How to manipulate in matrix

Assume matrix i:

i = [
10010  8
10010  5
10010  2
10065  8
10065  6
10099  4
10099  2
10099  9
10099  4
10099  1
];

The first column is ID. There are three different IDs and repeated for 10 rows. I want to sum the second column corresponding to each ID as follows:

j = [
10010  15
10010  15
10010  15
10065  14
10065  14
10099  20
10099  20
10099  20
10099  20
10099  20
];

[the cyclist]

Zhan, you have now asked 10 questions on this forum, many of which have correct answers. You have not accepted any answers yet (at least as I write this). So, those of us who have answered your questions have no way of knowing if we have been helpful, and are losing incentive to provide you with more answers.

[Zhan]

@the cyclist Thank you for your comment. How can I accept an answer?

[Guillaume]

How can I accept an answer?

By clicking on the big blue button that says "Accept this answer" above the answer you want to accept.

Answer 1

[Image Analyst]

Sounds like it might be homework. Here's a start. See if you can finish it.

out = accumarray(i(:, 1), i(:, 2));
out(out==0) = []

Returns [15;14;20]

[Zhan]

out = out(i(:,1),:)

% works if IDs are [1,1,1,2,2,3,3,3,3,3]. Any tips for the oroginal ID?

Answer 2

[Akira Agata]

How about using splitapply function.

A = [
10010  8
10010  5
10010  2
10065  8
10065  6
10099  4
10099  2
10099  9
10099  4
10099  1
];

 [G, ID] = findgroups(A(:,1));
 B = [ID, splitapply(@sum,A(:,2),G)];

Now, the output matrix B is as follows:

 B =
       10010          15
       10065          14
       10099          20

Answer 3

[Andrei Bobrov]

[~,~,c] = unique(ii(:,1));
D = accumarray(c,ii(:,2));
out = [ii(:,1), D(c)];