comparing two matrix element simultaneously and adding third column

the problem is extension of the question https://www.mathworks.com/matlabcentral/answers/106095-comparing-matrix-elements-and-performing-operation-if-equal

whenever the elemnts in column 1 and 3 both are repeated elsewhere in matrix then only third column should get added up

e.g

A=

18 18 1

17 20 2

18 18 1

17 25 2

19 17 3

18 18 1

19 17 3

then answer should be

18 18 1

17 20 2

18 18 2

17 25 2

19 17 3

18 18 3

19 17 6

Answer 1

[KSSV]

A = [ 18   18    1
   17   20    2
   18   18    1
   17   25    2
   19   17    3
   18   18    1
   19   17    3] ;

[c,ia,ib] = unique(A(:,1:2),'rows') ; 

B = A ; 
% [c,ia,ib] = unique(A(:,3)) ; 
for i = 1:length(c)
    B(ib==i,3) = cumsum(A(ib==i,3)) ;
end
B

[N/A]

thank you KSSV this works absolutely fine but since my actual matrix is going to be 3x11870 i want that the unique elements and their cumulative sum should be stored separately otherwise it would be too difficult to find them can you tell me how that will work thanks in advance.