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 -

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 -

-When writing GridWorld question -use the GridWorld quick reference -use original method code when overidding -use Bug and BoxBug code if extending Bug -use Critter and ChameleonCritter code if extending Critter -use Critter and Bug for extends demo -use Critter for ArrayList and Math.random() demo -use GW quick reference on the entire AP test © A+ Computer Science -

AP A Free Response Topics © A+ Computer Science - www.apluscompsci.com
ArrayList of References / Objects – get,set,remove,add,size – levels of abstraction Matrix / 2 D Array – nested loops, GridWorld ( grid ) GridWorld or Make a Class – location, actor, bug, critter, grid, super, abstract String / Array Question – find biggest, find smallest, etc. © A+ Computer Science -

AP A – Array / String One question on the A test free response is usually an array or string question. CustomerSort Robot Reservation StringCoder © A+ Computer Science -

AP A - HodgePodge This question usually involves an array and / or a string and many times has sorting and searching components. © 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 -

What is a String? String s = "compsci"; s c o m p s i A String is a group of characters. Strings are used to store words, which can consist of letters, numbers, and symbols. A string is a group of characters. The first character in the group is at spot 0. © 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 -

String frequently used methods Name Use equals(s) checks if this string has same chars as s compareTo(s) compares this string and s for >,<, and == trim() removes leading and trailing whitespace replaceAll(x,y) returns a new String with all x changed to y toUpperCase() returns a new String with uppercase chars toLowerCase() returns a new String with lowercase chars The chart above lists some very common and very useful String class methods. equals() and compareTo() are used quite often. trim() and replaceAll() are very useful, but that widely used. toUpperCase() and toLowerCase() can be very useful in certain situations. © A+ Computer Science -

2011 AP A Free Response © A+ Computer Science -

2011 Question 1 - part A public int limitAmplitude(int limit) {
int cnt = 0; for ( int i = 0; i < samples.length; i++ ) { if ( samples[i] < -limit ) { samples[i] = -limit; cnt++; } if ( samples[i] > limit ) { samples[i] = limit; return cnt;

2011 Question 1 - part B public void trimSilenceFromBeginning() {
int i = 0; while ( samples[i] == 0 ) { i++; } int[] ray = new int[ samples.length – i ]; for (int j = 0; j < ray.length; j++) { ray[j] = samples[j+i]; samples = ray;

AP A - Case Study Make a Class
One question on the A test free response will require that you create a class. Sometimes you have to extend an abstract class or implement an interface. Other times, you will extend another class like Critter. © A+ Computer Science -

Location frequently used methods Name Use Location(row, col) creates a new row,col Location getCol() gets the column value for this location getRow() gets the row value for this location Location is a class that stores row and column information. Location spot = new Location(4,5); spot has a row value of 4 and a column value of 5. import info.gridworld.grid.Location; © A+ Computer Science -

frequently used fields © A+ Computer Science - www.apluscompsci.com
Location frequently used fields Name Use NORTH indicates going north – value of 0 SOUTH indicates going south – value of 180 EAST indicates going east – value of 90 WEST indicates going west – value of 270 The location class contains the following 8 directions : Location.NORTH Location.SOUTH Location.EAST Location.WEST Location.NORTHWEST Location.SOUTHWEST Location.NORTHEAST All of the direction fields in the location class are final integers. import info.gridworld.grid.Location; © A+ Computer Science -

Location Location locOne = new Location(2,1); Location locTwo = new Location(1,3); out.println(locOne.getAdjacentLocation(Location.NORTH)); out.println(locOne.getAdjacentLocation(Location.SOUTH)); out.println(locOne.getAdjacentLocation(Location.EAST)); out.println(locOne.getAdjacentLocation(Location.WEST)); OUTPUT (1, 1) (3, 1) (2, 2) (2, 0) 0,0 0,1 0,2 0,3 0,4 1,0 1,1 1,2 1,3 1,4 2,0 2,1 2,2 2,3 2,4 3,0 3,1 3,2 3,3 3,4 getAdjacentLocation will return a nearby location based on a provided direction. getDirectionToward will look at two locations and return the direction required to go from a to b. © A+ Computer Science -

Location Location locOne = new Location(2,1); Location locTwo = new Location(1,3); out.println(locOne.getDirectionToward(locTwo)); 0,0 0,1 0,2 0,3 0,4 1,0 1,1 1,2 1,3 1,4 2,0 2,1 2,2 2,3 2,4 3,0 3,1 3,2 3,3 3,4 OUTPUT 45 getAdjacentLocation will return a nearby location based on a provided direction. getDirectionToward will look at two locations and return the direction required to go from a to b. © A+ Computer Science -

Actor frequently used methods Name Use Actor() creates new blue north bound actor act() reverses the direction for actor getColor() gets the actor’s color getDirection() gets the actor’s direction getLocation() gets the actor’s location setColor(col) sets the actor’s color to col setDirection(dir) sets the actor’s direction to dir moveTo(loc) moves the actor to new location loc Notice that actor has only one constructor and that that constructor takes no parameters. © A+ Computer Science -

Actor frequently used methods Name Use putSelfInGrid(grid, loc) put this actor in grid at loc removeSelfFromGrid() takes this actor out of the grid getGrid() gets the grid which contains this actor toString() gets actor data as a String import info.gridworld.actor.Actor; © A+ Computer Science -

Actor ActorWorld world = new ActorWorld(); Actor dude = new Actor(); dude.setColor(Color.GREEN); dude.setDirection(Location.SOUTH); Location loc = new Location(2,2); world.add(loc, dude); world.show(); If you want an actor have a color other than BLUE, simply call the setColor method and provide the color of your choice. What happens if you click on an empty location? © A+ Computer Science -

Grid abstract methods Name Use get(loc) returns the ref at location loc getEmptyAdjacentLocations(loc) gets the valid empty locs in 8 dirs getNeighbors(loc) returns the objs around this getNumCols() gets the # of cols for this grid getNumRows() gets the # of rows for this grid getOccupiedAdjacentLocations(loc) gets the valid locs in 8 dirs that contain objs getOccupiedLocations() gets locs that contain live objs getValidAdjacentLocations(loc) gets the valid locs in 8 dirs isValid(loc) checks to see if loc is valid put(loc, obj) put the obj in grid at location loc remove(loc) take the obj at location loc out of the grid import info.gridworld.grid.Grid; © A+ Computer Science -

Grid Grid is an interface that details the behaviors expected of a Grid. Grid was designed as an interface because many different structures could be used to store the grid values. An interface works perfectly due to the large number of unknowns. Grid is a row / column structure that stores Objects. The location of each Object is determined by the Location provided when putting the Object in the grid. © A+ Computer Science -

Grid Grid<String> grd; grd = new BoundedGrid<String>( 4, 5 ); grd.put( new Location( 2, 2 ) , "CAT" ); grd.put( new Location( 1, 3 ) , "DOG" ); System.out.println( grd.get( new Location( 0, 0 ) ); System.out.println( grd.get( new Location( 2, 2 ) ); OUTPUT null CAT getAdjacentLocation will return a nearby location based on a provided direction. getDirectionToward will look at two locations and return the direction required to go from a to b. null DOG CAT © A+ Computer Science -

Bug extends Actor frequently used methods Name Use act() move if possible or turn getColor() gets the bug’s color getDirection() gets the bug’s direction getLocation() gets the bug’s location setColor(col) sets the bug’s color to col setDirection(dir) sets the bug’s direction to dir The methods listed below were inherited from actor. The act method has been overridden as the behavior of a bug is quite different from an actor. The other methods listed above that were inherited have not been changed. import info.gridworld.actor.Bug; © A+ Computer Science -

Bug extends Actor frequently used methods – Bug only
Name Use Bug() make a new red bug going north Bug(color) make a new color bug act() move if possible or turn canMove() check to see if a move is possible move() move forward and leave a flower turn() turn 45 degrees without moving Bug is a suped up actor. Bug extends actor. Bug has a default parameter-less constructor and an additional constructor that receives a Color parameter. Bug has overridden the act method inherited from actor. Bug has some new methods that are unique to bug : canMove, move, and turn. import info.gridworld.actor.Bug; © A+ Computer Science -

Bug ActorWorld world = new ActorWorld(); Bug dude = new Bug(); world.add(new Location(3,3), dude); Bug sally = new Bug(Color.GREEN); sally.setDirection(Location.SOUTHEAST); world.add(new Location(2,2), sally); Bug ali = new Bug(Color.ORANGE); ali.setDirection(Location.NORTHEAST); world.add(new Location(1,1), ali); world.show(); In this example, a default bug is created. A default bug is red and facing NORTH. The bug moves NORTH until it reaches the top of the grid. The bug them turns in 45 degree increments until it finds an empty location to which to move. Two other bugs are created as well. One of the bugs is green and the other is orange. © A+ Computer Science -

canMove The bug act method looks to see if a move is possible by calling canMove. canMove looks at the location in front of this bug to see if it is empty or if it contains a flower. canMove returns true or false. Each time the bug act method is called the bug will move to the location/cell immediately in front of it. The bug will turn if the cell/location in front of it is occupied or invalid. A flower of the same color as the bug is left in the cell/location vacated by the bug when it moves. The default color of a bug is red and the default direction of a bug is NORTH. © A+ Computer Science -

move The bug act method calls move if canMove returns true. move calls moveTo to move the bug to the location in front of this bug. move leaves a flower in the old location. Each time the bug act method is called the bug will move to the location/cell immediately in front of it. The bug will turn if the cell/location in front of it is occupied or invalid. A flower of the same color as the bug is left in the cell/location vacated by the bug when it moves. The default color of a bug is red and the default direction of a bug is NORTH. © A+ Computer Science -

turn The bug act method calls turn if canMove returns false. turn changes the direction of the bug by 45 degrees to the right. Each time the bug act method is called the bug will move to the location/cell immediately in front of it. The bug will turn if the cell/location in front of it is occupied or invalid. A flower of the same color as the bug is left in the cell/location vacated by the bug when it moves. The default color of a bug is red and the default direction of a bug is NORTH. © A+ Computer Science -

Critter extends Actor frequently used methods Name Use getColor() gets the critter's color getDirection() gets the critter's direction getLocation() gets the critter's location setColor(col) sets the critter's color to col setDirection(dir) sets the critter's direction to dir The methods listed below were inherited from actor. The act method has been overridden as the behavior of a bug is quite different from an actor. The other methods listed above that were inherited have not been changed. import info.gridworld.actor.Critter; © A+ Computer Science -

Critter extends Actor frequently used methods – Critter specific
Name Use act() calls the methods listed below getActors() gets all actors around this location processActors(actors) do something to actors sent in getMoveLocations() gets list of possible move locs selectMoveLocation(locs) picks loc to move to makeMove(loc) moves this critter to loc Critter’s act method first calls the getActors method to get a list of actors around this critter. Act then calls processActors and sends it the ArrayList of actors built by getActors. processActors typically does something to some or all of the actors in the list. Act calls getMoveLocations next. getMoveLocations builds and returns an ArrayList of locations to which this critter could move. selectMoveLocation is called and sent the ArrayList built by getMoveLocations. selectMoveLocation randomly picks one of the locations and returns it. makeMove is called last by act and makes the critter move if possible. import info.gridworld.actor.Critter; © A+ Computer Science -

act if no grid present – stop call getActors to get list of actors to proces processActors received from getActors call getMoveLocations to get a list of locations to which the critter might move call selectMoveLocation to select new location move to the new loc Critter’s act method first calls the getActors method to get a list of actors around this critter. Act then calls processActors and sends it the ArrayList of actors built by getActors. processActors typically does something to some or all of the actors in the list. Act calls getMoveLocations next. getMoveLocations builds and returns an ArrayList of locations to which this critter could move. selectMoveLocation is called and sent the ArrayList built by getMoveLocations. selectMoveLocation randomly picks one of the locations and returns it. makeMove is called last by act and makes the critter move if possible. © A+ Computer Science -

getActors The getActors method returns an ArrayList containing all of the actors around this critter using the 4 cardinal(N,S,E,W) and 4 intercardinal directions(NE, NW, SE, SW). In order to change which actors are returned by getActors, override the method and provide a different method of selecting actors. getActors must not modify any actors. Critter’s act method first calls the getActors method to get a list of actors around this critter. Act then calls processActors and sends it the ArrayList of actors built by getActors. processActors typically does something to some or all of the actors. © 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 -

2011 Question 2 - part A You must know ArrayList!
public class AttractiveCritter extends Critter { public ArrayList<Actor> getActors() ArrayList<Actor> stuff; stuff = new ArrayList<Actor>(); for ( Location loc : getGrid().getOccupiedLocations() ) if ( !loc.equals(getLocation()) ) stuff.add( getGrid().get(loc) ); } return stuff; You must know ArrayList!

2011 Question 2 - part B You must know ArrayList!
public void processActors(ArrayList<Actor> actors) { for (Actor a : actors) Location loc = a.getLocation(); int dir = loc.getDirectionToward( getLocation() ); Location x = loc.getAdjacentLocation(dir); if (getGrid().get( x ) == null) { a.moveTo( x ); } You must know ArrayList!

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 -

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 -

2011 Question 3 - part A You must know ArrayList!
public int nextTankToFillA(int threshold) { int min = filler.getCurrentIndex(); for (int i = 0; i < tanks.size(); i++) { int curr = tanks.get(i).getFuelLevel() ; int min = tanks.get(min).getFuelLevel(); if ( curr <= threshold && curr < min ) { min = i; } return min; You must know ArrayList!

2011 Question 3 - part B You must know ArrayList!
public void moveToLocation(int locIndex) { if ( filler.getCurrentIndex() > locIndex) { if ( filler.isFacingRight()) { filler.changeDirection(); } filler.moveForward(filler.getCurrentIndex() - locIndex); if (filler.getCurrentIndex() < locIndex) { if (!filler.isFacingRight()) { filler.moveForward(locIndex - filler.getCurrentIndex()); } } You must know ArrayList!

Visit my site at www.apluscompsci.com Full Curriculum Solutions
Provided A+ Computer Science Visit my site at Full Curriculum Solutions M/C Review Question Banks Live Programming Problems Tons of great content! © A+ Computer Science -

AP A - Matrices One question on the A test free response will require you to manipulate a 2-dimensional array or a GridWorld grid. © 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 -

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 -

2011 Question 4 - part A private void fillBlock(String str) {
int pos = 0; for (int r = 0; r < numRows; r++ ) for (int c = 0; c < numCols; c++ ) if (pos < str.length()) { letterBlock[r][c] = str.substring(pos, pos+1); pos++; } else{ letterBlock[r][c] = "A"; } }

2011 Question 4 - part B public String encryptMessage(String message)
{ String mess = ""; int sect = numRows * numCols; while (message.length() > 0) if (sect > message.length()) sect = message.length(); fillBlock(message); mess += encryptBlock(); message = message.substring( sect ); } return mess;

AP A Free Response Topics © A+ Computer Science - www.apluscompsci.com
ArrayList of References / Objects – get,set,remove,add,size – levels of abstraction Matrix / 2 D Array – nested loops, GridWorld ( grid ) GridWorld or Make a Class – location, actor, bug, critter, grid, super, abstract String / Array Question – find biggest, find smallest, etc. © 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 -

-When writing GridWorld question -use the GridWorld quick reference -use original method code when overidding -use Bug and BoxBug code if extending Bug -use Critter and ChameleonCritter code if extending Critter -use Critter and Bug for extends demo -use Critter for ArrayList and Math.random() demo -use GW quick reference on the entire AP test © A+ Computer Science -

How to do well on the AP CS Free Response Questions

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