cs 106b, lecture 3 vector and grid - stanford university...example file, carroll.txt : 1 beware the...

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CS106B,Lecture3VectorandGrid

reading:ProgrammingAbstractionsinC++,Chapter4-5

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2

Plan for Today • Learnabouttwonew"ADTs"orcollections

–  Vector:adatastructureforrepresentinglists– Grid:adatastructureidealforrepresentingtwodimensionalinformation

3

STL vs. Stanford • collection:anobjectthatstoresdata;a.k.a."datastructure"

–  theobjectsstoredarecalledelements.–  Alsoknownas"ADTs"–abstractdatatypes

• StandardTemplateLibrary(STL):C++builtinstandardlibraryofcollections.– vector,map,list,...–  Powerfulbutsomewhathardtousefornewcoders(messysyntax)–take106L!

• StanfordC++library(SPL):CustomlibraryofcollectionsmadeforuseinCS106B/X.– Vector,Grid,Stack,Queue,Set,Map,...–  SimilartoSTL,butsimplerinterfaceanderrormessages.– Notethecapitalizedfirstletter

4

Vectors (Lists) #include"vector.h"

• vector(akalist):acollectionofelementswith0-basedindexes–  likeadynamically-resizingarray(JavaArrayListorPythonlist)–  Includethetypeofelementsinthe<>brackets//initializeavectorcontaining5integers//index01234Vector<int>nums{42,17,-6,0,28};

Vector<string>names;//{}names.add("Ashley");//{"Ashley"}names.add("Shreya");//{"Ashley","Shreya"}names.insert(0,"Ed");//{"Ed","Ashley","Shreya"}

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Why not arrays? //actualarraysinC++aremostlyawfulintnums[5]{42,17,-6,0,28};//no

• Arrayshavefixedsizeandcannotbeeasilyresized.–  InC++,anarraydoesn'tevenknowitssize.(no.lengthfield)

• C++letsyouindexoutofthearraybounds(garbagememory)withoutnecessarilycrashingorwarning.

• Anarraydoesnotsupportmanyoperationsthatyou'dwant:–  inserting/deletingelementsintothefront/middle/backofthearray,reversing,sortingtheelements,searchingforagivenvalue...

index 0 1 2 3 4value 42 17 -6 0 28

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Vector members (5.1) v.add(value);orv+=value;orv+=v1,v2,...,vN;

appendsvalue(s)atendofvector

v.clear(); removesallelementsv[i]orv.get(i) returnsthevalueatgivenindexv.insert(i,value); insertsgivenvaluejustbeforethegivenindex,shifting

subsequentvaluestotherightv.isEmpty() returnstrueifthevectorcontainsnoelementsv.remove(i); removes/returnsvalueatgivenindex,shifting

subsequentvaluestotheleftv[i]=value;orv.set(i,value);

replacesvalueatgivenindex

v.subList(start,length) returnsnewvectorofsub-rangeofindexes

v.size() returnsthenumberofelementsinvector

v.toString() returnsastringrepresentationofthevectorsuchas"{3,42,-7,15}"

ostr<<v printsvtogivenoutputstream(e.g.cout<<v)

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Iterating over a vector Vector<string>names{"Ed","Hal","Sue"};for(inti=0;i<names.size();i++){cout<<names[i]<<endl;//forloop}//EdHalSuefor(inti=names.size()-1;i>=0;i--){cout<<names[i]<<endl;//forloop,backward}//SueHalEd

for(stringname:names){cout<<name<<endl;//"for-each"loop}//EdHalSue//Can'tedit(insert/delete)infor-eachloop

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Vector insert/remove v.insert(2,42);

• shiftelementsrighttomakeroomforthenewelement

v.remove(1);• shiftelementslefttocoverthespaceleftbytheremovedelement

(Theseoperationsareslowerthemoreelementstheyneedtoshift.)

index 0 1 2 3 4value 3 8 9 7 5

index 0 1 2 3 4 5value 3 8 42 9 7 5

index 0 1 2 3 4 5value 3 8 42 9 7 5

index 0 1 2 3 4value 3 42 9 7 5

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Announcements • Assignment0dueFriday

–  Fillouttheexamsurveyby5PMonFriday–  IfyouneedhelpwithQtstopbyLaIRorAshley'sofficehours(lastchancetogethelpis12:15PMonThursday)

• Sectionsstarttoday!Shouldhavereceivedanemailfromcs198@cs.stanford.edu–  Youcanswitchyoursectionorsignuplateatcs198.stanford.edu–  EmailShreyaatshreya@cs.stanford.eduifyouwereassignedadifferentsectionthanyourpartner

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Grid (5.1) #include"grid.h"

•  likea2Darray,butmorepowerful• Goodforboardgames,matrices,images,citymaps,etc.• mustspecifyelementtypein<>(atemplateoratypeparameter)

//constructingaGridGrid<int>matrix(3,4);matrix[0][0]=75;...

//orspecifyelementsin{}Grid<int>matrix={{75,61,83,71},{94,89,98,100},{63,54,51,49}};

0 1 2 30 75 61 83 711 94 89 98 1002 63 54 51 49

row

column

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Grid members (5.1)* Grid<type>name(r,c);Grid<type>name;

creategridwithgivennumberofrows/cols;empty0x0gridifomitted

g[r][c]org.get(r,c) returnsvalueatgivenrow/colg.fill(value); seteverycelltostorethegivenvalueg.inBounds(r,c) returnstrueifgivenpositionisinthegridg.numCols()org.width() returnsnumberofcolumnsg.numRows()org.height() returnsnumberofrowsg.resize(nRows,nCols); resizesgridtonewsize,discardingoldcontentsg[r][c]=value;org.set(r,c,value);

storesvalueatgivenrow/col

g.toString() returnsastringrepresentationofthegridsuchas"{{3,42},{-7,1},{5,19}}"

ostr<<g prints,e.g.{{3,42},{-7,1},{5,19}}

*(apartiallist;seehttp://stanford.edu/~stepp/cppdoc/)

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Looping over a grid • Row-majororder:

for(intr=0;r<grid.numRows();r++){for(intc=0;c<grid.numCols();c++){dosomethingwithgrid[r][c];}}

//"for-each"loop(alsorow-major)for(intvalue:grid){dosomethingwithvalue;}

• Column-majororder:for(intc=0;c<grid.numCols();c++){for(intr=0;r<grid.numRows();r++){dosomethingwithgrid[r][c];}}

0 1 2 3

0 75 61 83 71

1 94 89 98 91

2 63 54 51 49

0 1 2 3

0 75 61 83 71

1 94 89 98 91

2 63 54 51 49

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Grid as parameter • WhenaGridispassedbyvalue,C++makesacopyofitscontents.

–  Copyingisslow;youshouldpassbyreferencewith&–  Ifthecodewon'tmodifythegrid,alsopassitasconst

//Whichoneisbest?A)intcomputeSum(Grid<int>g){B)intcomputeSum(Grid<int>&g){C)intcomputeSum(constGrid<int>g){D)intcomputeSum(constGrid<int>&g){//Whichoneisbest?A)voidinvert(Grid<double>matrix){B)voidinvert(Grid<double>&matrix){C)voidinvert(constGrid<double>matrix){D)voidinvert(constGrid<double>&matrix){

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Grid exercise •  WriteafunctionknightCanMovethatacceptsagridandtworow/columnpairs(r1,c1),(r2,c2)asparameters,andreturnstrueifthereisaknightatchessboardsquare(r1,c1)thatcanlegallymovetoemptysquare(r2,c2).–  Recallthataknightmakesan"L"shapedmove,going2squaresinonedimensionand1squareintheother.

–  knightCanMove(board,1,2,2,4)returnstrue0 1 2 3 4 5 6 7

0 "king"

1 "knight"

2

3 "rook"

4

5

6

7

knightCanMove

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Grid exercise solution boolknightCanMove(Grid<string>&board,intr1,intc1,intr2,intc2){if(!board.inBounds(r1,c1)||!board.inBounds(r2,c2)){returnfalse;}

if(board[r1][c1]!="knight"||board[r2][c2]!=""){returnfalse;}

intdr=abs(r1-r2);intdc=abs(c1-c2);if(!((dr==1&&dc==2)||(dr==2&&dc==1))){returnfalse;}

returntrue;}

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Grid solution 2 boolknightCanMove(Grid<string>&board,intr1,intc1,intr2,intc2){intdr=abs(r1-r2),dc=abs(c1-c2);returnboard.inBounds(r1,c1)&&board.inBounds(r2,c2)&&board[r1][c1]=="knight"&&board[r2][c2]==""&&((dr==1&&dc==2)||(dr==2&&dc==1));}

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

Overflow(extra)slides

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istringstream #include<sstream>

• Anistringstreamletsyoutokenizeastring.

//readspecificwordtokensfromastringistringstreaminput("JennySmith8675309");stringfirst,last;intphone;input>>first>>last;//first="Jenny",last="Smith"input>>phone;//8675309//readalltokensfromastringistringstreaminput2("Tobeornottobe");stringword;while(input2>>word){cout<<word<<endl;//To\nbe\nor\nnot\n...}

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ostringstream #include<sstream>

• Anostringstreamletsyouwriteoutputintoastringbuffer.– Usethestrmethodtoextractthestringthatwasbuilt.//produceaformattedstringofoutputintage=42,iq=95;ostringstreamoutput;output<<"Zoidberg'sageis"<<age<<endl;output<<"andhisIQis"<<iq<<"!"<<endl;stringresult=output.str();//result="Zoidberg'sageis42\nandhisIQis95!\n"

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Bug: Mix lines/tokens cout<<"Howoldareyou?";intage;cin>>age;

cout<<"Andwhat'syourname?";stringname;getline(cin,name);cout<<"Wow,"<<name<<"is"<<age<<"!"<<endl;//output://Howoldareyou:17//Andwhat'syourname:Stuart//Wow,is17!–  Advice:Don'tmixgetlineand>>onthesameinputstream.–  Advice:AlwaysuseStanfordgetXxxmethodstoreadfromcin.

userinput:

17\nStuart\n

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Exercise: inputStats2 • WriteafunctioninputStats2thatprintsstatisticsaboutthedatainafile.Examplefile,carroll.txt:

1BewaretheJabberwock,myson,2thejawsthatbite,theclawsthatcatch,34BewaretheJubJubbirdandshun5thefrumiousbandersnatch.

• ThecallofinputStats2("carroll.txt");shouldprint:

Line1:30chars,5wordsLine2:41chars,8wordsLine3:0chars,0wordsLine4:31chars,6wordsLine5:26chars,3wordslongest=41,average=25.6

inputStats2

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inputStats2 solution /*Printslength/countstatisticsaboutdatainthegivenfile.*/voidinputStats2(stringfilename){ifstreaminput;input.open(filename);intlineCount=0,longest=0,totalChars=0;stringline;while(getline(input,line)){lineCount++;totalChars+=line.length();longest=max(longest,line.length());intwordCount=countWords(line);//onnextslidecout<<"Line"<<lineCount<<":"<<line.length()<<"chars,"<<wordCount<<"words"<<endl;}doubleaverage=(double)totalChars/lineCount;cout<<longest="<<longest<<",average="<<average<<endl;}

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inputStats2 solution /*Returnsthenumberofwordsinthegivenstring.*/intcountWords(stringline){istringstreamwords(line);intwordCount=0;stringword;while(words>>word){wordCount++;}returnwordCount;}

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Formatted I/O #include<iomanip>

–  helpsproduceformattedoutput,alaprintf

for(inti=2;i<=2000;i*=10){//21.41cout<<left<<setw(4)<<i//204.47<<right<<setw(8)<<fixed//20014.14<<setprecision(2)<<sqrt(i)<<endl;//200044.72}

Membername Description

setw(n) right-alignsnexttokeninafieldncharswide

setfill(ch) setspaddingcharsinsertedbysetwtothegivenchar(default'')

setbase(b) printsfuturenumerictokensinbase-b

left,right left-orright-alignstokensifsetwisused

setprecision(d) printsfuturedoubleswithddigitsafterdecimal

fixed printsfuturedoubleswithafixednumberofdigits

scientific printsfuturedoublesinscientificnotation

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Exercise: Hours • Givenhours.txtofsectionleaderhoursworked,inthisformat:

1123Alex3241246Jessica8.51.55510637289Erik3644.684

• WritecodetooutputhoursworkedbyeachSLinthisformat:

Alex(ID#123)worked10.0hours(2.50/day)Jessica(ID#46)worked36.0hours(6.00/day)Erik(ID#7289)worked21.7hours(4.34/day)

hoursWorked

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Hours solution /*Thisprogramcomputesthe...*/#include<fstream>#include<iomanip>#include<iostream>#include<sstream>usingnamespacestd;intmain(){ifstreaminput;input.open("hours.txt");stringline;while(getline(input,line)){//"7289Erik3644.684"istringstreamtokens(line);intid;//7289stringname;//"Erik"tokens>>id>>name;//restoftokensaredaysdoubletotalHours=0.0;intdays=0;...

doublehours;while(tokens>>hours){totalHours+=hours;days++;}//Erik(ID#7289)worked//21.7hours(4.34/day)cout<<left<<setw(9)<<name<<"(ID#"<<right<<setw(5)<<id<<")worked"<<fixed<<setprecision(1)<<totalHours<<"hours("<<setprecision(2)<<totalHours/days<<"/day)"<<endl;}return0;}