Compute weighted averages for large numbers

Question

I'm trying to get the weighted average of a few numbers. Basically I have:

Price    - 134.42
Quantity - 15236545

There can be as few as one or two or as many as fifty or sixty pairs of prices and quantities. I need to figure out the weighted average of the price. Basically, the weighted average should give very little weight to pairs like

Price    - 100000000.00
Quantity - 3

and more to the pair above.

The formula I currently have is:

((price)(quantity) + (price)(quantity) + ...)/totalQuantity

So far I have this done:

        double optimalPrice = 0;
        int totalQuantity = 0;
        double rolling = 0;
        System.out.println(rolling);

        Iterator it = orders.entrySet().iterator();
        while(it.hasNext()) {
            System.out.println("inside");
            Map.Entry order = (Map.Entry)it.next();
            double price = (Double)order.getKey();
            int quantity = (Integer)order.getValue();
            System.out.println(price + " " + quantity);

            rolling += price * quantity;
            totalQuantity += quantity;
            System.out.println(rolling);
        }
        System.out.println(rolling);
        return rolling/totalQuantity;

The problem is I very quickly max out the "rolling" variable.

How can I actually get my weighted average?

Answer 1

A double can hold a pretty large number (about 1.7 x 10^308, according the docs), but you probably shouldn't use it for values where exact precision is required (such as monetary values).

Check out the BigDecimal class instead. This question on SO talks about it in more detail.