MATLAB: Filtering rows of a matrix based on some columns

matrix arraymatrix manipulation

This is an overview of the problem I am having:
Example matrix:
[100  101  32.5;
 200   205  50.0;
 201   201  0.47;
 100    101  60.0;
  200   205  25.0]
My actual matrix is thousands of rows long.
And I want to be able to find all rows with common entries in column 1 & 2 AND add corresponding values in column three together. i.e I want something like(adding row 1 & 4 since they have common elements on column 1 & 2; similarly row 2 &5):
[100 101 92.5; 200 205 75.0]
And I want to create a new matrix for any rows with same values on first two columns:
[201 201 0.47]
My data will have a mixture of these two row types.
Thank you

Best Answer

   [ua1, junk1, b1] = unique( A(:,1) );
   [ua2, junk2, b2] = unique( A(:,2) );
   tB = accumarray( [b1(:), b2(:]], A(:,3), [], @sum, NaN);
   idx = find( ~isnan(tB) );
   [r, c] = sub2idx( size(tB), idx);
   B = [ua1(r(:)), ua2(c(:)), tB(idx(:)) ];
   C = A( A(:,1) == A(:,2), : );
Related Question