Sum values corresponding to a number in a vector

 4   1
 4   10
 4   2
 8   20
 5   12
 5   4
 5   8
 5   6

Suppose I have two vectors, I need average of the digits in second vector corresponding to the digits in the first vector such that the result is

    4  13
    8  20
    5  30

Answer 1

[Stephen23]

>> M = [4,1;4,10;4,2;8,20;5,12;5,4;5,8;5,6]
M =
  4    1
  4   10
  4    2
  8   20
  5   12
  5    4
  5    8
  5    6
>> [U,~,X] = unique(M(:,1));
>> S = accumarray(X,M(:,2),[],@sum);
>> [U,S]
ans =
  4   13
  5   30
  8   20