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

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

Answer 1

[dpb]

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
>>

See

doc *accumarray*

too...

[Al Falamanki]

That's exactly what I was looking for!! Thank you very much!!