SparseVector[] itemRatings = new SparseVector[n];
for (int i = 0; i < n; i++) {
final long item = items.getKey(i);
ScoredIdListBuilder ratings = itemRatingData.get(item);
SparseVector v = ratings.buildVector();
assert v.size() == ratings.size();
itemRatings[i] = v;
// release some memory
ratings.clear();
}