16.10. ArrayTester - Part A¶

The following is a free response question from 2018. It was question 4 on the exam. You can see all the free response questions from past exams at https://apstudents.collegeboard.org/courses/ap-computer-science-a/free-response-questions-by-year.

Question 4. This question involves reasoning about arrays of integers. You will write two static methods, both of which are in a class named ArrayTester.

public class ArrayTester
{
    /**
     * Returns an array containing the elements of column c of arr2D in the same
     * order as they appear in arr2D. Precondition: c is a valid column index in
     * arr2D. Postcondition: arr2D is unchanged.
     */
    public static int[] getColumn(int[][] arr2D, int c)
    {
        /* to be implemented in part (a) */
    }

    /**
     * Returns true if and only if every value in arr1 appears in arr2.
     * Precondition: arr1 and arr2 have the same length. Postcondition: arr1 and
     * arr2 are unchanged.
     */
    public static boolean hasAllValues(int[] arr1, int[] arr2)
    {
        /* implementation not shown */
    }

    /** Returns true if arr contains any duplicate values; false otherwise. */
    public static boolean containsDuplicates(int[] arr)
    {
        /* implementation not shown) */
    }

    /**
     * Returns true if square is a Latin square as described in part (b); false
     * otherwise. Precondition: square has an equal number of rows and columns.
     * Precondition: square has at least one row.
     */
    public static boolean isLatin(int[][] square)
    {
        /* to be implemented in part (b) */
    }
}

Part a. Write a static method getColumn, which returns a one-dimensional array containing the elements of a single column in a two-dimensional array. The elements in the returned array should be in the same order as they appear in the given column. The notation arr2D [r][c] represents the array at row r and column c.

The following code segment initializes an array and calls the getColumn method.

int [] [] arr2D = { { 0, 1, 2 }, { 3, 4, 5 }, { 6, 7, 8 }, { 9, 5, 3 } }; int[] result = ArrayTester.getColumn (arr2D, 1);

When the code segment has completed execution, the variable result result will have the following contents. result: {1, 4, 7, 5}

16.10.1. Try and Solve It¶

Complete the method getColumn below.

public class ArrayTester
{

/**
     * Returns an array containing the elements of column c of arr2D in the same
     * order as they appear in arr2D. Precondition: c is a valid column index in
     * arr2D. Postcondition: arr2D is unchanged.
     */
    public static int[] getColumn(int[][] arr2D, int c)
    {
        /** Complete this method * */
    }

// Main method to test getColumn method
    public static void main(String[] args)
    {
        int[][] arr2D = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8}, {9, 5, 3}};
        int[] result = ArrayTester.getColumn(arr2D, 1);
        System.out.println(
                "It should print the values from the second column: 1 4 7 5.");
        for (int i = 0; i < result.length; i++)
        {
            System.out.print(result[i] + " ");
        }
    } // end of main
} // end of class

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;
import java.util.Arrays;

// import java.util.ArrayList;

public class RunestoneTests extends CodeTestHelper
{
    public RunestoneTests()
    {
        super("ArrayTester");
        // CodeTestHelper.sort = true;
    }

@Test
    public void testMain1()
    {
        boolean passed = false;

String expect = "It should print the values from the second column: 1 4 7 5.\n1 4 7 5";

String output = getMethodOutput("main");

passed = getResults(expect, output, "Checking for expected output from main");
        assertTrue(passed);
    }

@Test
    public void testMain2()
    {
        boolean passed = false;

int[][] arr2D = { {0, 1, 2}, {3, 4, 5}, {6, 7, 8}, {9, 5, 3}};

String arrayStr = "[[0, 1, 2],\n [3, 4, 5],\n [6, 7, 8],\n [9, 5, 3]]";

int[] result = ArrayTester.getColumn(arr2D, 0);

String expect = "[0, 3, 6, 9]";
        String output = Arrays.toString(result);

passed =
                getResults(
                        expect,
                        output,
                        "Checking for expected output for getColumn(arr2D, 0)\n" + arrayStr);
        assertTrue(passed);
    }

@Test
    public void testMain3()
    {
        boolean passed = false;

int[][] arr2D = { {0, 1, 2, 3, 4, 5}, {6, 7, 8, 9, 5, 3}};

String arrayStr = "[[0, 1, 2, 3, 4, 5],\n [6, 7, 8, 9, 5, 3]]";

int[] result = ArrayTester.getColumn(arr2D, 2);

String expect = "[2, 8]";
        String output = Arrays.toString(result);

passed =
                getResults(
                        expect,
                        output,
                        "Checking for expected output for getColumn(arr2D, 0)\n" + arrayStr);
        assertTrue(passed);
    }
}

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16.10. ArrayTester - Part A¶

16.10.1. Try and Solve It¶