C程序设计基础 英文版 课件 Chapter 2 Data Types and Expressions

Chapter2DataTypesandExpressionsFangWangOutlineChapter2DataTypesandExpressions2.1TheGeneralFormofaSimpleProgram2.2CharacterSetandKeywords2.3DataTypes2.4VariablesandConstants2.5OperatorsandExpressions2IntegratedDevelopmentEnvironmentsCode::Blocks32.1TheGeneralFormofaSimpleProgramNOTE:•Crequiresasemicolon(;)attheendofeverystatement.•statement.printfisastandardCfunction--calledfrommain.•Statementsshouldbeenclosedinbraces.•“\n”signifiesnewline.Formattedoutput--morelater.•Casesensitive,printfandPrintfaredifferent.•Linenumbercanbeshowninaneditor,itisnotthecontentofprogram.42.1TheGeneralFormofaSimpleProgram2.2TheGeneralFormofaSimpleProgramSimpleCprogramshavetheformwiththreeparts

directives

intmain()

{

statements

}EventhesimplestCprogramsrelyonthreekeylanguagefeatures:DirectivesFunctionsStatements52.2TheGeneralFormofaSimpleProgramDirectivesBeforeaCprogramiscompiled,itisfirsteditedbyapreprocessor.Commandsintendedforthepreprocessorarecalleddirectives.Example:

#include<stdio.h>stdio.h

isaheadercontaininginformationaboutC’sstandardI/Olibrary.Directivesalwaysbeginwitha#character.there’snosemicolonorotherspecialmarkerattheend.62.2TheGeneralFormofaSimpleProgramFunctionsAfunctionisaseriesofstatementsthathavebeengroupedtogetherandgivenaname.Afunctionisusuallydesignedtoperformaspecifictask.Libraryfunctionsare

providedaspartoftheCimplementation.7XXXXXX(XXX)

{

statements

}intmain()

{

statements

}2.2TheGeneralFormofaSimpleProgramThemainFunction

isspecialEveryCprogrammusthaveoneandonlyonemainfunctionsomewhere.main

isspecial:itgetscalledautomaticallywhentheprogramisexecuted.Thewordintjustbeforemainindicatesthemainfunctionreturnsanintegervalue.main

returnsacode;thevalue0indicatesnormalprogramtermination.Ifthere’snoreturn

statementattheendofthemain

function,manycompilerswillproduceawarningmessage.Thewordvoidinparentheses()indicatesthatmainhasnoarguments.(Youdon'thavetowritetheword“void”,justleaveitblank)82.2TheGeneralFormofaSimpleProgramStatements：Astatementisacommandtobeexecutedwhentheprogramruns.Crequiresthateachstatementendwithasemicolon(;)Helloworldusesonlytwokindsofstatements.Oneisthereturn

statement;theotheristhefunctioncall.Askingafunctiontoperformitsassignedtaskisknownascallingthefunction.main()callsprintf

todisplayastring:92.2TheGeneralFormofaSimpleProgramPrintingStringsTheprintf

functiondisplaysastring—charactersenclosedindoublequotationmarks(””).\n

(thenew-linecharacter)

10In-classassignment1:introduceyourselfCreateanewprojectnamed“ex01”Edityour“helloworld”anddisplay3statements:1.yournameandwhereareyoufrom2.Introduceyourcountry3.SaysomethingtoChina

Thefilenamedoesn’tmatter,butdon’tusespace.1112CommentsAcommentbeginswith/*andendwith*/.

/*Thisisthefirstlineofthecomment

Thisisthesecondlineofthecomment

*/

Commentscanalsobewritteninthefollowingway: //Thisisacomment1.CharacterLinesbetween/*and*/areacomment.2.Commentsexplainwhattheprogramdoes.3.Commentsareignoredbythecompiler.13In-classassignment2:commentsAddsomecommentsto“introduceyourself”

142.1.2TheBasicStepstoWriteaSimpleProgramWecanconsideranotherprogramthatinvolvesaddingtwonumbers,denotedasaandb15162.2CharacterSetandKeywordsAcharactersetisasetofalphabets,digits,andsomespecialcharactersthatarevalidintheClanguage.(1)AlphabetsUppercase:ABCDEFGHIJKLMNOPQRSTUVWXYZLowercase:abcdefghijklmnopqrstuvwxyz(2)Digits0123456789(3)SpecialCharacters<>._();$:%[]#?'&{}"^!*/|-\～+172.2.2Keywords182.2.3Identifiers---------NAMESNamesforvariables,functions….arecalledidentifiers.Anidentifiermaycontainletters,digits,andunderscores,butmustbeginwithaletterorunderscore: times10get_next_char

Examplesofillegalidentifiers:

10timesget-next-charCiscase-sensitive:itdistinguishesbetweenupper-caseandlower-caselettersinidentifiers.Forexample,thefollowingidentifiersarealldifferent:

jobjoBjObjOBJobJoBJObJOB192.3DataTypes20DataTypesvshotelrooms21A

bit

storesjusta0or1.

Inthecomputerit'sall0'sand1’s.Group8bitstogethertomake1byte.Onebyte=collectionof8bits.1

Byte

=8bitsTypes------thesizeoftheemptybox/roomEveryvariablemusthaveatype.eg.

int

、

float、double

andchar.sizeMLXLS

storedata3312.3or99999999999‘a’Avariableoftypefloat

(shortforfloating-point)、double

canstoremuchlargernumbersthananint

variable.Also,afloat/double

variablecanstorenumberswithdigitsafterthedecimalpoint,like379.125.22Rangeandsizetype[signed]intunsigned[int][signed]short[int]unsignedshort[int][signed]long[int]unsignedlong[int][signed]charunsignedcharfloatdoublelongdoubleSize(Byte)44224411488/12Range-2147483648～+2147483647

～0～4294967295

～-32768～+32767

～0～65535

～-2147483648～+2147483647

～0～4294967295

～-128～+127

～0～255

～3.4×10−381.7×10−308precision3.4×10381.7×103081.2×104932716191.2×10−4932/CProgrammingLanguageSomebasicdatatypesTypecharshortintlongfloatdoubleDescribeAcharacterofthelocalcharactersetshortintegerintegerlongintegersingle-precisionfloatingpointdouble-precisionfloatingpointCProgrammingLanguage522.3.1IntegerTypesTheintegertypes,aredividedintotwocategories:signedandunsigned.整型分有符号整型和无符号整型Theleftmostbitofasignedinteger(knownasthesignbit)is0ifthenumberispositiveorzero,1ifit’snegative.有符号的整型数，其二进制的最左边一位是符号位。0正1负Anintegerwithnosignbit(theleftmostbitisconsideredpartofthenumber’smagnitude)issaidtobeunsigned.无符号的整型数，其二进制的最左边一位也看成是数字的一部分。252.3.1IntegerTypesSignedandUnsignedIntegersBydefault,integervariablesinCaresigned—theleftmostbitisreservedforthesign.Totellthecompilerthatavariablehasnosignbit,declareittobeunsigned.C通常默认整型变量都是有符号的。（可以用来存储负数）如果要声明数据是无符号的，int前面要加上unsigned262.3.1IntegerTypesC’sintegertypecomesindifferentsizes.intintegerisusually32bits,butmaybe16bitsonolderCPUs.Longintegersmayhavemorebitsthanordinaryintegers;shortintegersmayhavefewerbits.Int型变量一般占4个字节32位二进制的存储空间。不同的CPU会有差异。272.3.1IntegerTypesTheCstandardrequiresthatshort

int,int,andlong

intmusteachcoveracertainminimumrangeofvalues.intmustnotbeshorterthanshort

int,andlong

intmustnotbeshorterthanint.通常他们所占的存储空间是short<=int<=long282.3.1IntegerTypesOnlysixcombinationsproducedifferenttypes:

short

int

unsigned

short

int

int

unsigned

int

long

int unsigned

long

intthewordintcanbedropped(long

intcanbeabbreviatedtojustlong).

Int可以省略。比如longint可以省略成long292.3.1IntegerTypesTherangeofvaluesrepresentedbyeachofthesixintegertypesvariesfromonemachinetoanother.Typicalrangesona64-bitmachine: Type SmallestValue LargestValue short

int –32,768 32,767 unsigned

short

int 0 65,535 int –2,147,483,648 2,147,483,647 unsigned

int 0 4,294,967,295 long

int –263 263–1 unsigned

long

int 0

264–1存储范围表示了该类型的变量能存放的最大和最小的数。不同的机器上范围是不同的。302.3.1IntegerTypesIntegerTypesinC99C99providestwoadditionalstandardintegertypes,long

long

intandunsigned

long

long

int.Bothlong

longtypesarerequiredtobeatleast64bitswide.Therangeoflong

long

intvaluesistypically–263(–9,223,372,036,854,775,808)to263–1(9,223,372,036,854,775,807).Therangeofunsigned

long

long

int

valuesisusually0to264–1(18,446,744,073,709,551,615).312.3.2FloatingTypes实型----浮点数（存储小数）Cprovidesthreefloatingtypes

Type SmallestPositiveValue LargestValue Precision

float 1.17549

10–38 3.40282

1038 6digits

double 2.22507

10–308 1.79769

10308 15digitsfloat Single-precisionfloating-point单精度浮点数doubledouble-precisionfloating-point双精度浮点数long

doubleExtended-precisionfloating-pointlong

doubleisrarelyused.实型变量一般用来存储带小数的数。计算机中通常以浮动小数点的形式来存储，所以实型也叫做浮点数。实型主要分两种，float和double。两者的不同在于后者存储空间大，大约双倍精度。322.3.3CharacterTypeschargender;CharacterSetsToday’smostpopularcharactersetisASCII(AmericanStandardCodeforInformationInterchange),a7-bitcodecapableofrepresenting128characters.Ctreatscharactersassmallintegers.Thecharacter'a'hasthevalue97'A'hasthevalue65;'0'hasthevalue48‘1'hasthevalue49.

Characterconstantsactuallyhaveint

typeratherthanchartype.332.4VariablesandConstantsVariable变量--------emptybox/roomMostprogramsneedawaytostoredatatemporarilyduringprogramexecution.Thesestoragelocationsarecalledvariables.34Variablenamevariable2.4VariablesandConstantsDeclarations定义变量-------分配相应的存储空间Todeclareavariable,wefirstspecifythetypeofthevariable,thenitsname.

类型变量名，变量名……；

intheight,length,width,volume;

floatprofit,loss;

doublevolume;

charch;作用：在内存中，为变量分配相应的存储空间，以便后面用来存放数据。35Declarations------checkinathotelVariablesmustbedeclared

beforetheyareused.

intmain()

{

intspam;//declarations定义

spam=15;//statements使用return0;

}变量一定要先定义（分配存储空间）后使用。36spam15AssignmentAssignment赋值

------往变量里面存放数据Avariablecanbegivenavaluebymeansofassignment:inth;

h=8;Thenumber8issaidtobeaconstant（常量）

. intspam;

spam=15;//youstoredthevalue15insidethevariablespam spam=3;Butwhenyouenterspam=3,thevalue15isoverwritten(thatis,replaced)withthevalue3.Theoldvalueinspamisforgotten.Aconstantassignedtoafloat/double

variableusuallycontainsadecimalpoint:

profit=2150.48;It’sbesttoappendtheletterftoafloating-pointconstantifitisassignedtoafloat

variable:

profit=2150.48f;Failingtoincludethe

f

maycauseawarningfromthecompiler.37AssignmentAnint

variableisnormallyassignedavalueoftypeint,adoublevariableisnormallyassignedavalueoftypedouble.intd;d=33;floatx;doublez;x=178.111;z=1899.111;charc1;c1=‘a’;通常：整数赋给整型变量（int），小数赋给实型变量(float,double)字符赋给字符型变量(char)Mixingtypes(suchasassigninganintvaluetoafloatvariableorassigningafloatvaluetoanintvariable)ispossiblebutnotalwayssafe.intd;d=33.6;???38AssignmentAssignmentOnceavariablehasbeenassignedavalue,itcanbeusedtohelpcomputethevalueofanothervariable:height=8;length=12;width=10;volume=height*length*width;/*volumeisnow960*/Therightsideofanassignmentcanbeaformula(orexpression,inCterminology).3940AssignmentInitialization（初始化）定义变量的同时赋初值Theinitialvalueofavariablemaybeincludedinitsdeclaration:

intheight=8;

Thevalue8issaidtobeaninitializer.

intheight=8,length=12,width=10;Eachvariablerequiresitsowninitializer.

intheight,length,width=10; /*initializesonlywidth*/40AssignmentPrintingtheValueofaVariable输出变量的值printf

canbeusedtodisplaythecurrentvalueofavariable.conversionspecification格式说明符:beginwith%

%d

isaplaceholderindicatingwherethevalueofi

istobefilledin.

ddisplayanintegerindecimalform.i=3;printf(“Ihave%dcats\n",i);41Classexercise3:introduceyourfamilyCreateanewprojectnamed“ex03”introduceyourfamilyoryourpet.Thefilenamedoesn’tmatter,butdon’tusespace.422.4.2ConstantsIntegerConstants整型常量,整数Constantsarenumbersthatappearinthetextofaprogram.Callowsintegerconstantstobewrittenindecimal(base10十进制),65octal(base8八进制),0101orhexadecimal(base16十六进制).0x41432.4.1IntegerTypesOctalandHexadecimalNumbers八、十六进制Octalnumbersuseonlythedigits0through7.Eachpositioninanoctalnumberrepresentsapowerof8.Theoctalnumber237representsthedecimalnumber

2×82+3×81+7×80=128+24+7=159.Ahexadecimal(orhex)numberiswrittenusingthedigits0through9plusthelettersAthroughF,whichstandfor10through15,respectively.Thehexnumber1AFhasthedecimalvalue1×162+10×161+15×160=256+160+15=431.44IntegerConstantsDecimalconstantscontaindigitsbetween0and9,butmustnotbeginwithazero:十进制整型常量

1525532767Octalconstantscontainonlydigitsbetween0and7,andmustbeginwithazero:八进制整型常量用0开头

0170377077777Hexadecimalconstantscontaindigitsbetween0and9andlettersbetweenaandf,andalwaysbeginwith0x:十六进制整型常量用0x开头

0xf0xff0x7fffThelettersinahexadecimalconstantmaybeeitherupperorlowercase:

0xff0xfF0xFf0xFF0Xff0XfF0XFf0XFF45IntegerConstantsThetypeofadecimalintegerconstantisnormallyint.Toforcethecompilertotreataconstantasalonginteger,justfollowitwiththeletterL(orl):

15L

0377L

0x7fffLToindicatethataconstantisunsigned,puttheletterU(oru)afterit:

15U

0377U

0x7fffULandUmaybeusedincombination:

0xffffffffUL

TheorderoftheLandUdoesn’tmatter,nordoestheircase.十进制整数比如15会被默认为int型，如果想强制编译器把该数认为long型或者无符号整型。就要在数字后面加L或者U.(L,U可以小写）46IntegerConstantsinC99InC99,integerconstantsthatendwitheitherLL

orll(thecaseofthetwolettersmustmatch)havetypelong

long

int.AddingtheletterU(oru)beforeoraftertheLLorlldenotesaconstantoftypeunsigned

long

long

int.C99’sgeneralrulesfordeterminingthetypeofanintegerconstantareabitdifferentfromthoseinC89.Thetypeofadecimalconstantwithnosuffix(U,u,L,l,LL,orll)isthe“smallest”ofthetypesint,long

int,orlong

long

intthatcanrepresentthevalueofthatconstant.47ReadingandWritingIntegers八、十六、无符号数输入输出格式符Readingandwritingunsigned,short,andlongintegersrequiresnewconversionspecifiers.Whenreadingorwritinganunsignedinteger,usetheletteru,o,x

insteadofdintheconversionspecification.

%u%o%x

unsignedintu;

scanf("%u",&u);/*readsuinbase10*/ printf("%u",u);/*writesuinbase10*/ scanf("%o",&u);/*readsuinbase8*/ printf("%o",u);/*writesuinbase8*/ scanf("%x",&u);/*readsuinbase16*/ printf("%x",u);/*writesuinbase16*/48ReadingandWritingIntegers十、八、十六、无符号数输入输出格式符Whenreadingorwritingashortinteger,puttheletterhinfrontofd,o,u,orx:%hd%ho%hu%hx

shorts;

scanf("%hd",&s); printf("%hd",s);Whenreadingorwritingalonginteger,puttheletterl

infrontofd,o,u,orx.%ld%lo%lu%lxWhenreadingorwritingalong

long

integer(C99only),putthelettersllinfrontofd,o,u,orx.49FloatingConstants实型常量（小数）Floatingconstantscanbewritteninavarietyofways.Validwaysofwritingthenumber57.0:

57.057.57.0e057E05.7e15.7e+1 .57e2570.e-1Afloatingconstantmustcontainadecimalpointand/oranexponent;theexponentindicatesthepowerof10bywhichthenumberistobescaled.Ifanexponentispresent,itmustbeprecededbytheletterE

(ore).Anoptional+or-signmayappearaftertheE(ore).TheremustbeanumberbeforeandafterE,andthenumberafterEmustbeaninteger小数就是实型常量。可以写成小数或者科学记数法两种形式。注意：E前后必须有数字，E后必须为整数50TheremustbeanumberbeforeandafterE,andthenumberafterEmustbeaninteger注意：E前必须有数字，E后必须为整数51FloatingConstantsBydefault,floatingconstantsarestoredasdouble-precisionnumbers.Toindicatethatonlysingleprecision(float)isdesired,puttheletterF(orf)attheendoftheconstant(forexample,57.0F).Toindicatethataconstantshouldbestoredinlong

doubleformat,puttheletterL(orl)attheend(57.0L).

小数比如57.0会被默认为double类型的常量，如果要强制编译器把它看成float类型。就在后面加f。52ReadingandWritingFloating-PointNumbers%e,%f,and%gareusedforreadingandwritingsingle-precisionfloating-pointnumbers.Whenreadingavalueoftypedouble,puttheletterlinfrontofe,f,org:

floatd; scanf("%f",&d);printf("%f",d); doubled; scanf("%lf",&d);printf("%f",d);Note:Uselonlyinascanfformatstring,notaprintfstring.53CharacterconstantsCharacterSetsAvariableoftypecharcanbeassignedanysinglecharacter: charch1，ch2,ch3,ch4;

ch1='a';/*lower-casea*/97 ch2='A';/*upper-caseA*/65 ch3='0';/*zero*/48 ch4='';/*space*/32Noticethatcharacterconstants

areenclosedinsinglequotes,notdoublequotes.一般形式的字符常量：用单引号括起来的单个字符54OperationsonCharactersCtreatscharactersassmallintegers.Thecharacter'a'hasthevalue97'A'hasthevalue65;'0'hasthevalue48''hasthevalue32.

Characterconstantsactuallyhaveint

typeratherthanchartype.ch=‘a’;这句话实际上等价于ch=97;因为实际上存放在变量ch里面的是a的ASCII值97.555656Charactersinmemory字符数据在内存中存放的是其ASCII值。charc1,c2;c1=‘a’;c2=‘b’

;c1c297980110000101100010在用户眼里是字母a，在内存中实际存储的是97OperationsonCharactersWhenacharacterappearsinacomputation,Cusesitsintegervalue. charch; inti;

i='a';/*iisnow97*/ ch=65;/*chisnow'A'*/ ch=ch+1;/*chisnow'B'*/ ch++;/*chisnow'C'*/C会把字符常量处理成整数，也就是他的ASCII值。所以，在一定程度上char和int是可以通用的。57EscapeSequencesAcharacterconstantisusuallyonecharacterenclosedinsinglequotes.However,certainspecialcharacters—includingthenew-linecharacter—can’tbewritteninthisway,becausethey’reinvisible(nonprinting)orbecausetheycan’tbeenteredfromthekeyboard.Escapesequencesprovideawaytorepresentthesecharacters.Therearetwokindsofescapesequences:characterescapesandnumericescapes.58EscapeSequencesAcompletelistofcharacterescapes:

Name EscapeSequence Alert(bell) \a

Backspace \b Formfeed \f

Newline \n Carriagereturn \r

Horizontaltab \t Verticaltab \v

Backslash \\

Questionmark \? Singlequote \' Doublequote \"59Characterescapesarehandy,buttheydon’texistforallnonprintingASCIIcharacters.Characterescapesarealsouselessforrepresentingcharactersbeyondthebasic128ASCIIcharacters.EscapeSequencesNumericescapescanrepresentanycharacter.Anumericescapeforaparticularcharacterusesthecharacter’soctalorhexadecimalvalue.octalescapesequencehexadecimalescapesequence60EscapeSequencesAnoctalescapesequenceconsistsofthe\characterfollowedbyanoctalnumberwithatmostthreedigits,suchas\33or\101.用字符的8进制ASCII码值代表字符；Ahexadecimalescapesequenceconsistsof\xfollowedbyahexadecimalnumber,suchas\x41or\x1B.用字符的16进制ASCII码值代表字符；Thexmustbeinlowercase,butthehexdigitscanbeupperorlowercase.61‘A’‘\101’‘\X41’62例：整型量、字符量互相赋值和通用格式符

charc;inti;c=97;i=’b’;printf(”%c,%d\n”,c,c);printf(”%c,%d\n”,i,i);

a,97 b,98Outlines2.5.1ArithmeticOperators算术运算符2.5.2AssignmentOperators赋值运算符2.5.3IncrementandDecrementOperators自增自减运算符2.5.4ExpressionEvaluation表达式2.5.5ExpressionStatement表达式语句63OperatorsExpressionsarebuiltfromvariables,constants,andoperators.Eachexpressionhasitsowntypeandvalue.Chasarichcollectionofoperators,includingarithmeticoperatorsrelationaloperatorslogicaloperatorsassignmentoperatorsincrementanddecrementoperators andmanyothers642.5.1ArithmeticOperatorsUnaryArithmeticOperators一元算术运算符Therearealsotwounaryarithmeticoperators:

+ unaryplus

- unaryminusTheunaryoperatorsrequireoneoperand:

i=+1; j=-i;652.5.1ArithmeticOperatorsOperatorAssociativity(操作符结合律，有左结合和右结合两种）Associativitycomesintoplaywhenanexpressioncontainstwoormoreoperatorswithequalprecedence.Anoperatorissaidtobeleftassociative（左结合）

ifitgroupsfromlefttoright.Thebinaryarithmeticoperators(*,/,%,+,and-)areallleftassociative,so

i

-

j

–

kisequivalentto(i

-

j)

-

k

i

*

j

/

kisequivalentto(i

*

j)

/

kAnoperatorisrightassociative（右结合）

ifitgroupsfromrighttoleft.Theunaryarithmeticoperators(+and-)arebothrightassociative,so

-

+

iisequivalentto-(+i)当表达式中有多个运算符时，先看优先级，如优先级相同，则看结合律，左结合就从左向右算，右结合就从右向左算。只有少数运算符是右结合的。662.5.1ArithmeticOperatorsOperatorPrecedence(操作符优先级)先乘除后加减Doesi

+

j

*

k

mean“addiandj,thenmultiplytheresultbyk”or“multiplyjandk,thenaddi”?Onesolutiontothisproblemistoaddparentheses,writingeither(i

+

j)

*

k

ori

+

(j

*

k).Iftheparenthesesareomitted,Cusesoperatorprecedencerulestodeterminethemeaningoftheexpression.672.5.1ArithmeticOperatorsOperatorPrecedence

(操作符优先级)Thearithmeticoperatorshavethefollowingrelativeprecedence:

Highest: +

-(unary)正负号

*

/

% Lowest: +

-(binary)

Examples:

i

+

j

*

k

isequivalenttoi

+

(j

*

k)

-i

*

-j

isequivalentto(-i)

*

(-j)

+i

+

j

/

k

isequivalentto(+i)

+

(j

/

k)682.5.1ArithmeticOperatorsBinaryArithmeticOperators混合运算Binaryarithmeticoperators(withtheexceptionof%remainder)alloweitherintegerorfloating-pointoperands,withmixingallowed.Whenint

anddouble

operandsaremixed,theresulthastypedouble.

9+2.5hasthevalue11.5,and6.7

/

2hasthevalue3.35.

整型数据和实型数据混合运算，结果为实型

整型和整型运算，结果为整型

实型和实型运算，结果为实型692.5.1ArithmeticOperatorsBinaryArithmeticOperators二元算术运算符Cprovidesfivebinaryarithmeticoperators:

+ addition

- subtraction

* multiplication

/ division

% remainder(整数相除取余数)RemainderoperatorThevalueofi

%

j

istheremainderwheniisdividedbyj.

iandjmustbeintegers整数

10

%

3hasthevalue1,and10

%

5hasthevalue0.702.5.1ArithmeticOperatorsBinaryArithmeticOperatorsThe/and%operatorsrequirespecialcare:Whenbothoperandsareintegers,/“truncates”theresult.Thevalueof1

/

2

is0,not0.5.Thevalueof1.0/

2

is0.5.The%operatorrequiresintegeroperands;InC99,thevalueofa

%

bhasthesamesignasa.--5%3-2--5%-3-2712.5.2AssignmentOperators赋值运算符Simpleassignment:usedforstoringavalueintoavariableCompoundassignment:usedforupdatingavaluealreadystoredinavariable722.5.2AssignmentOperators赋值运算符SimpleAssignment:

v

=

e变量=常数、变量、表达式….Theeffectoftheassignmentv

=

e

istocalculatetheexpressioneandcopyitsvalueintov.ecanbeaconstant,avariable,oramorecomplicatedexpression:

x=111;/*xisnow111*/ y=x;/*yisnow111*/ z=5*x+y;/*zisnow666*/732.5.2AssignmentOperatorsLvalues左值Theassignmentoperatorrequiresanlvalueasitsleftoperand.Anlvaluerepresentsanobjectstoredincomputermemory,notaconstantortheresultofacomputation.Variablesarelvalues;Variables=…..等号的左边只能是变量742.5.2AssignmentOperatorsIfvandedon’thavethesametype,thenthevalueofeisconvertedtothetypeofvastheassignmenttakesplace:

inti; doublef;

i=72.99;/*iisnow72*/ f=136;/*fisnow136.0*/SideEffectsAnoperatorsthatmodifiesoneofitsoperandsissaidtohaveasideeffect

.Thesimpleassignmentoperatorhasasideeffect:itmodifiesitsleftoperand.752.5.2AssignmentOperatorsSideEffectsSinceassignmentisanoperator,severalassignmentscanbechainedtogether:

a=b=c=0;The=

operatorisrightassociative,sothisassignmentisequivalentto

a=(b=(c=0));Watchoutforunexpectedresultsinchainedassignmentsasaresultoftypeconversion:

inta; doubleb;

b=a=123.3;iisassignedthevalue33,thenfisassigned33.0(not33.3).762.5.2AssignmentOperatorsSimpleAssignmentInmanyprogramminglanguages,assignmentisastatement;inC,however,assignmentisanoperator,justlike+.v=eisanexpression.Thevalueofanassignmentv

=

e

isthevalueofvaftertheassignment.赋值表达式的值就是左边变量的值。772.5.2AssignmentOperatorsSideEffectsAnassignmentoftheformv

=

e

isallowedwhereveravalueoftypevwouldbepermitted:

i=1; k=1+(j=i); printf("%d%d%d\n",i,j,k); /*prints112*/“赋值表达式的值就是左边变量的值。782.5.2AssignmentOperatorsLvaluesSincetheassignmentoperatorrequiresanlvalueasitsleftoperand,it’sillegaltoputanyotherkindofexpressionontheleftsideofanassignmentexpression:

12=i;/***WRONG***/ i+j=0;/***WRONG***