Filtering rows of a matrix based on some columns

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

Answer 1

[Walter Roberson]

   [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), : );

[MountainKing]

Walter, everything else seems to work other than [r, c] = sub2idx( size(tB), idx)

sub2idx is undefined ! Thanks again.

[Walter Roberson]

subs2idx() instead. I tend to drop the "s" at 3 AM!

[MountainKing]

I still get "Undefined function or method 'subs2idx' for input arguments of type double" with subs2idx

[Walter Roberson]

Sorry, really it should be ind2sub()

http://www.mathworks.com/help/techdoc/ref/ind2sub.html

[MountainKing]

Thank you bunch!

Answer 2

[Andrei Bobrov]

[a,b,b] = unique( A(:,1:2),'rows' );
B = [a,accumarray(b,A(:,3))];
C = B(abs(diff(B(:,1:2),1,2)) < eps(100),:);

[MountainKing]

It works! But what if I wanted to collect averages instead of sums in the third column?

[Andrei Bobrov]

please read about 'accumarray'

B = [a,accumarray(b,A(:,3),[],@mean)];