Dear All, First of all, thanks to the whole list for their help. I am still struggling with things I used to be able to do quite easily (though maybe not efficiently) with other languages. I still have to get used to the immutable nature of lists and the absence of for loops. Let us say you have two lists l = [1,1,1,1,2,2,2,2,2,2,2,3,3,5,6,7] (already sorted) and a list of corresponding values for every entry in l m= [2,2,2,2,4,4,4,4,6,6,4,4,4,10,12,14]. Also consider the list of the unique values in l l_un = nub l Now, what I would like to do is to get a third list, let us call it q, such that its length is equal to the length of l_un. On top of that, its i-th entry should be the sum of the entries of m corresponding to the i-th values of l_un. To fix the ideas, q should be q = [8,32, 8,10,12,14] . How would you do that? Cheers Lorenzo P.S.: I will need this function both with Integer and Double numbers (I may have situation in which the entries of one list or both are real numbers, though the goal stays the same)