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

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

Answer 1

[Andrei Bobrov]

[a,~,c] = unique(X(:,1));
out = [a, accumarray(c,X(:,2))];

[Stephen23]

Very tidy.

[Jonathan]

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!