ArrayList FRQ Key

There were two common solutions given to this problem. The first creates a new list to hold the “good” items before averaging.

public double averageWithinRange(double lower, double upper)
{
    double sum = 0.0;
    int count = 0;
    ArrayList<ItemInfo> goodItems = new ArrayList<ItemInfo>();

    for (int i = 0; i < inventory.size(); i++)
    {
        if (inventory.get(i).isAvailable() && inventory.get(i).getCost() >= lower
                    && inventory.get(i).getCost() <= upper)
        {
            goodItems.add(inventory.get(i));
        }
    }

    for (int i = 0; i < goodItems.size(); i++) {
        sum += goodItems.get(i).getCost();
        count++;
    }

    return sum / count;
}

The second uses the original list and just updates the sum and count as needed.

public double averageWithinRange(double lower, double upper)
{
    double sum = 0.0;
    int count = 0;
    for (int i = 0; i < inventory.size(); i++)
    {
        if (inventory.get(i).isAvailable() && inventory.get(i).getCost() >= lower
                    && inventory.get(i).getCost() <= upper)
        {
            sum += inventory.get(i).getCost();
            count++;
        }
    }
    return sum / count;
}

A third solution, which was rare, is the best, though. Why? Because it is shorter, easier to write and much less error prone! It uses enhanced for loops.

public double averageWithinRange(double lower, double upper)
{
    double sum = 0.0;
    int count = 0;
    for (ItemInfo item : inventory)
    {
        double cost = item.getCost();
        if (item.isAvailable() && cost >= lower && cost <= upper)
        {
            sum += cost;
            count++;
        }
    }
    return sum / count;
}

Scoring your solution

What to look for

• Traverses every element of inventory.
• Tests availability and both bounds, inclusive (>= and <=).
• Accumulates a sum and a count of only the qualifying items.
• Returns sum divided by count, as a double.

Common mistakes

• Using > or <, which excludes the endpoints.
• Dividing by inventory.size() instead of the count of qualifying items.
• Declaring sum as an int, which loses the cents.
• Forgetting the isAvailable() check (the watch is the test case for this).