Extract rows of a matrix containing the same value in the first column by a loop

Question

Al Falamanki 2015-2-24

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/180108-extract-rows-of-a-matrix-containing-the-same-value-in-the-first-column-by-a-loop

评论： Al Falamanki 2015-2-25

采纳的回答： dpb

在 MATLAB Online 中打开

Hello I have a matrix

A = [5 10; 5 13 ; 4 31; 6 10; 6 11; 6 15 ;6 0; ... ; m p; m q;].

I want to extract each row containing the same value in the first column and build new matrices, to get:

A(1) = [5 10; 5 13]
A(2) = [4 31]
A(3) = [6 10; 6 11; 6 15 ;6 0]
...
A(n) = [m p; m q]

Since the matrix A changes in number of rows and values in the first column, i want to keep it as general as possible. I tried the following code using a loop:

    A = [4 10; 4 13 ; 5 31; 6 10; 6 11; 6 15 ;6 0];  % matrix
    value_ind = unique(A(:,1));   % get all values from 1st column
    A = sortrows(A);              % sort the matrix ascending for 1st column
    for i = 1:numel(value_ind)
      for j = 1:numel(value_ind)
        A(j) = A(A(:,1) == i,:);
      end
   end

Unfortunately, MATLAB says Error.

Does anyone have a suitable solution for this problem?

Thanks a lot, Al Falamanki

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Answer 1

dpb 2015-2-24

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/180108-extract-rows-of-a-matrix-containing-the-same-value-in-the-first-column-by-a-loop#answer_169140

在 MATLAB Online 中打开

unique is your friend...

> [u,ia,ib]=unique(A(:,1));
>> for i=1:length(u)
     C(i)={A(ib==i,:)};
end
>> C{:}
ans =
   4    31
ans =
   5    10
   5    13
ans =
   6    10
   6    11
   6    15
   6     0
>>