# Sum elements in the first column according to repeated values in a second column

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Sim on 2 Mar 2022
Commented: Sim on 4 Mar 2022
Hi, I have a two columns matrix, where the elements of the second column are repeated (here below in my example, 55, 98 and 17 are repeated several times).
I would like to sum the elements of the first column, which have (in common) the same element of the second column. Here an example... any fast way to do it?
% Input
A = [ 4 55
3 55
7 55
1 55
9 98
1 98
2 98
8 98
0 98
1 98
6 17
3 17
0 17
4 17]
% desired output
B = [15 55
21 98
13 17];
where, for example, the value 15 in B(1,1) is given by 4 + 3 + 7 + 1 = 15

Star Strider on 2 Mar 2022
A = [ 4 55
3 55
7 55
1 55
9 98
1 98
2 98
8 98
0 98
1 98
6 17
3 17
0 17
4 17];
[A2u,~,ix] = unique(A(:,2));
A1sums = accumarray(ix,A(:,1),[],@sum);
Desired_Result = [A1sums A2u]
Desired_Result = 3×2
13 17 15 55 21 98
The loops are still there, however they are hidden inside the accumarray call.
.
Sim on 4 Mar 2022
Thanks a lot for your solution @Johan Pelloux-Prayer!! It is very interesting! :)
(I would accept all the answers/solutions :) )

Sim on 2 Mar 2022
I found this solution, but I would like to avoid the loop for:
[a,b] = unique(A(:,2));
[~,d] = unique(A(:,2),'last');
for i = 1 : size(a,1)
B1(i) = sum(A(b(i):d(i),1));
end
B = [B1' a]
% result
B =
13 17
15 55
21 98