Chapter8StringsFangWangOutlines8.1StringConstantsandVariables字符串常量和字符串變量（字符數組）8.2ReadingandWritingStrings字符串讀寫操作8.3AccessingtheCharactersinaString8.4UsingtheCStringLibrary字符串庫函數8.5ArraysofStrings字符串數組2IntroductionThischaptercoversbothstringconstants

(orliterals,asthey’recalledintheCstandard)字符串常量andstringvariables.字符串變量Stringsarearraysofcharactersinwhichaspecialcharacter—thenullcharacter—markstheend.字符串就是以空字符結束的字符數組38.1StringLiterals字符串常量Astring

constantsisasequenceofcharactersenclosedwithindoublequotes:“Helloworld."字符串常量就是雙引號括起來的多個字符。Stringconstantsmaycontainescapesequences.Characterescapesoftenappearinprintf

andscanf

formatstrings.字符串常量里面可以有\開頭的轉義字符Forexample,"Candy\nIs

dandy\nBut

liquor\nIs

quicker.\n

--Ogden

Nash\n"48.1StringLiteralsContinuingaStringLiteralEncounteraverylongstringliteralThebackslashcharacter(\)canbeusedtocontinueastringconstantfromonelinetothenext:

printf("When

you

come

to

a

fork

in

the

road,

take

it.

\ --Yogi

Berra");Whentwoormorestringliteralsareadjacent,thecompilerwilljointhemintoasinglestring.Thisruleallowsustosplitastringliteralovertwoormorelines: printf("When

you

come

to

a

fork

in

the

road,

take

it.

" "--YogiBerra");58.1StringLiteralsHowStringLiteralsAreStoredWhenaCcompilerencountersastringconstantsoflengthninaprogram,itsetsasiden+1bytesofmemoryforthestring.Thismemorywillcontainthecharactersinthestring,plusoneextracharacter—thenull

character—tomarktheendofthestring.Thenullcharacterisrepresentedbythe

’\0’

escapesequence.字符串在存儲的時候，末尾會自動加‘\0’68.1StringLiteralsHowStringLiteralsAreStoredThestring"abc"

isstoredasanarrayoffourcharacters:Thestring""isstoredasasinglenullcharacter:78.1StringLiteralsOperationsonStringLiteralsWecanuseastringconstantwhereverCallowsachar

*

pointer: char*p; p="abc";Thisassignmentmakesppointtothefirstcharacterofthestring.88.2StringVariablesInitializingaStringVariableAstringvariablecanbeinitializedatthesametimeit’sdeclared:

chardate1[8]="June14";Thecompilerwillautomaticallyaddanullcharactersothatdate1canbeusedasastring:

98.2StringVariablesInitializingaStringVariableIftheinitializeristooshorttofillthestringvariable,thecompileraddsextranullcharacters:

chardate2[9]="June14";

Appearanceofdate2:108.2StringVariablesInitializingaStringVariableAninitializerforastringvariablecan’tbelongerthanthevariable,butitcanbethesamelength:

chardate3[7]="June14";There’snoroomforthenullcharacter,sothecompilermakesnoattempttostoreone:

118.2StringVariablesInitializingaStringVariableThedeclarationofastringvariablemayomititslength,inwhichcasethecompilercomputesit:

chardate4[]="June14";Thecompilersetsasideeightcharactersfordate4,enoughtostorethecharactersin"June14"

plusanullcharacter.128.2StringVariablesCharacterArraysversusCharacterPointersThedeclaration

chardate[]="June14";

declaresdatetobeanarray,

Thesimilar-looking

char*date="June14";

declaresdatetobeapointer.138.3ReadingandWritingStringsWritingastringiseasyusingeitherprintforfputs.Toreadastringinasinglestep,wecanuseeitherscanforfgets.148.3ReadingandWritingStringsWritingStringsUsingprintfandputsThe%s

conversionspecificationallowsprintftowriteastring: charstr[]="Arewehavingfunyet?"; printf("%s\n",str); Theoutputwillbe Arewehavingfunyet?printfwritesthecharactersinastringonebyoneuntilitencountersanullcharacter\0.158.3ReadingandWritingStringsWritingStringsUsingprintfandputsToprintpartofastring,usetheconversionspecification%.ps.pisthenumberofcharacterstobedisplayed.Thestatement

printf("%.6s\n",str);

willprint

Arewe

168.3ReadingandWritingStringsWritingStringsUsingprintfandputsThe%ms

conversionwilldisplayastringinafieldofsizem.Ifthestringhasfewerthanmcharacters,itwillberight-justifiedwithinthefield.Toforceleftjustificationinstead,wecanputaminussigninfrontofm.Themandpvaluescanbeusedincombination.Aconversionspecificationoftheform%m.ps

causesthefirst

p

charactersofastringtobedisplayedinafieldofsizem.printf("%6.3s\n",str);

Are178.3ReadingandWritingStringsWritingStringsUsingprintfandputsprintfisn’ttheonlyfunctionthatcanwritestrings.TheClibraryalsoprovidesfputs:charstr[100]=“aaaaa”;

fputs(str,stdout);Afterwritingastring,fputsalwayswritesanadditionalnew-linecharacter.188.3ReadingandWritingStringsReadingStringsUsingscanfandgetsThe%s

conversionspecificationallowsscanftoreadastringintoacharacterarray:charstr[100];

scanf("%s",str);stristreatedasapointer,sothere’snoneedtoputthe&operatorinfrontofstr.Whenscanfiscalled,itskipswhitespace,thenreadscharactersandstorestheminstruntilitencountersawhite-spacecharacter.scanfalwaysstoresanullcharacterattheendofthestring.aaabbb198.3ReadingandWritingStringsReadingStringsUsingscanfandfgetsscanfwon’tusuallyreadafulllineofinput.Anew-linecharacterwillcausescanftostopreading,butsowillaspaceortabcharacter.Toreadanentirelineofinput,wecanusefgets.Propertiesoffgets:Doesn’tskipwhitespacebeforestartingtoreadinput.Readsuntilitfindsanew-linecharacter.Discardsthenew-linecharacterinsteadofstoringit;thenullcharactertakesitsplace.208.3ReadingandWritingStringsReadingStringsUsingscanfandfgetsfgets（str，n，stdin）；//readn-1characters+\0Considerthefollowingprogramfragment:charsentence[10];fgets(sentence,sizeof(sentence)-1,stdin);charstr[10];fgets(str,7,stdin);Input:ILOVECHINA!21Classassignment40:input/outputthestringDeclareanarray/stringvariableInput:%sfgetsOutput:%sfputs22#include<stdio.h>#include<stdlib.h>intmain(){charstr[100];fgets(str,99,stdin);fputs(str,stdout);return0;}charstr[100];fgets(str,99,stdin);scanf("%[^\n]%*c",str);ucanusescanf("%[^\n]%*c",str);insteadoffgets(str,n,stdin);^means"NOT".so%*[^\n]scanseverythinguntila\n,butdoesn'tscaninthe\n.%*cscansasinglecharacter,whichwillbethe\nleftoverby%*[^\n]inthiscase.The*instructsscanftodiscardthe\ncharacter.[]meansscanset.238.3ReadingandWritingStringsReadingStringsUsingscanfandgetsAstheyreadcharactersintoanarray,scanfandfgetshavenowaytodetectwhenit’sfull.Consequently,theymaystorecharacterspasttheendofthearray,causingundefinedbehavior.scanfcanbemadesaferbyusingtheconversionspecification%ns

insteadof%s.nisanintegerindicatingthemaximumnumberofcharacterstobestored.getsisinherentlyunsafe;fgetsisamuchbetteralternative.248.3ReadingandWritingStringsReadingStringsCharacterbyCharacter

Standardfunctionssuchasscanfandfgetsautomaticallyputanullcharacterattheendofaninputstring.Ifwe’rewritingourowninputfunction,wemusttakeonthatresponsibility.258.4AccessingtheCharactersinaStringSincestringsarestoredasarrays,wecanusesubscriptingtoaccessthecharactersinastring.Toprocesseverycharacterinastrings,wecansetupaloopthatincrementsacounteriandselectscharactersviatheexpressions[i].268.4AccessingtheCharactersinaStringAfunctionthatcountsthenumberofspacesinastring: intcount_spaces(constchars[]) { intcount=0,i; for(i=0;s[i]!='\0';i++) if(s[i]=='') count++; returncount; }278.5UsingtheCStringLibraryDirectattemptstocopyorcomparestringswillfail.Copyingastringintoacharacterarrayusingthe=operatorisnotpossible: charstr1[10],str2[10];

… str1="abc";/***WRONG***/ str2=str1;/***WRONG***/ Usinganarraynameastheleftoperandof=isillegal.Initializingacharacterarrayusing=islegal,though: charstr1[10]="abc"; Inthiscontext,=isnottheassignmentoperator.288.5UsingtheCStringLibraryAttemptingtocomparestringsusingarelationalorequalityoperatorislegalbutwon’tproducethedesiredresult: if(str1==str2)…/***WRONG***/Thisstatementcomparesstr1andstr2aspointers.Sincestr1andstr2havedifferentaddresses,theexpressionstr1

==

str2

musthavethevalue0.298.5UsingtheCStringLibraryTheClibraryprovidesarichsetoffunctionsforperformingoperationsonstrings.Programsthatneedstringoperationsshouldcontainthefollowingline:

#include<string.h>Insubsequentexamples,assumethatstr1andstr2arecharacterarraysusedasstrings.308.5UsingtheCStringLibraryThestrcpy(StringCopy)FunctionPrototypeforthestrcpyfunction:char*strcpy(char*s1,constchar*s2);strcpycopiesthestrings2intothestrings1.Tobeprecise,weshouldsay“strcpycopiesthestringpointedtobys2intothearraypointedtobys1.”strcpyreturnss1(apointertothedestinationstring).318.5UsingtheCStringLibraryThestrcpy(StringCopy)FunctionAcallofstrcpythatstoresthestring"abcd"

instr2:charstr2[20]; strcpy(str2,"abcd");

/*str2nowcontains"abcd"*/Acallthatcopiesthecontentsofstr2intostr1:charstr1[20];

strcpy(str1,str2); /*str1nowcontains"abcd"*/328.5UsingtheCStringLibraryThestrcpy(StringCopy)FunctionInthecallstrcpy(str1,

str2),

strcpyhasnowaytocheckthatthestr2stringwillfitinthearraypointedtobystr1.Ifitdoesn’t,undefinedbehavioroccurs.338.5UsingtheCStringLibraryThestrcpy(StringCopy)FunctionCallingthestrncpyfunctionisasafer,albeitslower,waytocopyastring.strncpyhasathirdargumentthatlimitsthenumberofcharactersthatwillbecopied.Acallofstrncpythatcopiesstr2intostr1:

strncpy(str1,str2,sizeof(str1));348.5UsingtheCStringLibraryThestrcpy(StringCopy)Functionstrncpywillleavestr1withoutaterminatingnullcharacterifthelengthofstr2

isgreaterthanorequaltothesizeofthestr1array.Asaferwaytousestrncpy:

strncpy(str1,str2,sizeof(str1)-1); str1[sizeof(str1)-1]='\0';Thesecondstatementguaranteesthatstr1isalwaysnull-terminated.358.5UsingtheCStringLibraryThestrlen(StringLength)FunctionPrototypeforthestrlenfunction:

size_tstrlen(constchar*s);size_tisatypedefnamethatrepresentsoneofC’sunsignedintegertypes.368.5UsingtheCStringLibraryThestrlen(StringLength)Functionstrlenreturnsthelengthofastrings,notincludingthenullcharacter.Examples: intlen; len

=

strlen("abc");

/*

len

is

now

3

*/ len

=

strlen("");

/*

len

is

now

0

*/ strcpy(str1,

"abc"); len

=

strlen(str1);

/*

len

is

now

3

*/378.5UsingtheCStringLibraryThestrcat(StringConcatenation)FunctionPrototypeforthestrcatfunction:

char*strcat(char*s1,constchar*s2);strcat

appendsthecontentsofthestrings2totheendofthestrings1.Itreturnss1(apointertotheresultingstring).strcatexamples:charstr1[20]，str2[20]; strcpy(str1,"abc"); strcat(str1,"def");

/*str1nowcontains"abcdef"*/ strcpy(str1,"abc"); strcpy(str2,"def"); strcat(str1,str2); /*str1nowcontains"abcdef"*/

388.5UsingtheCStringLibraryThestrcat(StringConcatenation)FunctionAswithstrcpy,thevaluereturnedbystrcatisnormallydiscarded.Thefollowingexampleshowshowthereturnvaluemightbeused: charstr1[20]，str2[20];strcpy(str1,"abc"); strcpy(str2,"def"); strcat(str1,strcat(str2,"ghi")); /*str1nowcontains"abcdefghi"; str2contains"defghi"*/398.5UsingtheCStringLibraryThestrcat(StringConcatenation)Functionstrcat(str1,

str2)

causesundefinedbehaviorifthestr1arrayisn’tlongenoughtoaccommodatethecharactersfromstr2.Example: charstr1[6]="abc"; strcat(str1,"def");/***WRONG***/str1islimitedtosixcharacters,causingstrcattowritepasttheendofthearray.408.5UsingtheCStringLibraryThestrcat(StringConcatenation)FunctionThestrncatfunctionisasaferbutslowerversionofstrcat.Likestrncpy,ithasathirdargumentthatlimitsthenumberofcharactersitwillcopy.Acallofstrncat:

strncat(str1,str2,sizeof(str1)-1);strncatwillterminatestr1

withanullcharacter,whichisn’tincludedinthethirdargument.418.5UsingtheCStringLibraryThestrcmp(StringComparison)FunctionPrototypeforthestrcmpfunction:

intstrcmp(constchar*s1,constchar*s2);strcmpcomparesthestringss1ands2,returningavaluelessthan,equalto,orgreaterthan0,dependingonwhethers1islessthan,equalto,orgreaterthans2.charstr1[20]=“abc”，str2[20]=“abdef”;if(strcmp(str1,str2)==0)…//same428.5UsingtheCStringLibraryThestrcmp(StringComparison)FunctionTestingwhetherstr1

islessthanstr2:if(strcmp(str1,str2)<0)/*isstr1<str2?*/ …Testingwhetherstr1islessthanorequaltostr2:if(strcmp(str1,str2)<=0)/*isstr1<=str2?*/ …Bychoosingtheproperoperator(<,<=,>,>=,==,!=),wecantestanypossiblerelationshipbetweenstr1andstr2.438.5UsingtheCStringLibraryThestrcmp(StringComparison)Functionstrcmpconsiderss1tobelessthan

s2ifeitheroneofthefollowingconditionsissatisfied:Thefirsticharactersofs1ands2match,butthe(i+1)stcharacterofs1islessthanthe(i+1)stcharacterofs2.Allcharactersofs1matchs2,buts1isshorterthans2.448.5UsingtheCStringLibraryThestrcmp(StringComparison)FunctionAsitcomparestwostrings,strcmplooksatthenumericalcodesforthecharactersinthestrings.Someknowledgeoftheunderlyingcharactersetishelpfultopredictwhatstrcmpwilldo.ImportantpropertiesofASCII:A–Z,a–z,and0–9haveconsecutivecodes.Allupper-caselettersarelessthanalllower-caseletters.Digitsarelessthanletters.Spacesarelessthanallprintingcharacters.458.5UsingtheCStringLibrarysprintfissimilartoprintf,exceptthatitwritesoutputintoastring.

inta=3,b=5;charstr1[20]="abcd";sprintf(str1,"%d%d",a,b);fputs(str1,stdout);//35//35\0d468.5UsingtheCStringLibrary

scanf("%2d",&day);sscanf("12345","%4s",str);478.7ArraysofStringsThereismorethanonewaytostoreanarrayofstrings.Oneoptionistouseatwo-dimensionalarrayofcharacters,withonestringperrow: char

planets[][8]

=

{"Mercury",

"Venus",

"Earth",

"Mars",

"Jupiter",

"Saturn",

"Uranus",

"Neptune",

"Pluto"};Thenumberofrowsinthearraycanbeomitted,butwemustspecifythenumberofcolumns.488.7ArraysofStringsUnfortunately,theplanetsarraycontainsafairbitofwastedspace(extranullcharacters):49Romantime[12][5]={"I","II","III","IV","V","VI","VII","VIII","IX