cs106a review session - stanford university...event-driven programming let’s write a program like...

CS106A Review Session

Saturday Feb. 11, 2017Maria Yang

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Topic List•Karel• Javaconstructs•Graphics+Animation•Memory (Pass-by-referencevs.passbyvalue)• Event-drivenprogramming•CharactersandStrings

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Karel

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Karel• Tips:• Pseudocodefirst• Decomposetheproblem•Mightbelimitationsonconstructs• E.g.noJavafeatures(variables,break,etc.)

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Trick-or-Treat Karel

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Karel Needs Love

Karelisinaworldwithwalkwaystohousesthathavevalentines.Karelshouldgotoeveryhouseinorder,goupthewalkwayandtakeallthevalentines(beepers).Housewalkwayscanbeanydistanceapart,andhaveguidewallsontheleftandrightuptothevalentine.

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Karel Needs Love

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Demo: Karel Needs Love

Loop:- Gotonexthouse- GetValentines

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Gotonexthouse:- movealongleftwall

GetValentines:- traversewalkway- takevalentine- traversewalkway

Karel Needs Love

public void run() {while (frontIsClear()) {

goToNextHouse();getValentines();if (frontIsClear()) {

move();}

}}

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Karel Needs Love

private void goToNextHouse() {while (leftIsBlocked()) {

move();}

}

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Karel Needs Love

private void getValentines() {turnLeft();traverseWalkway();takeValentine();turnAround();traverseWalkway();turnLeft();

}

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Karel Needs Love

private void traverseWalkway() {move();while (leftIsBlocked() && rightIsBlocked()) {

move();}

}

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Karel Needs Love

private void takeValentine() {while (beepersPresent()) {

pickBeeper();}

}

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Karel Needs Love

Java Constructs

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•Variabletypes:primitives(int,double,…)+objects(GRect,Goval,…)•Controlstatements:if,while,for,switch•Whatiseachusefulfor?

•Methods

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Java Constructs


•Methods

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Java Constructs

•ReadinuserinputuntilyouhittheSENTINEL• Iteratethroughastring•MoveKareltoawall

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WHILEFORWHILE

For or While?


•Methods

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Java Constructs

•Amethodisaroutineofinstructionsthatmaytakesomeinputandgivebacksomeoutput.

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MethodParameters Returnvalue

Java Constructs - Methods

public void run() {println(“Hypotenuse of 3 and 4 is: “);println(hypotenuse(3.0, 4.0));

}

private double hypotenuse(double a, double b) {return Math.sqrt(a*a + b*b);

}

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•Parameters arehowthecaller givesinformationtothecallee.Areturnvalueishowthecallee givesinformationbacktothecaller.

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Callee

Parameters Returnvalue

Caller


public void run() {println(“Hypotenuse of 3 and 4 is: “);println(hypotenuse(3, 4));

}

private double hypotenuse(double a, double b) {return Math.sqrt(a*a + b*b);

}

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•Approachingprogramtraces• Localvariablesinthecalleraredistinctfromlocalvariablesinthecallee• Parametersarejustassignednamesbytheorderinwhichthey’repassed•Writevaluesabovevariablenamesasyougothroughtheprogram(ordrawstackcardboxes)

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Program Trace

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Program Tracepublic void run() {

String str = “Yay!! It is Valentine’s Day.”;println(getValentine(str, 6));...

}

private String getValentine(String str, int num1) {

num1 *= 2;

return str.substring(num1, str.length() – 1);

}

run

str Yay!!ItisValentine’sDay.

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}


num1 *= 2;


}

run


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}


num1 *= 2;


}

run


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}


num1 *= 2;


}

run

str

getValentine

str

num1

Yay!!ItisValentine’sDay.

6


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}


num1 *= 2;


}

run

str

getValentine

str

num1


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}


num1 *= 2;


}

run

str

getValentine

str

num1


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}


num1 *= 2;


}

run

str

getValentine

str

num1


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“Valentine’sDay”



}

run


Valentine’sDay(Console)

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Program Tracerun

candy

public void run() {

...

int candy = 5;

int love= 6;

candy = howMuchCandy(candy, love);

println(“I got “ + candy + “ candy(ies)”);

}

private int howMuchCandy(int candy, int love) {

int num3 = love + candy / 2;

return num3 % 3;

}

5

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...

int candy = 5;

int love = 6;



}



return num3 % 3;

}

runcandy

love

5

6

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...

int candy = 5;

int love = 6;



}



return num3 % 3;

}

runcandy

love

5

6

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...

int candy = 5;

int love = 6;



}



return num3 % 3;

}

runcandy

love

5

6

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...

int candy = 5;

int love = 6;



}



return num3 % 3;

}

runcandy

love

5

6

howMuchCandy

candy

love

5

6

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...

int candy = 5;

int love = 6;



}



return num3 % 3;

}

runcandy

love

5

6

howMuchCandy

candy

love

num3

5

6

8

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...

int candy = 5;

int love = 6;



}



return num3 % 3; // 8 % 3 = 2

}

runcandy

love

5

6

howMuchCandy

candy

love

num3

5

6

8

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...

int candy = 5;

int love = 6;



}

runcandy

love

2

6

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...

int candy = 5;

int love = 6;



}

runcandy

love

2

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Valentine’sDayIgot2candy(ies)(Console)

• Trickyspots: precedence,parameter/variablenames…•Drawpictures!/Labelvariablevalues

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Program Trace

Graphics + Animation

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• LookatlectureslidesforlistsofdifferentGObjecttypesandtheirmethods•Remember:thexandyofGRect,GOval,etc.istheirupperleftcorner,butthexandyofGLabel isitsleftmostbaselinecoordinate.•Remember:alabel’sheightisgottenfromgetAscent().

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Graphics

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while (CONDITION) {updateGraphics(); // e.g. move GObjectperformChecks(); // e.g. check game conditionspause(PAUSE_TIME);

}

Standardformatforanimationcode:

Animation

Memory

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• Stackandheap• Stack iswherelocalvariableslive• Heap iswhereobjectslive

•Whenyoumakeanobject,thelocalvariable(whatyounamedit)isaboxthatstoresanaddressontheheap wheretheobjectactuallylives.•Whenyoumakeaprimitive,thelocalvariableisaboxthatstorestheactualvalue.

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Memory

• ==isdangerousbecauseitcompareswhat’sinthevariableboxes!• Forprimitives,ok• Forobjects,comparestheiraddresses!Soonlytrueifthey’retheexactsameobjectlivingintheexactsameplace.

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Memory

•Parameters: whenyoupassaparameter,Javapassesacopyofwhateverisinthevariable’sbox.• Forprimitives– acopyoftheirvalue• Forobjects– acopyoftheiraddress! Sothere’sstillonly1objectversion

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Memory

public void run() {GRect rect = new Grect(0,0,50,50);fillBlue(rect);add(rect); // rect is blue!

}

private void fillBlue(GRect rect) {rect.setFilled(true);rect.setColor(Color.BLUE);

}

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Memory

public void run() {int x = 2;addTwo(x);println(x); // x is still 2!

}

private void addTwo(int x) {x += 2; // this modifies addTwo’s COPY!

}

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Memory

public void run() {int x = 2;x = addTwo(x);println(x); // x is 4!

}private int addTwo(int x) {

x += 2; // this modifies addTwo’s COPY!return x;

}

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Memory – Getting Changes to Copies

Event-driven Programming

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•MouseEvents!• Two waysforJavatoexecuteyourcode:fromrun()andfromeventhandler(mouseClicked,mouseMoved,etc.).• Theseprogramsareasynchronous – codeisnotruninorderanymore,sinceyoudon’tknowwhentheuserwillinteractwithyourprogram!

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Event-Driven Programming

1. Signupfornotificationsforkeyormouseevents2. Implementthemethodcorrespondingtowhat

eventyoucareabout(e.g.mousePressed,mouseMoved).

3. Javawillcallthatmethodwheneverthecorrespondingeventoccurs.

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Let’swriteaprogramlike“mysterysquare”,butwhenyouclicktheshapechangesbetweenasquareandacircle.

Theshapesshouldalwaysbethesamerandomcolor,evenifyouclickinbetweencolorschanging.

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Refresher: Mystery Square

Modifications:•Haveasquareandcircle atthesametime•Changethecolorsofboth everyPAUSE_TIME•Onclick,addoneandremovetheother

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Color-Changing Shape

private static final int SQUARE_SIZE = 100;private static final int PAUSE_TIME = 100;private GRect square;private GOval circle;private boolean isShowingSquare = true;

private RandomGenerator rgen = RandomGenerator.getInstance();

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Constants + Instance Variables

public void run() {setup();while (true) {

Color c = rgen.nextColor();square.setColor(c);circle.setColor(c);pause(PAUSE_TIME);

}}

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private void setup() {double x = (getWidth() – SQUARE_SIZE) / 2.0;double y = (getHeight() – SQUARE_SIZE) / 2.0;square = new GRect(x, y, SQUARE_SIZE, SQUARE_SIZE);circle = new GOval(x, y, SQUARE_SIZE, SQUARE_SIZE);square.setFilled(true);circle.setFilled(true);add(square); // only add square!addMouseListeners();

}61


public void mouseClicked(MouseEvent e) {if (isShowingSquare) {

remove(square);

add(circle);

} else {

remove(circle);

add(square);

}

isShowingSquare = !isShowingSquare;

}62


Characters and Strings

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•Achar isaprimitivetype thatrepresentsasingleletter,digit,orsymbol.Usessinglequotes(‘’).•Computersrepresentchars asnumbersunderthehood(ASCIIencodingscheme).•Astringisanimmutableobject thatrepresentsasequenceofcharacters.Usesdoublequotes(“”).

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Characters and Strings

char uppercaseA = ‘A’;char uppercaseB = (char)(uppercaseA + 1);int lettersInAlphabet = ‘Z’ – ‘A’ + 1;// equivalent: ‘z’ – ‘a’ + 1// A to Z and a to z are sequential numbers.

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Characters

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Characters

•Note: charsareprimitives.Thismeanswecan’tcallmethodsonthem!• InsteadweusetheCharacter classandcallmethodsonit.Wepassinthecharacterofinterestasaparameter.• Thesemethodsdonotchangethechar!Theyreturnamodifiedchar.

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Characters

char ch = ‘a’;Character.toUpperCase(ch); // does nothing!ch.toUpperCase(); // won’t compile!ch = Character.toUpperCase(ch); // ✔

if (Character.isUpperCase(ch)) {println(ch + “ is upper case!”);

}

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Characters

•Note: stringsare(immutable)objects.Thismeanswecan callmethodsonthem!•Wecannotchangeastringaftercreatingit.• Stringscanbecombinedwithints,doubles,chars,etc.

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Strings

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Strings

String str = “Hello world!”; // no “new” neededstr.toUpperCase(); // does nothing!str = str.toUpperCase(); // ✔

for (int i = 0; i < str.length(); i++) {println(str.charAt(i));

}// prints each char on its own line

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Strings

String str = ”’ello mate!”;str = str.substring(1);str = ‘H’ + str; // str = “Hello mate!”String newStr = “”;for (int i = 0; i < str.length(); i++) {

newStr = str.charAt(i) + newStr;}// newStr = “!etam olleH”

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Putting it ALL together

println(“B” + 8 + 4);// prints “B84”println(“B” + (8 + 4));// prints “B12”println(’A’ + 5 + “ella”);// prints “70ella (note: ‘A’ corresponds to 65)”println((char)(‘A’ + 5) + “ella”);// prints “Fella”

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Type Conversion Mayhem

• Thisseemsnonsensical- butit’snot!(kindof)• Justuseprecedencerules andkeeptrackofthetypealongtheway.

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println(‘A’ + 5 + “ella”);// ‘A’ + 5 is int (70), int + “ella” is stringprintln((char)(‘A’ + 5) + “ella”);// ‘A’ + 5 is char (‘F’), char + “ella” is string

Type Conversion Mayhem

Letswriteamethodthat,givenastring,removesallstringswithinasterisksandreturnstheresult.

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String str = “int s = 2; * This is 2 *”;println(removeComments(str)); // “int s = 2; ”str = “Hi * Hello * Hello”;println(removeComments(str)); // “Hi Hello“str = “No comments!”;println(removeComments(str)); // “No comments!“

Strings Practice

• SuperhelpfulStringspattern:givenastring,iteratethroughandbuildupanewstring.(Sincestringsareimmutable!)

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String oldStr = ...String newStr = “”;for (int i = 0; i < oldStr.length(); i++) {

// build up newStr}

Strings Practice

private String removeComments(String str) {String newStr = “”;boolean inComment = false;for (int i = 0; i < str.length(); i++) {

if (str.charAt(i) == ‘*’) inComment = !inComment;

// Note: if at end of comment, inComment already false!if (!inComment && str.charAt(i) != ‘*’) {

newStr += str.charAt(i);}

}return newStr; // DON’T FORGET!!

}

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Strings Practice

Parting Words

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• Trytogettoeveryproblem•Don’trushtocodingtooquickly•Pseudocode!• Lookoverthepracticemidterm

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Parting Words

cs106a review session - stanford university...event-driven programming let’s write a program like...

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