1/28/16 c++ primer · 1/28/16 1 c++ primer part 1 cmsc 202 topics covered • our first “hello...

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C++ Primer Part 1

CMSC 202

Topics Covered •  Our first “Hello world” program •  Basic program structure •  main() •  Variables, identifiers, types •  Expressions, statements •  Operators, precedence, associativity

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ASampleC++Program

Copyright©2012PearsonAddison-Wesley.Allrightsreserved.

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Display1.1ASampleC++Program(2of2)

1-4Copyright©2012PearsonAddison-Wesley.Allrightsreserved.

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C++Variables

•  C++IdenGfiers–  Keywords/reservedwordsvs.IdenGfiers–  Case-sensiGvityandvalidityofidenGfiers– Meaningfulnames!

•  Variables–  AmemorylocaGontostoredataforaprogram– Mustdeclarealldatabeforeuseinprogram


Variable Declaration •  Syntax: <type> <legal identifier>; •  Examples:

int sum; float average; double grade = 98;

–  Must be declared before being used –  May appear in various places and contexts (described later) –  Must be declared of a given type (e.g. int, float, char, etc.)

Semicolon required!

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Variable Declarations (con’t)

When we declare a variable, we tell the compiler: •  When and where to set aside memory space for

the variable •  How much memory to set aside •  How to interpret the contents of that memory: the

specified data type •  What name we will be referring to that location by:

its identifier

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Naming Conventions •  Naming conventions are rules for names of

variables to improve readability •  CMSC 202 has its own standards, described in

detail on the course website Ø Start with a lowercase letter Ø  Indicate "word" boundaries with an uppercase letter Ø Restrict the remaining characters to digits and

lowercase letters

topSpeed bankRate1 timeOfArrival

•  Note: variable names are case sensitive!

DataTypes:Display1.2SimpleTypes(1of2)


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DataTypes:Display1.2SimpleTypes(2of2)


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AssigningData

•  IniGalizingdataindeclaraGonstatement–  Results"undefined"ifyoudon’t!

•  intmyValue=0;

•  AssigningdataduringexecuGon–  Lvalues(le\-side)&Rvalues(right-side)

•  Lvaluesmustbevariables•  Rvaluescanbeanyexpression•  Example:distance=rate*Gme;Lvalue:"distance"Rvalue:"rate*Gme"


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DataAssignmentRules

•  CompaGbilityofDataAssignments–  Typemismatches

•  GeneralRule:Cannotplacevalueofonetypeintovariableofanothertype

–  intVar=2.99; //2isassignedtointVar!•  Onlyintegerpart"fits",sothat’sallthatgoes•  Called"implicit"or"automaGctypeconversion"

–  Literals•  2,5.75,"Z","HelloWorld"•  Considered"constants":can’tchangeinprogram


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EscapeSequences

•  "Extend"characterset•  Backslash,\precedingacharacter

–  Instructscompiler:aspecial"escapecharacter"iscoming

– Followingcharactertreatedas"escapesequencechar"

– Display1.3nextslide


Display1.3SomeEscapeSequences(1of2)


Display1.3SomeEscapeSequences(2of2)


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LiteralData

•  Literals–  Examples:

•  2 //Literalconstantint•  5.75 //Literalconstantdouble•  'Z' //Literalconstantchar•  "HelloWorld\n" //Literalconstantstring

•  CannotchangevaluesduringexecuGon•  Called"literals"becauseyou"literallytyped"theminyourprogram!


Constants

•  Youshouldnotuseliteralconstantsdirectlyinyourcode–  Itmightseemobvioustoyou,butnotso:

•  “limit=52”:isthisweeksperyear…orcardsinadeck?

•  Instead,youshouldusenamedconstants– Representtheconstantwithameaningfulname– AlsoallowsyoutochangemulGpleinstancesinacentralplace


Constants

•  Therearetwowaystodothis:– Oldway:preprocessordefiniGon:

#define WEEKS_PER_YEAR 52

(Note:thereisno“=“)– Newway:constantvariable:

•  Justaddthekeyword“const”tothedeclaraGon

const float PI = 3.14159;


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ArithmeGcOperators:Display1.4NamedConstant(1of2)

•  StandardArithmeGcOperators– Precedencerules–standardrules


ArithmeGcOperators:Display1.4NamedConstant(2of2)


Operators,Expressions

•  Recall:mostprogramminglanguageshaveavarietyofoperators:calledunary,binary,andeventernary,dependingonthenumberofoperands(thingstheyoperateon)

•  Usuallyrepresentedbyspecialsymboliccharacters:e.g.,‘+’foraddiGon,‘*’formulGplicaGon


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Operators,Expressions

•  TherearealsorelaGonaloperators,andBooleanoperators

•  Simpleunitsofoperandsandoperatorscombineintolargerunits,accordingtostrictrulesofprecedenceandassocia-vity

•  Eachcomputableunit(bothsimpleandlargeraggregates)arecalledexpressions


BinaryOperators

•  Whatisabinaryoperator?–  Anoperatorthathastwooperands

<operand><operator><operand>

–  ArithmeGcOperators+-*/%

–  RelaGonalOperators<>==<=>=

–  LogicalOperators&&||

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RelaGonalOperators•  InC++,allrelaGonaloperatorsevaluatetoabooleanvalueofeithertrueorfalse. x = 5; y = 6;

x>ywillalwaysevaluatetofalse.

•  C++hasaternaryoperator–thegeneralformis:(condiGonalexpression)?truecase:falsecase;

•  Ternaryexample:Cout << (( x > y ) ? "X is greater" : "Y is greater");

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UnaryOperators•  Unaryoperatorsonlyhaveoneoperand.

!++--

!islogicalnegaGon,!trueisfalse,!falseistrue++and--aretheincrementanddecrementoperatorsx++apost-increment(pos|ix)operaGon++xapre-increment(prefix)operaGon

•  ++and--are“shorthand”operators.Moreontheselater…

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Precedence,AssociaGvity•  OrderofoperatorapplicaGontooperands:

•  Pos|ixoperators:++--(righttole\)•  Unaryoperators:+-++--!(righttole\)•  */%(le\toright)•  +-(le\toright)•  <><=>=•  ==!=•  &&•  ||•  ?:•  Assignmentoperator:=(righttole\)

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AssociaGvity

•  Whatisthevalueoftheexpression?3*6/9(3*6)/918/92

•  Whataboutthisone?intx,y,z;x=y=z=0;


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ArithmeGcPrecision

•  PrecisionofCalculaGons– VERYimportantconsideraGon!

•  ExpressionsinC++mightnotevaluateasyou’d"expect"!

– "Highest-orderoperand"determinestypeofarithmeGc"precision"performed

– Commonpi|all!


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ArithmeGcPrecisionExamples

•  Examples:–  17/5evaluatesto3inC++!

•  Bothoperandsareintegers•  Integerdivisionisperformed!

–  17.0/5equals3.4inC++!•  Highest-orderoperandis"doubletype"•  Double"precision"divisionisperformed!

–  intintVar1=1,intVar2=2;intVar1/intVar2;

•  Performsintegerdivision!•  Result:0!


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IndividualArithmeGcPrecision

•  CalculaGonsdone"one-by-one"–  1/2/3.0/4performs3separatedivisions.

•  Firstà1/2equals0•  Thenà0/3.0equals0.0•  Thenà0.0/4equals0.0!

•  Sonotnecessarilysufficienttochangejust"oneoperand"inalargeexpression– MustkeepinmindallindividualcalculaGonsthatwillbeperformedduringevaluaGon!


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TypeCasGng

•  CasGngforVariables–  Canadd".0"toliteralstoforceprecisionarithmeGc,butwhataboutvariables?

•  Wecan’tuse"myInt.0"!–  staGc_cast<double>intVar –  Explicitly"casts"or"converts"intVartodoubletype

•  Resultofconversionisthenused•  Exampleexpression:doubleVar=staGc_cast<double>intVar1/intVar2;

–  CasGngforcesdouble-precisiondivisiontotakeplaceamongtwointegervariables!


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TypeCasGng

•  Twotypes–  Implicit—alsocalled"AutomaGc"

•  DoneFORyou,automaGcally17/5.5Thisexpressioncausesan"implicittypecast"totakeplace,casGngthe17à17.0

–  Explicittypeconversion•  ProgrammerspecifiesconversionwithcastoperatorstaGc_cast<double>17/5.5

Sameexpressionasabove,usingexplicitcaststaGc_cast<double>myInt/myDouble

Moretypicaluse;castoperatoronvariable


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ShorthandOperators

•  Increment&DecrementOperators–  Justshort-handnotaGon–  Incrementoperator,++intVar++;isequivalenttointVar=intVar+1;

– Decrementoperator,--intVar--;isequivalenttointVar=intVar–1;


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ShorthandOperators:TwoOpGons

•  Post-IncrementintVar++–  Usescurrentvalueofvariable,THENincrementsit

•  Pre-Increment++intVar–  Incrementsvariablefirst,THENusesnewvalue

•  "Use"isdefinedaswhatever"context"variableiscurrentlyin

•  Nodifferenceif"alone"instatement:intVar++;and++intVar;àidenGcalresult


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Post-IncrementinAcGon

•  Post-IncrementinExpressions:int n=2,

valueProduced;valueProduced=2*(n++);cout<<valueProduced<<endl;cout<<n<<endl;–  Thiscodesegmentproducestheoutput:43

–  Sincepost-incrementwasused


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Pre-IncrementinAcGon

•  NowusingPre-increment:int n=2,

valueProduced;valueProduced=2*(++n);cout<<valueProduced<<endl;cout<<n<<endl;–  Thiscodesegmentproducestheoutput:63

–  Becausepre-incrementwasused


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AssigningData:ShorthandNotaGons

•  Display,page14


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ConsoleInput/Output

•  I/Oobjectscin,cout,cerr•  DefinedintheC++librarycalled<iostream>

•  Musthavetheselines(calledpre-processordirecGves)nearstartoffile:–  #include<iostream>usingnamespacestd;

–  TellsC++touseappropriatelibrarysowecanusetheI/Oobjectscin,cout,cerr


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ConsoleOutput

•  Whatcanbeoutpu�ed?–  Anydatacanbeoutpu�edtodisplayscreen

•  Variables•  Constants•  Literals•  Expressions(whichcanincludeallofabove)

–  cout<<numberOfGames<<"gamesplayed.";2valuesareoutpu�ed:

"value"ofvariablenumberOfGames,literalstring"gamesplayed."

•  Cascading:mulGplevaluesinonecout


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SeparaGngLinesofOutput

•  Newlinesinoutput–  Recall:"\n"isescapesequenceforthechar"newline"

•  Asecondmethod:objectendl

•  Examples:cout<<"HelloWorld\n";

•  Sendsstring"HelloWorld"todisplay,&escapesequence"\n",skippingtonextline

cout<<"HelloWorld"<<endl;•  Sameresultasabove


Forma�ngOutput

•  Forma�ngnumericvaluesforoutput– Valuesmaynotdisplayasexpected cout << "The price is $" << price << endl;

•  Ifprice(declaredadouble)hasthevalue78.5,youmightget

–  Thepriceis$78.5000000–  Thepriceis$78.5

•  Neitheriswhatyouwant– HavetotellC++howtooutputnumbers.


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Forma�ngNumbers

•  "MagicFormula"toforcedecimalsizes:cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(2);

•  Thesestatementsforceallfuturecout’edvaluestohaveexactlytwodigitsa\erthedecimalplace:–  Example:cout<<"Thepriceis$"<<price<<endl;

•  Nowresultsinthefollowing:The price is $78.50

•  Canmodifyprecision"asyougo"aswell.


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Forma�ngIntegers

•  Fieldwidthandfillcharacters– Must#include <iomanip> –  setw(n)setsfieldwidthton–  cout.fill(c)sets“fill”charactertoc

•  Example:int x = 7;

cout.fill('0'); // set fill character to zero

cout << setw(3) << x << endl;

Outputs007 (le\-padswithzeros)


C-strings

•  C++hastwodifferentkindsof“stringofcharacters”:–  theoriginalC-string:arrayofcharacters–  Theobject-orientedstringclass

•  C-stringsareterminatedwithanullcharacter(‘\0’)char myString[80];

declaresavariablewithenoughspaceforastringwith79usablecharacters,plusnull.


C-strings

•  YoucaniniGalizeaC-stringvariable:charmyString[80]=“Helloworld”;

Thiswillsetthefirst11charactersasgiven,makethe12thcharacter‘\0’,andtherestunusedfornow.


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Stringtype•  C++addedadatatypeof“string”

– NotaprimiGvedatatype;disGncGonwillbemadelater

– Mayneed#include <string> atthetopoftheprogram

– The“+”operatoronstringsconcatenatestwostringstogether

– cin>>strwherestrisastringonlyreadsuptothefirstwhitespacecharacter


StringEquality

•  InPython,youcanusethesimple“==“operatortocomparetwostrings:

ifname==“Fred”:•  InC++,youcanuse“==“tocomparetwostringclassitems,butnotC-strings!

•  TocomparetwoC-strings,youhavetousethefuncGonstrcmp();itisnotsyntacGcallyincorrecttocomparetwoC-stringswith“==“,butitdoesnotdowhatyouexpect…


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InputUsingcin

•  cinforinput,coutforoutput•  Differences:

–  ">>"(extracGonoperator)pointsopposite•  Thinkofitas"poinGngtowardwherethedatagoes"

–  Objectname"cin"usedinsteadof"cout"–  Noliteralsallowedforcin

•  Mustinput"toavariable"

•  cin>>num;– Waitson-screenforkeyboardentry–  Valueenteredatkeyboardis"assigned"tonum


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PrompGngforInput:cinandcout

•  Always"prompt"userforinputcout<<"Enternumberofdragons:";cin>>numOfDragons;

–  Noteno"\n"incout.Prompt"waits"onsamelineforkeyboardinputasfollows:Enternumberofdragons:____

–  Underscoreabovedenoteswherekeyboardentryismade

•  Everycinshouldhavecoutprompt– Maximizesuser-friendlyinput/output


Input/Output(1of2)


Input/Output(2of2)


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ErrorOutput

•  Outputwithcerr– cerrworksalmostthesameascout– ProvidesmechanismfordisGnguishingbetweenregularoutputanderroroutput

•  Re-directoutputstreams– Mostsystemsallowcoutandcerrtobe"redirected"tootherdevices

•  e.g.,lineprinter,outputfile,errorconsole,etc.


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ProgramStyle•  Bo�om-line:Makeprogramseasytoreadandmodify

•  Comments,twomethods:–  //TwoslashesindicateenGrelineistobeignored–  /*Delimitersindicateseverythingbetweenisignored*/–  Bothmethodscommonlyused

•  IdenGfiernaming–  ALL_CAPSforconstants–  lowerToUpperforvariables–  Mostimportant:MEANINGFULNAMES!


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Libraries

•  C++StandardLibraries•  #include<Library_Name>

– DirecGveto"add"contentsoflibraryfiletoyourprogram

– Called"preprocessordirecGve"•  Executesbeforecompiler,andsimply"copies"libraryfileintoyourprogramfile

•  C++hasmanylibraries–  Input/output,math,strings,etc.


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Namespaces•  Namespacesdefined:

–  CollecGonofnamedefiniGons•  Fornow:interestedinnamespace"std"

–  HasallstandardlibrarydefiniGonsweneed•  Examples:#include<iostream>usingnamespacestd;

•  IncludesenGrestandardlibraryofnamedefiniGons

•  #include<iostream>usingstd::cin; usingstd::cout;

•  Canspecifyjusttheobjectswewant


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Summary1

•  C++iscase-sensiGve•  Usemeaningfulnames

– Forvariablesandconstants•  Variablesmustbedeclaredbeforeuse

– ShouldalsobeiniGalized•  UsecareinnumericmanipulaGon

– Precision,parentheses,orderofoperaGons•  #includeC++librariesasneeded


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Summary2

•  Objectcout–  Usedforconsoleoutput

•  Objectcin–  Usedforconsoleinput

•  Objectcerr–  Usedforerrormessages

•  Usecommentstoaidunderstandingofyourprogram–  Donotovercomment


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UsingtheCCompileratUMBC

•  Invokingthecompilerissystemdependent.

– AtUMBC,wehavetwoCcompilersavailable,ccandgcc.

– Forthisclass,wewillusethegcccompilerasitisthecompileravailableontheLinuxsystem.

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InvokingthegccCompiler

  Attheprompt,type

  g++ -Wall program.cpp –o program.out

  whereprogram.cppistheC++programsourcefile(thecompileralsoaccepts“.cc”asafileextensionforC++source)

•  -WallisanopGontoturnonallcompilerwarnings(bestfornewprogrammers).

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TheResult:a.out

•  Iftherearenoerrorsinprogram.cpp,thiscommandproducesanexecutablefile,whichisonethatcanbeexecuted(run).

•  Ifyoudonotusethe“-o”opGon,thecompilernamestheexecutablefilea.out.

•  Toexecutetheprogram,attheprompt,type ./program.out

•  Althoughwecallthisprocess“compilingaprogram,”whatactuallyhappensismorecomplicated.

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UNIXProgrammingTools

•  Wewillbeusingthe“make”systemtoautomatewhatwasshowninthepreviousfewslides

•  Thiswillbediscussedinlab


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