# Unit 4B - Free Response Question (FRQ) 3 - ItemInventory

Each coding exercise has its own folder. Open its `.java` file and press **Run** to try it. **Run Tests** checks every graded exercise in this lesson at once.

The third FRQ on the AP CSA Exam as of 2026 will be on Data Analysis with ArrayList and worth 5 points. Students will be provided with a scenario and its associated class(es). Students will write one method of a given class based on provided specifications and examples. The method requires students to use, analyze,and manipulate data in an **ArrayList** structure. (Arrays will not be tested in an FRQ.)

Here are two common patterns for `ArrayList` traversal loops that you will need to use in this FRQ:

```java
for (Type obj : list)
{
    if (obj ....)
        ...
}

for(int i=0; i < list.size(); i++)
{
   if (list.get(i) ....)
       ...
}
```

The following problem is a good example FRQ 3 from  the [2025-2026 AP CSA Course and Exam Description (CED)](https://apcentral.collegeboard.org/media/pdf/ap-computer-science-a-course-and-exam-description-effective-fall-2025.pdf).

## FRQ ItemInventory Description

The `ItemInfo` class is used to store information about an item at a store. A partial declaration of the `ItemInfo` class is shown below.

```java
public class ItemInfo
{
    /**
    * Returns the name of the item
    */
    public String getName()
    { /* implementation not shown */ }

    /**
    * Returns a value greater than 0.0 that represents the
    * cost of a single unit of the item, in dollars
    */
    public double getCost()
    { /* implementation not shown */ }

    /**
    * Returns true if the item is currently available and
    * returns false otherwise
    */
    public boolean isAvailable()
    { /* implementation not shown */ }

    /* There may be instance variables, constructors, and
    methods that are not shown. */
}
```

The `ItemInventory` class maintains an `ArrayList` named inventory that contains all items at the store. A partial declaration of the `ItemInventory` class is shown below. (Note that it contains an `ArrayList` of `ItemInfo` objects.)

```java
public class ItemInventory
{
    /** The list of all items at the store */
    private ArrayList<ItemInfo> inventory;

    /**
    * Returns the average cost of the available items
    * whose cost is between lower and upper, inclusive
    * Precondition: lower <= upper
    * At least one available element of
    * inventory has a cost between
    * lower and upper, inclusive.
    * No elements of inventory are null.
    */
    public double averageWithinRange(double lower, double upper)
    { /* to be implemented */}

    /* There may be instance variables, constructors, and methods
    that are not shown. */
}
```

Write the `ItemInventory` method `averageWithinRange`. The method should return the average cost of the available items in inventory whose cost is between the parameters `lower` and `upper`, inclusive.

Suppose `inventory` contains the following seven `ItemInfo` objects.

![ItemInventory with ArrayList of 7 ItemInfo objects](images/itemInventory.png)

*ItemInventory with ArrayList of 7 ItemInfo objects*

For the inventory shown, `averageWithinRange(10.0, 50.0)` should return `25.0`, which is equal to the average cost of the available items within the specified range (a $20 action figure, a $45 frying pan, and a $10 coffee mug). Although the watch is within the specified range, it is not available.

Complete method `averageWithinRange`.

```java
/**
 * Returns the average cost of the available items
 * whose cost is between lower and upper, inclusive
 * Precondition: lower <= upper
 * At least one available element of
 * inventory has a cost between
 * lower and upper, inclusive.
 * No elements of inventory are null.
*/
public double averageWithinRange(double lower, double upper)
```

## The Algorithm for averageWithinRange

The following exercises will help you to develop the algorithm for the method `averageWithinRange`.

> Check your understanding: Which of the following loops is the most appropriate for iterating through `inventory` (an `ArrayList` of `ItemInfo` objects) to access each `ItemInfo` object when the index is not directly needed? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/FRQ3ArrayList.html#inventory-loop-mcq

> Check your understanding: You are iterating through the `inventory` `ArrayList` using an enhanced for-each loop, and `currentItem` represents the `ItemInfo` object in the current iteration:
>
> ```java
> for (ItemInfo currentItem : inventory)
> {
>     // ...
> }
> ```
>
> Inside this loop, how would you get the cost of the `currentItem`?
>
> Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/FRQ3ArrayList.html#iteminfo-get-cost

> Check your understanding: You are iterating through the `inventory` `ArrayList`, and `currentItem` is the `ItemInfo` object in the current iteration. Which of the following condition correctly checks if `currentItem` is **available** AND its cost is **between `lower` and `upper`, inclusive**? — answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/FRQ3ArrayList.html#iteminventory-conditions

The method `averageWithinRange` should iterate through the `inventory` list, checking each `ItemInfo` object to see if it is available and if its cost is within the specified range. If both conditions are met, the cost of the item should be added to a running total and counted. Remember that to compute the average, we need to add up the costs of all items that meet the criteria and then divide by the number of such items.

## Solve FRQ ItemInventory

### Exercise 1

Write the `averageWithinRange` method for the `ItemInventory` class.

> Open the `01-frq-average-inventory` folder and edit `ItemInventory.java`. Press **Run** to try it; **Run Tests** checks it (2 tests).

## Grading Rubric for FRQ 3

The following AP rubric is used to grade the `averageWithinRange` method in the `ItemInventory` class. Each item is worth 1 point, for a total of 5 points. Did you receive all of the points? In class, your teacher may have you grade each others’ code.

![Rubric for `averageWithinRange`](images/itemInventoryRubric1.png)

![Rubric for `averageWithinRange`](images/itemInventoryRubric2.png)

*Rubric for `averageWithinRange`*