How to sum/group values in a matrix based on their index values in first column?!

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Mohammed
Mohammed 2014-9-28
评论： Jonathan 2019-2-27
I have the following 2-D array and would like to sum the values in second column based on their index values in frist column. Please Note that I don't want to pre-define a vector of index values!
Any Ideas? Thanks!
X =
0 1
100 89
0 2
50 24
100 140
0 11
33 72
67 317
100 209

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采纳的回答

Andrei Bobrov
Andrei Bobrov 2014-9-28
[a,~,c] = unique(X(:,1));
out = [a, accumarray(c,X(:,2))];
  2 个评论
Jonathan
Jonathan 2019-2-27
I went from a sparse matrix representation followed by a summation, to your solution and it is tens of times faster!
Previous implementation:
ValueReps = sparse(rows_i,cols_j,values(cols_j))
out = sum(ValueReps,2);
Great solution! Very efficient!

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