comparing matrix elements and performing operation if equal

If I have a matrix

A=

   18   18    1
   17   20    2
   18   18    1
   17   25    2
   19   16    3
   18   18    1
   19   17    3

I want to scan through the matrix and when ever the first column elements are equal, the 3rd column elements should get added up

e.g: the resultant matrix should be

18 18 1

17 20 2

18 18 2

17 25 4

19 16 3

18 18 3

19 17 6

Please help me in this

[Maaz Rao]

My problem is also the same i just have one little extension that 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

[Simon]

Hi!

% get indices of unique values
[~, ~, c] = unique(A(:, 1));

% loop over all unique values
for n = 1:max(c)
   ind = n==c;
    % cumulative sum
    cs = cumsum(A(ind, 3));
    % set third column
    A(ind, 3) = cs;
end

[Mahi Nazir]

Brilliant! Thanks a ton