MATLAB: Removing of duplicates from first column and summing of elements in second column

Question

Dear All,

I need your help as I am not sure how to solve this problem. Namely, from a matrix with N rows and 2 columns, for example:

X =
     1     2
     2     3
     3     4
     4     1
     4     3
     4     2
     5     5
     6     8
     6     1

I want to get new matrix where in the first column will be shown only unique values, and in the second column (in those rows where were duplicates in the first column) will be shown sum of values of second column.

Result:

Y =
     1     2
     2     3
     3     4
     4     6
     5     5
     6     9

I hope I was clear enough.

Thank you in advance.

Answer 1

This works:

[Xu1,~,Ic2] = unique(X(:,1), 'stable');
Tally = accumarray(Ic2, X(:,2));
Y = [Xu1 Tally]
Y =
     1     2
     2     3
     3     4
     4     6
     5     5
     6     9