pick arrays from matrix with condition

Hi, I have a matrix:

     2     2     2     1
     2     2     1     1
     1     1     9     1
     2     2     0     1

I need to pick rows that have same first number and sum them, but order of picking is important. In this case i need to pick first two rows and sum, then pick third and finaly fourth row; after that all the first numbers of column of new matrix should be the same depending on matrix sumed, so in the end there should be:

     2     4     3     2
     1     1     9     1
     2     2     0     1

Any suggestions?

Answer 1

[Andrei Bobrov]

I = cumsum([true;diff(a(:,1))~=0]);
ii = fullfact(size(a));
out = accumarray([I(ii(:,1)), ii(:,2)],a(:))