How to do well on the AP CS Free Response Questions

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How to do well on the AP CS Free Response Questions © A+ Computer Science -

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Free Response Questions © A+ Computer Science -

AP A Free Response Topics © A+ Computer Science - www.apluscompsci.com
Array / Array or Arrays – for and for each loops, nested loops, array of arrays concepts Make a Class - Basic – create a basic class with instance variables, constructors, and methods ArrayList of References / Strings – get,set,remove,add,size – levels of abstraction Make a Class - Interface / Abstract – implement / extend – looks like this topic is back © A+ Computer Science -

Free Response Strategies © A+ Computer Science - www.apluscompsci.com
-Read all 4 questions before writing anything -answer the easiest question 1st -most times question 1 is the easiest -see if part B calls part A and so on -many times part C consists of A and B calls -write something on every question -write legibly / use PENCIL!!!!!!!!!! -keep track of your time © A+ Computer Science -

-When writing methods -use parameter types and names as provided -do not redefine the parameters listed -do not redefine the methods provided -return from all return methods -return correct data type from return methods © A+ Computer Science -

-When writing a class or methods for a class -know which methods you have -know which instance variables you have -check for public/private on methods/variables -return from all return methods -return correct data type from return methods © A+ Computer Science -

-When extending a class -know which methods the parent contains -have the original class where you can see it -make sure you have super calls -check for public/private on methods/variables -make super calls in sub class methods as needed © A+ Computer Science -

-When extending abstract / implementing interface -know which methods the parent contains -have the original class where you can see it -make sure you have super calls -check for public/private on methods/variables -make super calls in sub class methods as needed -implement all abstract methods in sub class © A+ Computer Science -

Arrays and Array of Arrays
One question on the A test free response will require you to manipulate an array and / or an array of arrays. © A+ Computer Science -

What is a matrix? A matrix is an array of arrays. int[][] mat = new int[3][3]; 2 Each spot in an matrix stores the location/address of an array. mat[0] stores the location / address of a one-dimensional array. © A+ Computer Science -

AP A - Arrays int[] nums = new int[10]; //Java int array nums An array is a group of items all of the same type which are accessed through a single identifier. © A+ Computer Science -

AP A - Arrays String[] s = new String[5]; s null Arrays are automatically filled with values when instantiated. © A+ Computer Science -

Matrix Variables int size = 40; int[][] mat = {{5,7,9,2}, {5,3,4,6}, {7,0,8,9}}; int[][] intMat = new int[size][size]; //intMat is filled with zeros - 0s © A+ Computer Science -

Matrix Variables String[][] words = new String[4][4]; //words is filled with null double[][] dMat = new double[3][3]; //dMat is filled with 0.0 © A+ Computer Science -

Printing Spots int[][] mat = {{5,7,9,2,1,9}, {5,3,4}, {3,7,0,8,9}}; out.println(mat[2][1]); out.println(mat[1][2]); out.println(mat[0][3]); out.println(mat[2][4]); OUTPUT When printing out a spot in a matrix, the row and column must be provided. out.println(mat[2][1]); This line prints spot 1 of the array referred to by mat[2]. mat[2] stores the location/address of an array. © A+ Computer Science -

Printing Spots int[][] mat = {{5,7,9,2,1,9}, {5,3,4}, {3,7,0,8,9}}; out.println(mat[7/4][0]); out.println(mat[1*2][2]); out.println(mat.length); out.println(mat[0].length); OUTPUT When printing out a spot in a matrix, the row and column must be provided. out.println(mat[2][1]); This line prints spot 1 of the array referred to by mat[2]. mat[2] stores the location/address of an array. © A+ Computer Science -

What is a matrix? A matrix is an array of arrays. int[][] mat = new int[3][3]; mat[0][1]=2; 2 2 Which array? Each spot in an matrix stores the location/address of an array. mat[0] stores the location / address of a one-dimensional array. mat[0][1]=2; This line sets mat[0] spot 1 to 2. Which spot? © A+ Computer Science -

Assigning Matrix Values mat[2][2]=7; mat[0][3]=5; mat[4][1]=3 5 7 3 mat[2] stores the location / address of a one-dimensional array. mat[2][2]=7; This line sets mat[2] spot 2 to 7. © A+ Computer Science -

Assigning Matrix Values for( int r = 0; r < mat.length; r++) { for( int c = 0; c < mat[r].length; c++) mat[r][c] = r*c; } 1 2 4 if mat was 3x3 © A+ Computer Science -

Nested Loop Review int outer=1; //start //stop //increment for(outer=1; outer<=2; outer++) { } //start //stop //increment for(int inner=1; inner<=2; inner++) out.println(outer + " " + inner); out.println(); Nested loops are very important when accessing all spots in a matrix. The outer loop is used to access each array. The inner loop is used to move from column to column across each array. OUTPUT 1 1 1 2 OUTPUT 1 1 1 2 2 1 2 2 © A+ Computer Science -

What is a matrix? A matrix is an array of arrays. int[][] mat = new int[3][3]; # of arrays size of each array 2 © A+ Computer Science -

Printing an Array int[][] mat = {{5,7},{5,3,4,6},{0,8,9}}; out.println(Arrays.toString(mat[0])); out.println(Arrays.toString(mat[1])); Arrays.toString() can be used to print out each array in a matrix. OUTPUT [5, 7] [5, 3, 4, 6] © A+ Computer Science -

Printing an Array int[] nums = {1,2,3,4,5,6,7}; for(int r=0; r<nums.length; r++) { out.println(nums[r]); } OUTPUT To print all spots in an array, some type of loop must be used that contains a variable that increases or decreases so that each spot in the array may be visited. length returns the # of elements/items/spots in the array!!! © A+ Computer Science -

Printing a Matrix Printing a row int[][] mat = {{5,7},{5,3,4,6},{0,8,9}}; for(int r=0; r<mat.length; r++) { } for(int c=0; c<mat[r].length; c++) { out.print(mat[r][c]); } out.println(); for(int c=0; c<mat[1].length; c++) { out.print(mat[1][c]); } out.println(); When printing a matrix, the inner loop is used to move across the columns. The outer loop is used to move from row to row. Each row is an array. OUTPUT OUTPUT © A+ Computer Science -

Printing a Matrix int[][] mat = {{5,7},{5,3,4,6},{0,8,9}}; for( int[] row : mat ) { for( int num : row ) System.out.print( num + " "); } System.out.println(); The for each loop works quite well as tool to print a matrix. OUTPUT © A+ Computer Science -

2015 AP A Free Response © A+ Computer Science -

2015 Question 1 part A public static int arraySum( int[] arr ) {
int sum = 0; for( int item : arr ) sum = sum + item; return sum; } 2015 Question 1 part A

2015 Question 1 part B public static int[] rowSums( int[][] arr2D ) {
int[] ret = new int[arr2D.length]; for( int i = 0; i < ret.length; i++ ) ret[i] = arraySum(arr2D[i]); return ret; } 2015 Question 1 part B

2015 Question 1 part C – ver 1 { int[] totals = rowSums( arr2D );
public static boolean isDiverse( int[][] arr2D ) { int[] totals = rowSums( arr2D ); for( int i = 0; i < totals.length-1; i++ ) for( int j = i+1; j < totals.length; j++) if( totals[i] == totals[j] ) return false; } return true; 2015 Question 1 part C – ver 1 This version is kinda lame, but I assume most students will do something like this.

2015 Question 1 part C – ver 2 { int[] totals = rowSums( arr2D );
public static boolean isDiverse( int[][] arr2D ) { int[] totals = rowSums( arr2D ); ArrayList<Integer> used; used = new ArrayList<Integer>(); for( int item : totals ) if( used.contains( item) ) return false; else used.add( item ); } return true; 2015 Question 1 part C – ver 2 This algorithm is more interesting and more of what I would expect from my students that participate in contests.

Provided by A+ Computer Science A+ Lab Assignments that cover the same matrix concepts as FR question # 1 :: • Magic Square and Spiral Matrix • Forest and Tic-Tac-Toe • Codes and Ciphers © A+ Computer Science -

AP A Make a Class One question on the A test free response will require that you create a class sometimes using inheritance and sometimes not. You will need to create instance variables, constructors, and methods. The 2015 question was a class question that involved strings. © A+ Computer Science -

Class Components © A+ Computer Science -

classes public class Triangle { private int sideA; private int sideB; private int sideC; Instance variables Instance variables store the state information for an object. © A+ Computer Science -

constructors public Triangle(int a, int b, int c) { sideA=a; sideB=b; sideC=c; } Initialization Constructor Constructors are similar to methods. Constructors set the properties of an object to an initial state. © A+ Computer Science -

modifier methods public void setSideA(int a ) { sideA=a; } Modifier methods are methods that change the properties of an object. © A+ Computer Science -

accessor methods public int getSideA() { return sideA; } Accessor methods are methods that retrieve or grant access to the properties of an object, but do not make any changes. © A+ Computer Science -

String © A+ Computer Science -

frequently used methods © A+ Computer Science - www.apluscompsci.com
String frequently used methods Name Use substring(x,y) returns a section of the string from x to y not including y substring(x) returns a section of the string from x to length-1 length() returns the # of chars charAt(x) returns the char at spot x indexOf(c) returns the loc of char c in the string, searching from spot 0 to spot length-1 lastIndexOf(c) returns the loc of char c in the string, searching from spot length-1 to spot 0 String is an immutable Object. String cannot be changed. All of the String methods are accessor method. All of the String methods are return methods. © A+ Computer Science -

Strings OUTPUT 4 -1 iga tors rule String sent = "alligators rule"; String find = "gato"; System.out.println( sent.indexOf( find ) ); System.out.println( sent.indexOf( "dog" ) ); System.out.println( sent.substring( 3 , 6 ) ); System.out.println( sent.substring( 6 ) ); © A+ Computer Science -

2015 Question 2 public class HiddenWord { private String word;
public HiddenWord( String w ) word = w; } public String getHint( String guess ) String ret = ""; for( int i = 0; i < word.length(); i++) if( word.substring(i,i+1).equals(guess.substring(i,i+1)) ) ret += word.substring( i, i+1); else if( word.indexOf( guess.substring(i,i+1))!=-1) ret += "+"; else ret += "*"; return ret; 2015 Question 2

Provided by A+ Computer Science A+ Lab Assignments that cover the same string concepts as FR question # 2 :: • HexChecker, MorseCode • Rock-Paper-Scissors • Etch-A-Sketch © A+ Computer Science -

ArrayList © A+ Computer Science -

AP A - ArrayList A typical ArrayList question involves putting something into an ArrayList and removing something from an ArrayList. © A+ Computer Science -

AP A - ArrayList Arraylist is a class that houses an array. An ArrayList can store any type. All ArrayLists store the first reference at spot / index position 0. ArrayList can store a reference to any type of Object. ArrayList was built using an array[] of object references. © A+ Computer Science -

AP A - ArrayList int[] nums = new int[10]; //Java int array nums An array is a group of items all of the same type which are accessed through a single identifier. © A+ Computer Science -

frequently used methods © A+ Computer Science - www.apluscompsci.com
ArrayList frequently used methods Name Use add(item) adds item to the end of the list add(spot,item) adds item at spot – shifts items up-> set(spot,item) put item at spot z[spot]=item get(spot) returns the item at spot return z[spot] size() returns the # of items in the list remove() removes an item from the list clear() removes all items from the list import java.util.ArrayList; © A+ Computer Science -

AP A - ArrayList OUTPUT h c List ray = new ArrayList(); ray.add("hello"); ray.add("whoot"); ray.add("contests"); out.println(((String)ray.get(0)).charAt(0)); out.println(((String)ray.get(2)).charAt(0)); In the example above, ray is an ArrayList that stores Object references. In order to call non-Object methods on a spot in ray, casting would be required. ray.add(0,"hello"); out.println(((String)ray.get(0)).charAt(0)); ray stores Object references. © A+ Computer Science -

AP A - ArrayList List<String> ray; ray = new ArrayList<String>(); ray.add("hello"); ray.add("whoot"); ray.add("contests"); out.println(ray.get(0).charAt(0)); out.println(ray.get(2).charAt(0)); OUTPUT h c In the example above, ray is an ArrayList that stores String references. Casting would not be required to call non-Object methods on ray. ray.add(0,"hello"); ray.add(1,"chicken"); out.println(ray.get(0).charAt(0)); out.println(ray.get(1).charAt(5)); ray stores String references. © A+ Computer Science -

AP A - ArrayList Removal © A+ Computer Science - www.apluscompsci.com
int spot=list.size()-1; while(spot>=0) { if(list.get(spot).equals("killIt")) list.remove(spot); spot--; } © A+ Computer Science -

for(int i=list.size()-1; i>=0; i--) { if(list.get(i).equals("killIt")) list.remove(i); } © A+ Computer Science -

int spot=0; while(spot<list.size()) { if(list.get(spot).equals("killIt")) list.remove(spot); else spot++; } © A+ Computer Science -

2015 Question 3 part A You must know ArrayList!
public int getValueAt( int row , int col ) { for( SparseArrayEntry item : entries ) if( item.getRow()==row && item.getCol()==col) return item.getValue(); } return 0; You must know ArrayList!

2015 Question 3 - part B You must know ArrayList!
public void removeCol( int col ) { for( int i = entries.size()-1; i>=0; i--) SparseArrayEntry e = entries.get(i); if( e.getCol()==col) entries.remove(i); } else if ( e.getCol()>col) SparseArrayEntry up; up = new SparseArrayEntry( e.getRow(), e.getCol()-1, e.getValue() ); entries.set( i, up); numCols--; You must know ArrayList!

Provided by A+ Computer Science A+ Lab Assignments that cover the same list concepts as FR question # 3 :: • MadLib, WordList, Numbers, and Toys • Moving Shapes • BlackJack – 21 © A+ Computer Science -

AP A - Abstract/Interface © A+ Computer Science - www.apluscompsci.com
A typical Abstract/Interface question requires that a class be written that extends the abstract class or implements the interface and that all abstract method(s) be implemented. © A+ Computer Science -

Abstract classes are used to define a class that will be used only to build new classes. No objects will ever be instantiated from an abstract class. Abstract Classes are used to create hierarchies of classes. Abstract classes are used to setup future classes. An abstract class can not be instantiated. © A+ Computer Science -

Mammal (abstract class) Human Whale Cow
AP A - Abstract/Interface Mammal (abstract class) In this example, Mammal is the abstract class. You would never have just a Mammal. For instance, you would not walk outside and go “Hey, look at that Mammal!” Mammal would be used to create something more specific, like a Dog, Human, or Whale. Human Whale Cow © A+ Computer Science -

Any sub class that extends a super abstract class must implement all methods defined as abstract in the super class. All abstract methods in the Abstract class must be implemented by the sub class extending the abstract class. This process is very similar to implementing an interface. When implementing an interface, all abstract methods in the interface must be implemented in the class. All methods in an interface are Abstract. © A+ Computer Science -

abstract class Monster{ private String myName; public Monster() { myName =""; } public Monster(String name) { myName =name; public String getName() { return myName; public abstract void talk( ); public String toString() { return myName + "\n"; AP A - Abstract / Interface Monster is an abstract class. Monster x = new Monster(); //illegal Monster y = new Ghost(); //legal Monster cannot be instantiated. Monster contains one abstract method, method talk(); The assumption is that all Monsters will have a name and name related methods. Also, it is assumed that all Monsters will talk. There is no way to know at the Monster level what exactly a particular type of Monster will say. The talk() method is abstract because it makes no sense it implement talk() at the Monster level. It makes more sense to write talk() for each specific Monster as each specific Monster will say something specific. © A+ Computer Science -

Why define talk as abstract? public abstract void talk( ); Does each Monster say the exact same thing? Each Monster will say something specific. A Ghost will say something different than a Werewolf and a Yetti will say something different from a Vampire. Thus, the talk() method is abstract as it will be written in each of the specific Monsters in a way specific to each Monster. © A+ Computer Science -

Vampire class Vampire extends Monster { public Vampire( String name ) { super(name); } public void talk() out.println("Vampire " + getName() + " say \"I want to drink your blood!\""); Vampire is a Monster, but it is a Monster that talks in its own specific way. A Vampire says “I want to drink your blood!”. Sub Class © A+ Computer Science -

public abstract class APlus { public APlus(int x) //constructor code not shown public abstract double goForIt(); //other fields/methods not shown } Pet Item © A+ Computer Science -

public class PassAPTest extends APlus { public PassAPTest(int x) { super(x); } public double goForIt() { double run=0.0; //write some code run = x*y/z return run; } //other fields/methods not shown } public abstract class APlus { public APlus(int x) //constructor code not shown public abstract double goForIt(); //other fields/methods not shown } © A+ Computer Science -

public interface Exampleable { int writeIt(Object o); int x = 123; } All methods listed in an interface are public abstract. Abstract methods have no code. Each abstract method listed in an interface must be implemented in the class that implements the interface. All variables listed in an interface are public static final, making them final class variables. Interfaces cannot contain implemented methods, constructors, or instance variables. Methods are public abstract! Variables are public static final! © A+ Computer Science -

public interface Exampleable { public abstract int writeIt(Object o); public static final int x = 123; } All methods listed in an interface are public abstract. Abstract methods have no code. Each abstract method listed in an interface must be implemented in the class that implements the interface. All variables listed in an interface are public static final, making them final class variables. Interfaces cannot contain implemented methods, constructors, or instance variables. Methods are public abstract! Variables are public static final! © A+ Computer Science -

An interface is a list of abstract methods that must be implemented. An interface may not contain any implemented methods. Interfaces cannot have constructors!!! All methods listed in an interface are public abstract. Abstract methods have no code. Each abstract method listed in an interface must be implemented in the class that implements the interface. All variables listed in an interface are public static final, making them final class variables. Interfaces cannot contain implemented methods, constructors, or instance variables. © A+ Computer Science -

Interfaces are typically used when you know what you want an Object to do, but do not know how it will be done. If only the behavior is known, use an interface. Interfaces are used to detail what things an Object should do. Interfaces are used typically when the way an Object will do things is unknown. Comparable is a great example. With Comparable, it is clear that each Object should be compared to another Object of the same type. Comparable is an interface because it is not known what the Objects that implement Comparable will contain. It is known that the Object should be compared to other Obejcts of the same type in a certain way. © A+ Computer Science -

Abstract classes are typically used when you know what you want an Object to do and have a bit of an idea how it will be done. If the behavior is known and some properties are known, use an abstract class. © A+ Computer Science -

2015 Question 4 Part A public interface NumberGroup {
boolean contains( int val ); } 2015 Question 4 Part A

2015 Question 4 Part B public class Range implements NumberGroup {
private int low; private int high; public Range( int lo, int hi) low = lo; high = hi; } public boolean contains( int val ) return val >= low && val <= high; 2015 Question 4 Part B

public class MultipleGroups implements NumberGroup
{ private List<NumberGroup> groupList; //constructors , other methods, and such not shown public boolean contains( int val ) for( NumberGroup item : groupList ) if( item.contains( val) ) return true; } return false; 2015 Question 4 Part C

Provided by A+ Computer Science A+ Lab Assignments that cover the same class concepts as FR question # 4 :: • Tic Tac Toe • Lots of games © A+ Computer Science -

-Read all 4 questions before writing anything -answer the easiest question 1st -most times question 1 is the easiest -see if part B calls part A and so on -many times part C consists of A and B calls -write something on every question -write legibly / use PENCIL!!!!!!!!!! -keep track of your time © A+ Computer Science -

-When writing methods -use parameter types and names as provided -do not redefine the parameters listed -do not redefine the methods provided -return from all return methods -return correct data type from return methods © A+ Computer Science -

-When writing a class or methods for a class -know which methods you have -know which instance variables you have -check for public/private on methods/variables -return from all return methods -return correct data type from return methods © A+ Computer Science -

-When extending a class -know which methods the parent contains -have the original class where you can see it -make sure you have super calls -check for public/private on methods/variables -make super calls in sub class methods as needed © A+ Computer Science -

-When extending abstract / implementing interface -know which methods the parent contains -have the original class where you can see it -make sure you have super calls -check for public/private on methods/variables -make super calls in sub class methods as needed -implement all abstract methods in sub class © A+ Computer Science -

Provided by A+ Computer Science Visit us at Full Curriculum Solutions Slides and Examples Labs and Worksheets Tests and Quizzes Solutions provided for all © A+ Computer Science -

How to do well on the AP CS Free Response Questions

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