# FRQ4 - 2D Arrays - Part 1

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.

From the 2025 Course and Exam Description:

**Free-Response Question 4: 2D Array**. 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 a 2D array structure.

FRQ 4 requires the following skills:

- Iterate through a 2D array
- Access elements of a 2D array
- Call methods on elements of a 2D array

## 2013 FRQ 4 - SkyView - Part (a)

The following is a free response question from 2013. It was question 4 on the exam on [https://secure-media.collegeboard.org/digitalServices/pdf/ap/apcentral/ap13_frq_comp_sci.pdf](https://secure-media.collegeboard.org/digitalServices/pdf/ap/apcentral/ap13_frq_comp_sci.pdf).

**Question 4.** A telescope scans a rectangular area of the night sky and collects the data into a 1-dimensional array. Each data value scanned is a number representing the amount of light detected by the telescope. The telescope scans back and forth across the sky (alternating between left to right and right to left) in the pattern indicated below by the arrows. The back-and-forth ordering of the values received from the scan is called telescope order.

![The first row is left to right and the second is right to left and so on.](images/SkyViewOrder.png)

*The first row is left to right and the second is right to left and so on.*

The telescope records the data in telescope order into a 1-dimensional array of `double` values. This 1-dimensional array of information received from a single scan will be transferred into a 2-dimensional array, which reconstructs the original view of the rectangular area of the sky. This 2-dimensional array is part of the SkyView class, shown below. In this question you will write a constructor and a method for this class.

```java
public class SkyView
{
    /**
     * A rectangular array that holds the data representing a rectangular area of
     * the sky.
     */
    private double[][] view;

    /**
     * Constructs a SkyView object from a 1-dimensional array of scan data.
     *
     * @param numRows the number of rows represented in the view Precondition:
     *     numRows > 0
     * @param numCols the number of columns represented in the view Precondition:
     *     numCols > 0
     * @param scanned the scan data received from the telescope, stored in
     *     telescope order Precondition: scanned.length == numRows * numCols
     *     Postcondition: view has been created as a rectangular 2-dimensional
     *     array with numRows rows and numCols columns and the values in scanned
     *     have been copied to view and are ordered as in the original rectangular
     *     area of sky.
     */
    public SkyView(int numRows, int numCols, double[] scanned)
    {
        /* to be implemented in part (a) */
    }

    /**
     * Returns the average of the values in a rectangular section of view.
     *
     * @param startRow the first row index of the section
     * @param endRow the last row index of the section
     * @param startCol the first column index of the section
     * @param endCol the last column index of the section Precondition: 0 <=
     *     startRow <= endRow < view.length Precondition: 0 <= startCol <= endCol <
     *     view[0].length
     * @return the average of the values in the specified section of view
     */
    public double getAverage(int startRow, int endRow, int startCol, int endCol)
    {
        /* to be implemented in part (b) */
    }

    // There may be other instance variables, constructors, and methods
}
```

**Part a.** Write the constructor for the `SkyView` class. The constructor initializes the `view` instance variable to a 2-dimensional array with `numRows` rows and `numCols` columns. The information from `scanned`, which is stored in the telescope order, is copied into `view` to reconstruct the sky view as originally seen by the telescope. The information in `scanned` must be rearranged as it is stored into `view` so that the sky view is oriented properly.

For example, suppose `scanned` contains values, as shown in the following array.

![First example scanned array values](images/SkyViewExArray.png)

*First example scanned array values*

Using the scanned array above, a `SkyView` object created with `new SkyView(4, 3, values)`, would have `view` initialized with the following values.

![The resulting view from the first example scanned array](images/SkyViewRes1.png)

*The resulting view from the first example scanned array*

For another example, suppose `scanned` contains the following values.

![Second example scanned array values](images/SkyViewEx2Array.png)

*Second example scanned array values*

A `SkyView` object created with `new SkyView(3, 2, values)`, would have `view` initialized with the following values.

![The resulting view from the second example scanned array](images/SkyViewRes2.png)

*The resulting view from the second example scanned array*

Complete the `SkyView` constructor in the class below.

The code below declares the class, the view, and a constructor for you to finish writing. It also has a main method for testing the constructor.

### Exercise 1

Complete the `SkyView` constructor in the class.

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

## 2013 FRQ 4 - SkyView - Part (b)

SkyView Part B is from the 2103 AP Computer Science A Exam at [https://secure-media.collegeboard.org/digitalServices/pdf/ap/apcentral/ap13_frq_comp_sci.pdf](https://secure-media.collegeboard.org/digitalServices/pdf/ap/apcentral/ap13_frq_comp_sci.pdf).

**Part b.** Write the `SkyView` method `getAverage`, which returns the average of the elements of the section of `view` with row indexes from `startRow` through `endRow`, inclusive, and column indexes from `startCol` through `endCol`, inclusive.

For example, if `nightSky` is a `SkyView` object where `view` contains the values shown below, the call `nightSky.getAverage(1, 2, 0, 1)` should return `0.8`. (The average is (1.1 + 1.4 + 0.2 + 0.5) / 4, which equals 0.8). The section being averaged is indicated by the dark outline in the table below.

![Example values with the region for the getAverage outlined](images/SkyViewBEx.png)

*Example values with the region for the getAverage outlined*

![The class information](images/SkyViewClassInfo.png)

*The class information*

### Exercise 2

Finish writing the `getAverage` method in the class.

> Open the `02-SkyViewB` folder and edit `SkyView.java`. Press **Run** to try it; **Run Tests** checks it (2 tests).