Thisdocumentiscopyright(C)StanfordComputerScienceandMartyStepp,licensedunderCreativeCommonsAttribution2.5License.Allrightsreserved.BasedonslidescreatedbyKeithSchwarz,MehranSahami,EricRoberts,StuartReges,andothers.

CS106A,Lecture16Arrays

suggestedreading:JavaCh.11.1-11.5

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Where Are We in CS 106A?•KareltheRobot•Java•ConsolePrograms•TextProcessing•GraphicsPrograms•DataStructures•DefiningourownVariableTypes•GUIs

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Midterm!Arrays

HW5:ImageShop

Youarehere

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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What are Data Structures?

Datastructuresarevariabletypesthatcanstoredataininteresting

ways.

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Why Are Data Structures Useful?ConsideraprogramsimilartoWeatherfromHW2thatpromptsfordailytemperaturesandprintsaverages,high/lows,etc.

– Whyisthishardtowritewithwhatwe'velearnedsofar?

How many days' temperatures? 7Day 1's high temp: 45Day 2's high temp: 44Day 3's high temp: 39Day 4's high temp: 48Day 5's high temp: 37Day 6's high temp: 46Day 7's high temp: 53All temperatures: [45, 44, 39, 48, 37, 46, 53]Average temp = 44.64 days were above average.Two coldest days: 37, 39Two hottest days: 53, 48

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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ArraysAnewvariabletypethatisanobjectthatrepresentsanordered,homogeneouslist ofdata.

– Arrayshavemanyelements thatyoucanaccessusingindices

index 0 1 2 3 4 5 6 7 8 9

value 12 49 -2 26 5 17 -6 84 72 3

element0 element4 element9

length=10

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Creating an Arraytype[] name = new type[length];

int[] numbers = new int[5];

index 0 1 2 3 4value 0 0 0 0 0

Javaautomaticallyinitializeselementsto0.

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Accessing Data In An Array

name[index] // get element at index

• LikeStrings,indicesgofrom0 tothearray'slength- 1.for (int i = 0; i < 7; i++) {

println(numbers[i]);}println(numbers[9]); // exceptionprintln(numbers[-1]); // exception

index 0 1 2 3 4 5 6value 0 1 2 3 4 5 6

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Putting Data In An Array

name[index] = value; // set element at index

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Putting Data In An Array

name[index] = value; // set element at index

• LikeStrings,indicesgofrom0 tothearray'slength- 1.int[] numbers = new int[7];for (int i = 0; i < 7; i++) {

numbers[i] = i;}numbers[8] = 2; // exceptionnumbers[-1] = 5; // exception

index 0 1 2 3 4 5 6value 0 1 2 3 4 5 6

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PracticeQ:Whatarethecontentsofnumbers afterexecutingthiscode?

int[] numbers = new int[8];numbers[1] = 3;numbers[4] = 7;numbers[6] = 5;int x = numbers[1];numbers[x] = 2;numbers[numbers[4]] = 9;

// 0 1 2 3 4 5 6 7A. {0, 3, 0, 2, 7, 0, 5, 9}B. {0, 3, 0, 0, 7, 0, 5, 0}C. {3, 3, 5, 2, 7, 4, 5, 0}D. {0, 3, 0, 2, 7, 6, 4, 4}

arrayElements1

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Arrays Of Other TypesYoucancreatearraysofanyvariabletype.Forexample:

double[] results = new double[5];

String[] names = new String[3];

boolean[] switches = new boolean[4];

GRect[] rects = new GRect[5];

• Javainitializeseachelementofanewarraytoitsdefaultvalue,whichis0 forint anddouble,‘\0’ forchar,false forboolean,andnull forobjects.

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Array LengthSimilartoaString,youcangetthelengthofanarraybysaying

myArray.length

Notethattherearenoparenthesesattheend!

Practice:•Whatistheindexofthelastelement ofanarrayintermsofitslength?

•Whatistheindexofthemiddleelement ofanarrayintermsofitslength?

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Arrays + For Loops = ❤JustlikewithStrings,wecanuseanarray’slength,alongwithitsindices,toperformcooloperations.

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Arrays + For Loops = ❤JustlikewithStrings,wecanuseanarray’slength,alongwithitsindices,toperformcooloperations.Forinstance,wecanefficientlyinitializearrays.

int[] numbers = new int[8];for (int i = 0; i < numbers.length; i++) {

numbers[i] = 2 * i;}

index 0 1 2 3 4 5 6 7

value 0 2 4 6 8 10 12 14

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Arrays + For Loops = ❤JustlikewithStrings,wecanuseanarray’slength,alongwithitsindices,toperformcooloperations.Forinstance,wecanreadinnumbersfromtheuser:

int length = readInt("# of numbers? ");int[] numbers = new int[length];for (int i = 0; i < numbers.length; i++) {

numbers[i] = readInt("Elem " + i + ": ");}

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Arrays + For Loops = ❤JustlikewithStrings,wecanuseanarray’slength,alongwithitsindices,toperformcooloperations.Forinstance,wecansumup allofanarray’selements.

int sum = 0;for (int i = 0; i < numbers.length; i++) {

sum += numbers[i];}println(sum);

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Brief Aside: Creating ArraysSometimes,wewanttohardcodetheelementsofanarray.

int numbers = new int[7];numbers[0] = 5;numbers[1] = 32;numbers[3] = 12;...

// This is tedious!

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Brief Aside: Creating ArraysSometimes,wewanttohardcodetheelementsofanarray.Luckily,Javahasaspecialsyntaxforinitializingarraystohardcodednumbers.

type[] name = { elements };

// Java infers the array lengthint[] numbers = {5, 32, 12, 2, 1, -1, 9};

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Limitations of Arrays• Anarray’slengthisfixed.Youcannotresizeanexistingarray:int[] a = new int[4];a.length = 10; // error

• Youcannotcomparearrayswith== orequals :int[] a1 = {42, -7, 1, 15};int[] a2 = {42, -7, 1, 15};if (a1 == a2) { ... } // false!if (a1.equals(a2)) { ... } // false!

• Anarraydoesnotknowhowtoprintitself:println(a1); // [I@98f8c4]

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Arrays Methods To The Rescue!• TheclassArrays inpackagejava.util hasusefulmethodsformanipulatingarrays:

Methodname DescriptionArrays.binarySearch(array, value) returnstheindexofthegivenvalueinasorted

array(or<0ifnotfound)Arrays.copyOf(array, length) returnsanewcopyofarrayofgivenlengthArrays.equals(array1, array2) returnstrue ifthetwoarrayscontainsame

elementsinthesameorderArrays.fill(array, value); setseveryelementtothegivenvalueArrays.sort(array); arrangestheelementsintosortedorderArrays.toString(array) returnsastringrepresentingthearray,suchas

"[10, 30, -25, 17]"

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Example: Arrays.toStringArrays.toString acceptsanarrayasaparameterandreturnsastringrepresentationofitselements.

int[] e = {0, 2, 4, 6, 8};e[1] = e[3] + e[4]; println("e is " + Arrays.toString(e));

Output:e is [0, 14, 4, 6, 8]

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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Passing Arrays Between Methods

•Arraysarejustanothervariabletype,somethodscantakearraysasparametersandreturnanarray.

private int sumArray(int[] numbers) {...

}

private int[] makeSpecialArray(...) {...return myArray;

}

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Passing Arrays Between Methods

•Arraysarejustanothervariabletype,somethodscantakearraysasparametersandreturnanarray.

•However,arraysareobjects,soperAVariableOriginStory,anarrayvariableboxactuallystoresitslocation.

• Thismeanschangestoanarraypassedasaparameteraffecttheoriginalarray!

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Arrays: Pass By Referencepublic void run() {

int[] numbers = new int[7];fillArray(numbers);println(Arrays.toString(numbers));

}

private void fillArray(int[] arr) {for (int i = 0; i < arr.length; i++) {

arr[i] = 2 * i;}

}

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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Practice: Swapping ElementsLet’swriteamethodcalledswapElements thatswapstwoelementsofanarray.Howcanwedothis?

Whatparametersshouldittake(ifany)?Whatshoulditreturn(ifanything)?

private ??? swapElements(???) {...

}

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Swap: Take 1public void run() {

int[] array = new int[5];...swapElements(array[0], array[1]);...

}

private void swapElements(int x, int y) {int temp = x;x = y;y = temp;

}

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Swap: Take 1public void run() {

int[] array = new int[5];...swapElements(array[0], array[1]);...

}

private void swapElements(int x, int y) {int temp = x;x = y;y = temp;

}

Ints areprimitives,sotheyarepassedbyvalue!Theirvariableboxesstoretheiractualvalues.Sochangestotheparameterdonotaffecttheoriginal.

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Swap: Take 2public void run() {

int[] array = new int[5];...swapElements(array, 0, 1);...

}

private void swapElements(int[] arr, int pos1, int pos2) {int temp = arr[pos1];arr[pos1] = arr[pos2];arr[pos2] = temp;

}

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Swap: Take 2public void run() {

int[] array = new int[5];...swapElements(array, 0, 1);...

}

private void swapElements(int[] arr, int pos1, int pos2) {int temp = arr[pos1];arr[pos1] = arr[pos2];arr[pos2] = temp;

}

Arraysareobjects,sotheyarepassedbyreference!Theirvariableboxesstoretheirlocation.Sochangestotheparameterdoaffecttheoriginal.

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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WeatherStation• WriteaWeatherStation programthatpromptstheusertoenterdailytemperatures,andusesanarraytoproducethisoutput:

How many days' temperatures? 7Day 1's high temp: 45Day 2's high temp: 44Day 3's high temp: 39Day 4's high temp: 48Day 5's high temp: 37Day 6's high temp: 46Day 7's high temp: 53

All temperatures: [45, 44, 39, 48, 37, 46, 53]Average temp = 44.64 days were above average.Two coldest days: 37, 39Two hottest days: 53, 48

Weather

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Plan for Today•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation•Recap

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Recap: Arrays•Anarrayisanordered,homogeneouslist ofdata.• Arrayscanstorebothprimitivesandobjects•Anarray’slengthcannotbechanged onceitiscreated.• Therearenomethodsyoucancallonanarray;however,thereisthehelpfulArrays class,withmethodssuchasArrays.toString.

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Recap•DataStructures•Arrays•ArraysasParametersandReturnValues•Announcements•Practice:SwappingElements•Practice:WeatherStation

Nexttime:2DArrays

Thisdocumentiscopyright(C)StanfordComputerScienceandMartyStepp,licensedunderCreativeCommonsAttribution2.5License.Allrightsreserved.BasedonslidescreatedbyKeithSchwarz,MehranSahami,EricRoberts,StuartReges,andothers.

ExtraSlides

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Array reverse exercise• Writeareversemethodthatreversestheelementsofanarray.

– Example:int[] numbers = {11, 42, -5, 27, 0, 89};reverse(numbers);

– Afterthecall,itshouldstore:[89, 0, 27, -5, 42, 11]

– Thecodeshouldworkforanarrayofanysize.

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Algorithm idea• Swappairsofelementsfromtheedges;workinwards:

index 0 1 2 3 4 5value 11 42 -5 27 0 89index 0 1 2 3 4 5value 89 42 -5 27 0 11index 0 1 2 3 4 5value 89 0 -5 27 42 11index 0 1 2 3 4 5value 89 0 27 -5 42 11

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Possible algorithm• Q:Whatistheeffectofthecodebelow?Doesitreversethearray?int[] numbers = {11, 42, -5, 27, 0, 89};// reverse the arrayfor (int i = 0; i < numbers.length; i++) {

int temp = numbers[i];numbers[i] = numbers[numbers.length - 1 - i];numbers[numbers.length - 1 - i] = temp;

}

– A. Codeiscorrectandreversesthearrayproperly.– B. Elementsarereversed,butsomearelost/missing.– C. Indexesareoff-by-1.– D. Arraycontentsarethesameattheend;thecodedoesnothing.– E. Noneoftheabove

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Correct algorithm• Correctedversion:int[] numbers = {11, 42, -5, 27, 0, 89};// reverse the arrayfor (int i = 0; i < numbers.length / 2; i++) {

int temp = numbers[i];numbers[i] = numbers[numbers.length - 1 - i];numbers[numbers.length - 1 - i] = temp;

}

index 0 1 2 3 4 5value 11 42 -5 27 0 89index 0 1 2 3 4 5value 89 42 -5 27 0 11index 0 1 2 3 4 5value 89 0 -5 27 42 11index 0 1 2 3 4 5value 89 0 27 -5 42 11