class Item {
    public:
        int id;
        void do(int);
    };

    Array<Item> items;
    Array<int>  data;


    /*
     * ... filling arrays with items here ...
     */

//section ONE
{
    ArrayMap<int, int> idx;
    for(int i = 0; i < items.GetCount(); ++i) {
        idx.Add(i, items[i].id);
    }

    Vector<int> idx_order = GetSortOrder(idx.GetValues));
    int ni = 0;	    // count of the items with unique id
    for(int i = 1; i < idx_order.GetCount(); ++i) {
        ni += items[idx_order[i - 1]].id ==
              items[idx_order[i    ]].id ? 0 : 1;
    }
}

// section TWO
    for(int d = 0; d < data.GetCount(); ++d) {
        for(int i = 0; i < items.GetCount(); ++i) {
            if(items[i].id == d) {
                items[i].do(d);
            }
	}
    }