AP CSP Practice Test on Arrays (ArrayList and 2D arrays)

Consider the following method that will alter the matrix mat:

/** @param mat the initialized matrix
* @param row the row number
*/
public static void matStuff(int[][] mat, int row)
{

int numCols = mat[0].length;
for (int col = 0; col < numCols; col++)
mat[row][col] = row;

}

Suppose mat is originally

1 4 9 0
2 7 8 6
5 1 4 3

After the method call matStuff(mat,2), matrix mat will be

Questions are based on the Coin and Purse classes given below:

/* A simple coin class */
public class Coin
{

private double value;
private String name;
//constructor
public Coin(double coinValue, String coinName)
{
value = coinValue;
name = coinName;
}
/** @return the value of this coin */
public double getValue()
{ return value; }
/** @return the name of this coin */
public String getName()
{ return name; }
/** @param obj a Coin object
* @return true if this coin equals obj; otherwise false
*/
public boolean equals(Object obj)
{ return name.equals(((Coin) obj).name); }
//Other methods are not shown.

}

/* A purse holds a collection of coins */
public class Purse
{

private List coins;
/** Creates an empty purse. */
public Purse()
{ coins = new ArrayList(); }
/** Adds aCoin to the purse.
* @param aCoin the coin to be added to the purse
*/
public void add(Coin aCoin)
{ coins.add(aCoin); }
/** @return the total value of coins in purse */
public double getTotal()
{ /* implementation not shown */}

}

Here is the getTotal method from the Purse class:

/** @return the total value of coins in purse */
public double getTotal()
{
double total = 0;
/* more code */
return total;
}

Which of the following is a correct replacement for / more code /?

This question refers to the following method:

public static boolean isThere(String[][] mat, int row, int col,
String symbol)
{

boolean yes;
int i, count = 0;
for (i = 0; i < SIZE; i++)
if (mat[i][col].equals(symbol))
count++;
yes = (count == SIZE);
count = 0;
for (i = 0; i < SIZE; i++)
if (mat[row][i].equals(symbol))
count++;
return (yes || count == SIZE);

}

Now consider this code segment:

public final int SIZE = 8;
String[][] mat = new String[SIZE][SIZE];

Which of the following conditions on a matrix mat of the type declared in the code segment will by itself guarantee that

isThere(mat, 2, 2, "$")

will have the value true when evaluated?

I The element in row 2 and column 2 is "$"
II All elements in both diagonals are "$"
III All elements in column 2 are "$"

Let list be an ArrayList containing these elements: 2 5 7 6 0 1.

Which of the following statements would not cause an error to occur? Assume that each statement applies to the given list, independent of the other statements.

The following code fragment is intended to find the smallest value in arr[0] ... arr[n-1]:

/** Precondition:
* - arr is an array, arr.length = n.
* - arr[0]...arr[n-1] initialized with integers.
* Postcondition: min = smallest value in arr[0]...arr[n-1].
*/
int min = arr[0];
int i = 1;
while (i < n)
{
i++;
if (arr[i] < min)
min = arr[i];
}

This code is incorrect. For the segment to work as intended, which of the following modifications could be made?

• I: Change the line int i = 1; to `int i = 0; Make no other changes.
• II: Change the body of the while loop to:

{
if (arr[i] < min)
min = arr[i];
i++;
}

• III: Change the test for the while loop as follows: while (i <= n) Make no other changes.

Questions refer to the Ticket and Transaction classes below

public class Ticket
{

private String row;
private int seat;
private double price;
//constructor
public Ticket(String aRow, int aSeat, double aPrice)
{
    row = aRow;
   seat = aSeat;
   price = aPrice;
}
//accessors getRow(), getSeat(), and getPrice()
...

}

public class Transaction
{

private int numTickets;
private Ticket[] tickList;
//constructor
public Transaction(int numTicks)
{

numTickets = numTicks;
tickList = new Ticket[numTicks];
String theRow;
int theSeat;
double thePrice;
for (int i = 0; i < numTicks; i++)
{

< read user input for theRow, theSeat, and thePrice >
...
/* more code */

}

}

/** @return total amount paid for this transaction */
public double totalPaid()
{

double total = 0.0;
/* code to calculate amount */
return total;

}

}

Suppose it is necessary to keep a list of all ticket transactions. Assuming that there are NUMSALES transactions, a suitable declaration would be:

Consider this program segment:

for (int i = 2; i <= k; i++)
if (arr[i] < someValue)
System.out.print("SMALL");

What is the maximum number of times that "SMALL" can be printed?

Assume that a square matrix mat is defined by

int[][] mat = new int[SIZE][SIZE];
//SIZE is an integer constant >= 2

What does the following code segment do?

for (int i = 0; i < SIZE - 1; i++)
for (int j = 0; j < SIZE - i - 1; j++)
swap(mat, i, j, SIZE - j - 1, SIZE - i - 1);

You may assume the existence of this swap method:

/** Interchange mat[a][b] and mat[c][d]. */
public void swap(int[][] mat, int a, int b, int c, int d)

A simple Tic-Tac-Toe board is a 3 × 3 array filled with either X’s, O’s, or blanks. Here is a class for a game of Tic-Tac-Toe:

public class TicTacToe
{

private String[][] board;
private static final int ROWS = 3;
private static final int COLS = 3;
/** Construct an empty board. */
public TicTacToe()
{

board = new String[ROWS][COLS];
for (int r = 0; r < ROWS; r++)
for (int c = 0; c < COLS; c++)
board[r][c] = " ";

}
/** @param r the row number
* @param c the column number
* @param symbol the symbol to be placed on board[r][c]
* Precondition: The square board[r][c] is empty.
* Postcondition: symbol placed in that square.
*/
public void makeMove(int r, int c, String symbol)
{
   board[r][c] = symbol;
}
/** Creates a string representation of the board, e.g.
   * |o |
   * |xx |
   * | o|
   * @return the string representation of board
*/
public String toString()
{
String s = ""; //empty string
/* more code */
return s;
}

}

Which segment represents a correct replacement for /* more code */ for the toString method?

Option A

for (int r = 0; r < ROWS; r++)
{
for (int c = 0; c < COLS; c++)
{
s = s + "|";
s = s + board[r][c];
s = s + "|\n";
}
}

Option B

for (int r = 0; r < ROWS; r++)
{
s = s + "|";
for (int c = 0; c < COLS; c++)
{
s = s + board[r][c];
s = s + "|\n";
}

Option C

for (int r = 0; r < ROWS; r++)
{
s = s + "|";
for (int c = 0; c < COLS; c++)
s = s + board[r][c];
}
s = s + "|\n";

Option D

for (int r = 0; r < ROWS; r++)
s = s + "|";
for (int c = 0; c < COLS; c++)
{
s = s + board[r][c];
s = s + "|\n";

Option E

for (int r = 0; r < ROWS; r++)
{
s = s + "|";
for (int c = 0; c < COLS; c++)
s = s + board[r][c];
s = s + "|\n";
}

public class Address
{
   private String name;
   private String street;
   private String city;
   private String state;
   private String zip;
//constructors
...
//accessors
  public String getName()
  { return name; }
  public String getStreet()
  { return street; }
  public String getCity()
  { return city; }
  public String getState()
  { return state; }
  public String getZip()
  { return zip; }
}
public class Student
{
  private int idNum;
  private double gpa;
  private Address address;
  //constructors
  ...
  //accessors
public Address getAddress()
{ return address; }
  public int getIdNum()
  { return idNum; }
  public double getGpa()
  { return gpa; }
}

Consider the following code segment:

for (Address addr : list)
{
/* more code */
}

Which is a correct replacement for /* more code */?

I System.out.println(list[i].getName());
System.out.println(list[i].getStreet());
System.out.print(list[i].getCity() + ", ");
System.out.print(list[i].getState() + " ");
System.out.println(list[i].getZip());

II System.out.println(addr.getName());
System.out.println(addr.getStreet());
System.out.print(addr.getCity() + ", ");
System.out.print(addr.getState() + " ");
System.out.println(addr.getZip());

III System.out.println(addr);

Consider this class:

public class Book
{

private String title;
private String author;
private boolean checkoutStatus;
public Book(String bookTitle, String bookAuthor)
{
title = bookTitle;
author = bookAuthor;
checkoutStatus = false;
}
/** Change checkout status. */
public void changeStatus()
{ checkoutStatus = !checkoutStatus; }
// Other methods are not shown.

}

A client program has this declaration:

Book[] bookList = new Book[SOME_NUMBER];

Suppose bookList is initialized so that each Book in the list has a title, author, and checkout status. The following piece of code is written, whose intent is to change the checkout status of each book in bookList:

for (Book b : bookList)
b.changeStatus();

Which is true about this code?

Consider a class that has this private instance variable:

private int[][] mat;

The class has the following method, alter.

public void alter(int c)
{
for (int i = 0; i < mat.length; i++)
for (int j = c + 1; j < mat[0].length; j++)
mat[i][j-1] = mat[i][j];
}

If a 3 × 4 matrix mat is

1 3 5 7
2 4 6 8
3 5 7 9

then alter(1) will change mat to

Questions refer to the Ticket and Transaction classes below

public class Ticket
{

private String row;
private int seat;
private double price;
//constructor
public Ticket(String aRow, int aSeat, double aPrice)
{
    row = aRow;
   seat = aSeat;
   price = aPrice;
}
//accessors getRow(), getSeat(), and getPrice()
...

}

public class Transaction
{

private int numTickets;
private Ticket[] tickList;
//constructor
public Transaction(int numTicks)
{

numTickets = numTicks;
tickList = new Ticket[numTicks];
String theRow;
int theSeat;
double thePrice;
for (int i = 0; i < numTicks; i++)
{

< read user input for theRow, theSeat, and thePrice >
...
/* more code */

}

}

/** @return total amount paid for this transaction */
public double totalPaid()
{

double total = 0.0;
/* code to calculate amount */
return total;

}

}

Which represents correct /* code to calculate amount */ in the totalPaid method?

public class Address
{
   private String name;
   private String street;
   private String city;
   private String state;
   private String zip;
//constructors
...
//accessors
  public String getName()
  { return name; }
  public String getStreet()
  { return street; }
  public String getCity()
  { return city; }
  public String getState()
  { return state; }
  public String getZip()
  { return zip; }
}
public class Student
{
  private int idNum;
  private double gpa;
  private Address address;
  //constructors
  ...
  //accessors
public Address getAddress()
{ return address; }
  public int getIdNum()
  { return idNum; }
  public double getGpa()
  { return gpa; }
}

A client method has this declaration, followed by code to initialize the list:

Address[] list = new Address[100];

Here is a code segment to generate a list of names only.

for (Address a : list)
/* line of code */

Which is a correct /* line of code */?

Which declaration will cause an error?

• I: List stringList = new ArrayList();
• II: List intList = new ArrayList();
• III: ArrayList compList = new ArrayList();

Consider this class:

public class BingoCard
{

private int[] card;
/** Default constructor: Creates BingoCard with
* 20 random digits in the range 1 - 90.
*/
public BingoCard()
{ /* implementation not shown */ }
/* Display BingoCard. */
public void display()
{ /* implementation not shown */ }
...

}

A program that simulates a bingo game declares an array of BingoCard. The array has NUMPLAYERS elements, where each element represents the card of a different player. Here is a code segment that creates all the bingo cards in the game:

/* declare array of BingoCard */
/* construct each BingoCard */

Which of the following is a correct replacement for /* declare array of BingoCard */?

Questions refer to the Ticket and Transaction classes below

public class Ticket
{

private String row;
private int seat;
private double price;
//constructor
public Ticket(String aRow, int aSeat, double aPrice)
{
    row = aRow;
   seat = aSeat;
   price = aPrice;
}
//accessors getRow(), getSeat(), and getPrice()
...

}

public class Transaction
{

private int numTickets;
private Ticket[] tickList;
//constructor
public Transaction(int numTicks)
{

numTickets = numTicks;
tickList = new Ticket[numTicks];
String theRow;
int theSeat;
double thePrice;
for (int i = 0; i < numTicks; i++)
{

< read user input for theRow, theSeat, and thePrice >
...
/* more code */

}

}

/** @return total amount paid for this transaction */
public double totalPaid()
{

double total = 0.0;
/* code to calculate amount */
return total;

}

}

Which of the following correctly replaces /* more code */ in the Transaction constructor to initialize the tickList array?

Questions are based on the Coin and Purse classes given below:

/* A simple coin class */
public class Coin
{

private double value;
private String name;
//constructor
public Coin(double coinValue, String coinName)
{
value = coinValue;
name = coinName;
}
/** @return the value of this coin */
public double getValue()
{ return value; }
/** @return the name of this coin */
public String getName()
{ return name; }
/** @param obj a Coin object
* @return true if this coin equals obj; otherwise false
*/
public boolean equals(Object obj)
{ return name.equals(((Coin) obj).name); }
//Other methods are not shown.

}

/* A purse holds a collection of coins */
public class Purse
{

private List coins;
/** Creates an empty purse. */
public Purse()
{ coins = new ArrayList(); }
/** Adds aCoin to the purse.
* @param aCoin the coin to be added to the purse
*/
public void add(Coin aCoin)
{ coins.add(aCoin); }
/** @return the total value of coins in purse */
public double getTotal()
{ /* implementation not shown */}

}

Two coins are said to match each other if they have the same name or the same value. You may assume that coins with the same name have the same value and coins with the same value have the same name. A boolean method find is added to the Purse class:

/** @return true if the purse has a coin that matches aCoin,
* false otherwise
*/
public boolean find(Coin aCoin)
{
for (Coin c : coins)
{
/* code to find match */
}
return false;
}

Which is a correct replacement for / code to find match /?

• I if (c.equals(aCoin)) return true;
• II if ((c.getName()).equals(aCoin.getName())) return true;
• III if ((c.getValue()).equals(aCoin.getValue())) return true;

Consider this class:

public class BingoCard
{

private int[] card;
/** Default constructor: Creates BingoCard with
* 20 random digits in the range 1 - 90.
*/
public BingoCard()
{ /* implementation not shown */ }
/* Display BingoCard. */
public void display()
{ /* implementation not shown */ }
...

}

Assuming that players has been declared as an array of BingoCard, which of the following is a correct replacement for /* construct each BingoCard */?

I

for (BingoCard card : players)
card = new BingoCard();

II

for (BingoCard card : players)
players[card] = new BingoCard();

III

for (int i = 0; i < players.length; i++)
players[i] = new BingoCard();

public class Address
{
   private String name;
   private String street;
   private String city;
   private String state;
   private String zip;
//constructors
...
//accessors
  public String getName()
  { return name; }
  public String getStreet()
  { return street; }
  public String getCity()
  { return city; }
  public String getState()
  { return state; }
  public String getZip()
  { return zip; }
}
public class Student
{
  private int idNum;
  private double gpa;
  private Address address;
  //constructors
  ...
  //accessors
public Address getAddress()
{ return address; }
  public int getIdNum()
  { return idNum; }
  public double getGpa()
  { return gpa; }
}

A client method has this declaration:

Student[] allStudents = new Student[NUM_STUDS]; // NUM_STUDS is an int constant

Here is a code segment to generate a list of Student names only. (You may assume that allStudents has been initialized.)

for (Student student : allStudents)
/* code to print list of names */

Which is a correct replacement for /* code to print list of names */?

Consider writing a program that produces statistics for long lists of numerical data. Which of the following is the best reason to implement each list with an array of int (or double), rather than an ArrayList of Integer (or Double) objects?

Consider writing a program that reads the lines of any text file into a sequential list of lines. Which of the following is a good reason to implement the list with an ArrayList of String objects rather than an array of String objects?

Consider the following code segment, applied to list, an ArrayList of Integer values.

int len = list.size();
for (int i = 0; i < len; i++)
{
list.add(i + 1, new Integer(i));
Object x = list.set(i, new Integer(i + 2));
}

If list is initially 6 1 8, what will it be following execution of the code segment?

Consider these declarations:

List strList = new ArrayList();
String ch = " ";
Integer intOb = new Integer(5);

Which statement will cause an error?

Refer to the following code segment. You may assume that array arr1 contains elements arr1[0], arr1[1], . . . , arr1[N-1], where N = arr1.length.

int count = 0;
for (int i = 0; i < N; i++) {
if (arr1[i] != 0) {
arr1[count] = arr1[i];
count++;
}
}
int[] arr2 = new int[count];
for (int i = 0; i < count; i++) {
arr2[i] = arr1[i];
}

If array arr1 initially contains the elements 0, 6, 0, 4, 0, 0, 2 in this order, what will arr2 contain after execution of the code segment?

Consider a class MatrixStuff that has a private instance variable:

private int[][] mat;

Refer to method alter below that occurs in the MatrixStuff class. (The lines are numbered for reference.)

Line 1: /** @param mat the matrix initialized with integers
Line 2: * @param c the column to be removed
Line 3: * Postcondition:
Line 4: * - Column c has been removed.
Line 5: * - The last column is filled with zeros.
Line 6: */
Line 7:   public void alter(int[][] mat, int c)
Line 8:   {
Line 9:      for (int i = 0; i < mat.length; i++)
Line 10:     for (int j = c; j < mat[0].length; j++)
Line 11:     mat[i][j] = mat[i][j+1];
Line 12:   //code to insert zeros in rightmost column
Line 13: ...
Line 14:  }

The intent of the method alter is to remove column c. Thus, if the input matrix mat is

2 6 8 9
1 5 4 3
0 7 3 2

the method call alter(mat, 1) should change mat to

2 8 9 0
1 4 3 0
0 3 2 0

The method does not work as intended. Which of the following changes will correct the problem?

I

Change line 10 to
for (int j = c; j < mat[0].length - 1; j++)
and make no other changes.

II

Change lines 10 and 11 to
for (int j = c + 1; j < mat[0].length; j++)
mat[i][j-1] = mat[i][j];
and make no other changes.

III

Change lines 10 and 11 to
for (int j = mat[0].length - 1; j > c; j--)
mat[i][j-1] = mat[i][j];
and make no other changes.

Which of the following initializes an 8 × 10 matrix with integer values that are perfect squares? (0 is a perfect square.)

I int[][] mat = new int[8][10];

II int[][] mat = new int[8][10];
for (int r = 0; r < mat.length; r++) for (int c = 0; c < mat[r].length; c++) mat[r][c] = r * r;

III int[][] mat = new int[8][10];
for (int c = 0; c < mat[r].length; c++) for (int r = 0; r < mat.length; r++) mat[r][c] = c * c;

A two-dimensional array of double, rainfall, will be used to represent the daily rainfall for a given year. In this scheme, rainfall[month][day] represents the amount of rain on the given day and month. For example,

rainfall[1][15] is the amount of rain on Jan. 15
rainfall[12][25] is the amount of rain on Dec. 25

The array can be declared as follows:

double[][] rainfall = new double[13][32];

This creates 13 rows indexed from 0 to 12 and 32 columns indexed from 0 to 31, all initialized to 0.0. Row 0 and column 0 will be ignored. Column 31 in row 4 will be ignored, since April 31 is not a valid day. In years that are not leap years, columns 29, 30, and 31 in row 2 will be ignored since Feb. 29, 30, and 31 are not valid days.

Consider the method averageRainfall below:

/** Precondition:
* - rainfall is initialized with values representing amounts
* of rain on all valid days.
* - Invalid days are initialized to 0.0.
* - Feb 29 is not a valid day.
* Postcondition: Returns average rainfall for the year.
*/
public double averageRainfall(double rainfall[][])
{
double total = 0.0;
/* more code */
}

Which of the following is a correct replacement for /* more code */ so that the postcondition for the method is satisfied?

I

for (int month = 1; month < rainfall.length; month++)
for (int day = 1; day < rainfall[month].length; day++)
total += rainfall[month][day];
return total / (13 * 32);

II

for (int month = 1; month < rainfall.length; month++)
for (int day = 1; day < rainfall[month].length; day++)
total += rainfall[month][day];
return total / 365;

III

for (double[] month : rainfall)
for (double rainAmt : month)
total += rainAmt;
return total / 365;

Finding Index of First Negative Integer

The following program segment is intended to find the index of the first negative integer in arr[0] . . . arr[N-1], where arr is an array of N integers:

int i = 0;
while (arr[i] >= 0) {
i++;
}
location = i;

This segment will work as intended

Which of the following correctly initializes an array arr to contain four elements each with value 0?

I. int[] arr = {0, 0, 0, 0};
II. int[] arr = new int[4];
III. int[] arr = new int[4]; for (int i = 0; i < arr.length; i++) arr[i] = 0;

Refer to the match method below:

/** @param v an array of int sorted in increasing order
* @param w an array of int sorted in increasing order
* @param N the number of elements in array v
* @param M the number of elements in array w
* @return true if there is an integer k that occurs
* in both arrays; otherwise returns false
* Precondition:
* v[0]..v[N-1] and w[0]..w[M-1] initialized with integers.
* v[0] < v[1] < .. < v[N-1] and w[0] < w[1] < .. < w[M-1].
*/
public static boolean match(int[] v, int[] w, int N, int M)
{

int vIndex = 0, wIndex = 0;
while (vIndex < N && wIndex < M)
{
if (v[vIndex] == w[wIndex])
return true;
else if (v[vIndex] < w[wIndex])
vIndex++;
else
wIndex++;

}
return false;
}

Assuming that the method has not been exited, which assertion is true at the end of every execution of the while loop?

The method changeNegs below should replace every occurrence of a negative integer in its matrix parameter with 0.

/** @param mat the matrix
* Precondition: mat is initialized with integers.
* Postcondition: All negative values in mat replaced with 0.
*/
public static void changeNegs(int[][] mat)
{
/* code */
}

Which is correct replacement for /* code */?

I

for (int r = 0; r < mat.length; r++)
for (int c = 0; c < mat[r].length; c++)
if (mat[r][c] < 0)
mat[r][c] = 0;

II

for (int c = 0; c < mat[0].length; c++)
for (int r = 0; r < mat.length; r++)
if (mat[r][c] < 0)
mat[r][c] = 0;

III

for (int[] row : mat)
for (int element : row)
if (element < 0)
element = 0;

Refer to the method insert below:

/** @param list an ArrayList of String objects
* @param element a String object
* Precondition: list contains String values sorted
* in decreasing order.
* Postcondition: element inserted in its correct position in list.
*/

public void insert(List list, String element)
{

int index = 0;
while (element.compareTo(list.get(index)) < 0)
index++;
list.add(index, element);

}

Assuming that the type of element is compatible with the objects in the list, which is a true statement about the insert method?

Refer to the following code segment. You may assume that arr is an array of int values.

int sum = arr[0], i = 0;
while (i < arr.length) {
i++;
sum += arr[i];
}

Which of the following will be the result of executing the segment?

public class Address
{
   private String name;
   private String street;
   private String city;
   private String state;
   private String zip;
//constructors
...
//accessors
  public String getName()
  { return name; }
  public String getStreet()
  { return street; }
  public String getCity()
  { return city; }
  public String getState()
  { return state; }
  public String getZip()
  { return zip; }
}
public class Student
{
  private int idNum;
  private double gpa;
  private Address address;
  //constructors
  ...
  //accessors
public Address getAddress()
{ return address; }
  public int getIdNum()
  { return idNum; }
  public double getGpa()
  { return gpa; }
}

Here is a method that locates the Student with the highest idNum:

/** Precondition: Array stuArr of Student is initialized.
* @return Student with highest idNum
*/
public static Student locate(Student[] stuArr) {
/* method body */
}

Which of the following could replace /* method body */ so that the method works as intended?

I

int max = stuArr[0].getIdNum();
for (Student student : stuArr)
if (student.getIdNum() > max) {
max = student.getIdNum();
return student;
}
return stuArr[0];

II

Student highestSoFar = stuArr[0];
int max = stuArr[0].getIdNum();
for (Student student : stuArr)
if (student.getIdNum() > max) {
max = student.getIdNum();
highestSoFar = student;
}
return highestSoFar;

III

int maxPos = 0;
for (int i = 1; i < stuArr.length; i++)
if (stuArr[i].getIdNum() > stuArr[maxPos].getIdNum())
maxPos = i;
return stuArr[maxPos];

What will be output from the following code segment, assuming it is in the same class as the doSomething method?

int[] arr = {1, 2, 3, 4};
doSomething(arr);
System.out.print(arr[1] + " ");
System.out.print(arr[3]);

public void doSomething(int[] list) {
int[] b = list;
for (int i = 0; i < b.length; i++)
b[i] = i;
}