for loop, change matrix dimensions

Question

Giovanni Gardan 2020-8-5

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https://ww2.mathworks.cn/matlabcentral/answers/575752-for-loop-change-matrix-dimensions

回答： Rik 2020-8-5

I have a matrix (A) which is 2156x25 in dimension.

I want to find if there are more than one equal element in column 2 that in first column have the same values (=67). For these elements I want to keep the corresponding first row and sum to this row the numbers of the third and fourth column of all these elements. Then I want to delete from the matrix the elements of the rows with same elements in second and first column(except the first row).

I have a problem in my code: the message is: Index in position 1 exceeds array bounds (must not exceed 958).

Why? Is this Because the matrix dimension change? Could I do something?

% Input: A is a matrix which is 2156x25
for k = 1:size(A,1)
    dop = find(A(k,2) == CAR_GEN(:,2));
              if size(dop,1) > 1
                ccc = find(A(dop,1) ==  67); 
                                      if(size(ccc,1)) > 1  
                                       A(ccc(1),3) = sum(A(ccc,3));
                                       A(ccc(1),4) = sum(A(ccc,4));
                                       A(ccc(2:end),:)= [];
                                       end
               end
end

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Answer 1

Rik 2020-8-5

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In general you're better off using a separate logical array to mark which rows should be deleted and doing the deletion in one go. If you want to keep your current code structure, a while loop is a better choice:

k=0;
while k<size(A,1)
    k=k+1;
    dop = find(A(k,2) == CAR_GEN(:,2));
    if size(dop,1) > 1
        ccc = find(A(dop,1) ==  67); 
        if(size(ccc,1)) > 1  
            A(ccc(1),3) = sum(A(ccc,3));
            A(ccc(1),4) = sum(A(ccc,4));
            A(ccc(2:end),:)= [];
        end
    end
end