrices:
A=[1 2 1
3 4 1
7 8 1]
B=[1 2 0
2 6 0
3 4 0
4 9 0
5 2 0
6 1 0
7 8 0]
I need to add matrix A to matrix B with remaining the rows of matrix B. Only when the first two elements are the same, the row should be replaced. I need a matrix:
C=[1 2 1
2 6 0
3 4 1
4 9 0
5 2 0
6 1 0
7 8 1]
It has to work for a 41120x3 matrix.
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Answers (3)
Roger Stafford
on 11 Jun 2013
[tf,loc] = ismember(B(:,1:2),A(:,1:2),'rows');
t = loc(tf);
C = B;
C(t,:) = A(t,:);
Note that if B contains more than one row with the same first two elements, this will select the one with the higher index.
5 Comments
Roger Stafford
on 11 Jun 2013
A thousand pardons! My own code is in error. I should have written this:
[tf,loc] = ismember(B(:,1:2),A(:,1:2),'rows');
C = B;
C(tf,3:end) = A(loc(tf),3:end);
Roger Stafford
on 11 Jun 2013
If I understand you correctly, the reason you are getting a 41120 x 5 array is that your system is NOT handling commas as you expect! It is interpreting them is dividing symbols between separate elements of a five-element row. Try this simple experiment on your machine:
x = [1,2 3,4]
and see how many elements x has and what they are. If there are two and they are fractional, your system is dealing with commas properly. If there are four and they are all integers, it is not using commas properly.
Azzi Abdelmalek
on 11 Jun 2013
for k=1:size(B,1)
ii=find(ismember(A(:,1:2),B(k,1:2),'rows'),1);
if ~isempty(ii)
B(k,:)=A(ii,:);
end
end
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