CFundamentals&FormattedInput/OutputadoptedfromKNKCProgramming:AModernApproach

CFundamentals

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Program:PrintingaPun• Thefilenamedoesn’tmatter,butthe.c extensionisoftenrequired.• forexample:pun.c

• tocompile

• torun

#include <stdio.h> // directive

int main(void) // function{ /* statements begin */printf("To C, or not to C: that is the question.\n");return 0;

} /* statements end */

% cc -o pun pun.c % gcc -o pun pun.cor

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% ./pun

TheGeneralFormofaSimpleProgram

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ThreekeylanguagefeaturesofCprograms

statements

directives

int main(void){

}

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2

3begin

end

Example:#include <stdio.h>

TheGeneralForm:Directives

• BeforeaCprogramiscompiled,itisfirsteditedbyapreprocessor.• Commandsintendedforthepreprocessorarecalleddirectives.

• <stdio.h> isaheader containinginformationaboutC’sstandardI/Olibrary.• Directivesalwaysbeginwitha# character.• Bydefault,directivesareonelinelong;• there’snosemicolonorotherspecialmarkerattheend.

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ThreekeylanguagefeaturesofCprograms

directives1 Example:#include <stdio.h>

TheGeneralForm:Directives

• Libraryfunctionsare providedaspartoftheCimplementation.• Themain functionismandatory.• main isspecial:itgetscalledautomaticallywhentheprogramisexecuted.• main returnsastatuscode;thevalue0indicatesnormalprogramtermination.

• Ifthere’snoreturn statementattheendofthemain function,manycompilerswillproduceawarningmessage.

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ThreekeylanguagefeaturesofCprograms

int main(void){

# of statementsreturn x + 1;

}

2 aseriesofstatementsthathavebeengroupedtogetherandgivenaname.

Afunctionthatcomputesavalueusesareturn statementtospecifywhatvalueit“returns”:

TheGeneralForm:Statements

• pun.c usesonlytwokindsofstatements.• Oneisthereturn statement;theotheristhefunctioncall.

• Askingafunctiontoperformitsassignedtaskisknownascalling thefunction.• pun.c callsprintf todisplayastring:printf("To C, or not to C: that is the question.\n");

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statements3

ThreekeylanguagefeaturesofCprograms

acommandtobeexecutedwhentheprogramruns.Crequiresthateachstatementendwithasemicolon.

PrintingStrings:printf• Whentheprintf functiondisplaysastringliteral—charactersenclosedindoublequotationmarks—itdoesn’tshowthequotationmarks.

• printf doesn’tautomaticallyadvancetothenextoutputlinewhenitfinishesprinting.

• Tomakeprintf advanceoneline,include\n (thenew-linecharacter)inthestringtobeprinted.

printf("To C, or not to C: that is the question.\n");

printf("To C, or not to C: ");printf("that is the question.\n");

printf("Brevity is the soul of wit.\n --Shakespeare\n");

Sameeffect

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moreonprintf onfollowingsection

Comments• Acomment beginswith/* andendwith*/./* This is a comment */

• Commentsmayappearalmostanywhereinaprogram,eitheronseparatelinesoronthesamelinesasotherprogramtext.

• Commentsmayextendovermorethanoneline./* Name: pun.c

Purpose: Prints a bad pun.Author: K. N. King */

• InC99,commentscanalsobewritteninthefollowingway:// A comment, which ends automatically at the end of a line

• Advantagesof// comments:• Safer:there’snochancethatanunterminatedcommentwillaccidentallyconsumepartofaprogram.

• Multilinecommentsstandoutbetter.

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VariablesandAssignment• Mostprogramsneedtoawaytostoredatatemporarilyduringprogramexecution.

• Thesestoragelocationsarecalledvariables.

• Tousevariablesandassignments,youneedtoknow1. type2. declaration3. initialization

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int height = 183;1 32

VariablesandAssignment:Types• Everyvariablemusthaveatype.• Chasawidevarietyoftypes,includingint andfloat.

• Avariableoftypeint (shortforinteger)canstoreawholenumbersuchas0,1,392,or–2553.

• Avariableoftypefloat (shortforfloating-point)canstoremuchlargernumbersthananint variable.• Also,afloat variablecanstorenumberswithdigitsafterthedecimalpoint,like379.125.

• Drawbacksoffloat variables:• Slowerarithmetic

• Approximatenatureoffloat values

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VariablesandAssignment:Declarations• Variablesmustbedeclared beforetheyareused.

• Botharelegal

• Whenmain containsdeclarations,thesemustprecedestatements:

int height;float profit;

int height, length, width, volume;float profit, loss;

int main(void){

declarationsstatements

}

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VariablesandAssignment:Assignment(1/2)• Avariablecanbegivenavaluebymeansofassignment:height = 8; // Thenumber8 issaidtobeaconstant.

• Beforeavariablecanbeassignedavalue—orusedinanyotherway—itmustfirstbedeclared.

• Aconstantassignedtoafloat variableusuallycontainsadecimalpoint:profit = 2150.48;

• It’sbesttoappendtheletterf toafloating-pointconstantifitisassignedtoafloat variable:profit = 2150.48f;

Failingtoincludethefmaycauseawarningfromthecompiler.

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VariablesandAssignment:Assignment(2/2)• Anint variableisnormallyassignedavalueoftypeint,andafloatvariableisnormallyassignedavalueoftypefloat.• Mixingtypes(suchasassigninganint valuetoafloat variable)ispossiblebutnotalwayssafe.

• Onceavariablehasbeenassignedavalue,itcanbeusedtohelpcomputethevalueofanothervariable:

• Therightsideofanassignmentcanbeaformula(orexpression, inCterminology)involvingconstants,variables,andoperators.

height = 8;length = 12;width = 10;volume = height * length * width; // volume is now 960

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VariablesandAssignment:Initialization• Somevariablesareautomaticallysettozerowhenaprogrambeginstoexecute,butmostarenot.• Avariablethatdoesn’thaveadefaultvalueandhasn’tyetbeenassignedavaluebytheprogramissaidtobeuninitialized.

• Accessingthevalueofanuninitializedvariablecausesanunpredictableresult.• Withsomecompilers,worsebehavior—evenaprogramcrash—mayoccur.

• Theinitialvalueofavariablemaybeincludedinitsdeclaration:int height = 8; // The value 8 is said to be an initializer.

int height = 8, length = 12, width = 10;

int height, length, width = 10; // initializes only width

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PrintingtheValueofaVariable• printf canbeusedtodisplaythecurrentvalueofavariable.• Towritethefollowingmessage,we’dusethefollowingcallofprintf:

• %d isaplaceholderindicatingwherethevalueofheight istobefilledin.

• There’snolimittothenumberofvariablesthatcanbeprintedbyasinglecallofprintf:printf("Height: %d Length: %d\n", height, length);

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Detailsofprintf onfollowingsection

Height: h

printf("Height: %d\n", height);

ReadingInput• scanf istheClibrary’scounterparttoprintf.• scanf requiresaformatstringtospecifytheappearanceoftheinputdata.

• Exampleofusingscanf toreadanint value:scanf("%d", &i); /* reads an integer; stores into i */

• The& symbolisusually(butnotalways)requiredwhenusingscanf.

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Detailsofscanf onfollowingsection

DefiningNamesforConstants• Usingafeatureknownasmacrodefinition,wecannamethisconstant:#define INCHES_PER_POUND 166

• Whenaprogramiscompiled,thepreprocessorreplaceseachmacrobythevaluethatitrepresents.

• Duringpreprocessing,thestatement

willbecome

• Thevalueofamacrocanbeanexpression:#define RECIPROCAL_OF_PI (1.0f / 3.14159f)

• Ifitcontainsoperators,theexpressionshouldbeenclosedinparentheses.

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weight = (volume + INCHES_PER_POUND - 1) / INCHES_PER_POUND;

weight = (volume + 166 - 1) / 166;

Itiscommonconventiontouseonlyupper-caselettersinmacronames

Identifiers• Identifiers: Namesforvariables,functions,macros,andotherentities• Letters,digits,andunderscores,butmustbeginwithaletterorunderscore:

• Cplacesnolimitonthemaximumlengthofanidentifier.

• Examplesofillegalidentifiers:10times get-next-char

• Ciscase-sensitive

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allaredifferentjob joB jOb jOB Job JoB JOb JOB

times10 _done symbol_table current_page

symbolTable currentPage

Keywords• Thefollowingkeywords can’tbeusedasidentifiers:

• Keywords(withtheexceptionof_Bool,_Complex,and_Imaginary)mustbewrittenusingonlylower-caseletters.

• Namesoflibraryfunctions(e.g.,printf)arealsolower-case.

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autobreakcasecharconstcontinuedefault

dodoubleelseenumexternfloatfor

gotoifintlongregister

returnshortsignedsizeofstaticstructswitch

typedefunionunsignedvoidvolatilewhile

inline*restrict*_Bool*_Complex*_Imaginary**C99only

LayoutofaCProgram(1/2)• ACprogramis

aseriesoftokens.• Tokensinclude:• Identifiers• Keywords• Operators• Punctuation• Constants• Stringliterals

• Thestatementprintf("Height: %d\n", height);

consistsofseventokens:

printf Identifier( Punctuation"Height: %d\n" Stringliteral, Punctuationheight Identifier) Punctuation; Punctuation

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LayoutofaCProgram (2/2)• Statementscanbedividedoveranynumberoflines.

• Spacebetweentokens(suchasbeforeandaftereachoperator,andaftereachcomma)makesiteasierfortheeyetoseparatethem.

• Indentation canmakenestingeasiertospot.

• Blanklines candivideaprogramintologicalunits

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FormattedInputandOutput

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Theprintf Function(1/3)• Theprintf functionmustbesuppliedwithaformatstring,followedbyanyvaluesthataretobeinsertedintothestringduringprinting:

• Theformatstringmaycontainbothordinarycharactersandconversionspecifications,whichbeginwiththe% character.

• Aconversionspecificationisaplaceholderrepresentingavaluetobefilledinduringprinting.• %d isusedforint values• %f isusedforfloat values

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printf(format_string, expr1, expr2, …);

Theprintf Function (2/3)• Ordinarycharactersinaformatstringareprintedastheyappearinthestring;conversionspecificationsarereplaced.

• Example:

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int i, j;float x, y;

i = 10;j = 20;x = 43.2892f;y = 5527.0f;

printf("i = %d, j = %d, x = %f, y = %f\n", i, j, x, y);

i = 10, j = 20, x = 43.289200, y = 5527.000000Result:

Theprintf Function (3/3)• Compilersaren’trequiredtocheckthatthenumberofconversionspecificationsinaformatstringmatchesthenumberofoutputitems.

• Compilersaren’trequiredtocheckthataconversionspecificationisappropriate.• Anincorrectspecificationwillproducemeaninglessoutput:

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printf("%d %d\n", i); /*** WRONG ***/printf("%d\n", i, j); /*** WRONG ***/

int i;

float x;

printf("%f %d\n", i, x); /*** WRONG ***/

ConversionSpecifications(1/2)

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%m.pXminimumfieldwidth precision

conversionspecifier

optional optional

%5.3f%8d%-8d

12345.678912345.678

1234512345

%d - Integer%e - Exponentialformat%f - Fixeddecimal%g - Eitherexponentialformator

fixeddecimalformat

ConversionSpecifications(1/2)

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Formatspecifier Description Supporteddatatypes

%c Character charunsigned char

%d SignedInteger

shortunsigned shortintlong

%e or %E Scientificnotationoffloatvalues

floatdouble

%f Floatingpoint float

%g or %G Similaras%eor%E

floatdouble

%hi SignedInteger(Short)

short

%hu UnsignedInteger(Short)

unsigned short

%i SignedInteger

shortunsigned shortintlong

%l or %ld or %li SignedInteger long

%lf Floatingpoint double%Lf Floatingpoint long double

%lu Unsignedinteger unsigned intunsigned long

Formatspecifier Description Supporteddatatypes

%lli, %lld SignedInteger long long

%llu UnsignedInteger unsigned long long

%o OctalrepresentationofInteger.

shortunsigned shortintunsigned intlong

%p Addressofpointertovoidvoid*

void *

%s String char *

%u UnsignedInteger unsigned intunsigned long

%x or %XHexadecimalrepresentationofUnsignedInteger

shortunsigned shortintunsigned intlong

%n Printsnothing

%% Prints%character

EscapeSequences(1/2)• The\n codethatusedinformatstringsiscalledanescapesequence.• Escapesequencesenablestringstocontainnonprinting(control)charactersandcharactersthathaveaspecialmeaning(suchas").

• Apartiallistofescapesequences:

Alert(bell) \a

Backspace \b

Newline \n

Horizontaltab \t

• Astringmaycontainanynumberofescapesequences:

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printf("Item\tUnit\tPurchase\n\tPrice\tDate\n");

Item Unit Purchase

Price Date

EscapeSequences(2/2)• Astringmaycontainanynumberofescapesequences:

• Anothercommonescapesequenceis\",whichrepresentsthe" character:

• Toprintasingle\ character,puttwo\ charactersinthestring:

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printf("Item\tUnit\tPurchase\n\tPrice\tDate\n");

Item Unit Purchase

Price Date

printf("\"Hello!\""); /* prints "Hello!" */

printf("\\"); /* prints one \ character */

Thescanf Function• scanf readsinputaccordingtoaparticularformat.

• Ascanf formatstringmaycontainbothordinarycharactersandconversionspecifications.

• Theconversionsallowedwithscanf areessentiallythesameasthoseusedwithprintf.

• Inmanycases,ascanf formatstringwillcontainonlyconversionspecifications:

• Sampleinput:scanf willassign1,–20,0.3,and–4000.0toi,j,x,andy,respectively.

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int i, j;float x, y;scanf("%d%d%f%f", &i, &j, &x, &y);

1 -20 .3 -4.0e3

scanf(format_string, &var1, &var2, …);

HowscanfWorks(1/4)• scanf triestomatchgroupsofinputcharacterswithconversionspecificationsintheformatstring.

• Foreachconversionspecification,scanf triestolocateanitemoftheappropriatetypeintheinputdata,skippingblankspaceifnecessary.

• scanf thenreadstheitem,stoppingwhenitreachesacharacterthatcan’tbelongtotheitem.• Iftheitemwasreadsuccessfully,scanf continuesprocessingtherestoftheformatstring.

• Ifnot,scanf returnsimmediately.

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HowscanfWorks (2/4)• Asitsearchesforanumber,scanf ignoreswhite-spacecharacters• space,horizontalandverticaltab,form-feed,andnew-line

• Acallofscanf thatreadsfournumbers:scanf("%d%d%f%f", &i, &j, &x, &y);

• Thenumberscanbeononelineorspreadoverseverallines:

• scanf “peeks”atthefinalnew-linewithoutreadingit.

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1-20 .3

-4.0e3••1¤-20•••.3¤•••-4.0e3¤ssrsrrrsssrrssssrrrrrr

(s =skipped;r =read)

HowscanfWorks(3/4)• Whenaskedtoreadaninteger,scanf firstsearchesforadigit,aplussign,oraminussign;itthenreadsdigitsuntilitreachesanondigit.

• Whenaskedtoreadafloating-pointnumber,scanf looksfor• aplusorminussign(optional),followedby• digits(possiblycontainingadecimalpoint),followedby• anexponent(optional).Anexponentconsistsofthelettere (orE),anoptionalsign,andoneormoredigits.

• %e,%f,and%g areinterchangeablewhenusedwithscanf.

• Whenscanf encountersacharacterthatcan’tbepartofthecurrentitem,thecharacteris“putback”tobereadagainduringthescanningofthenextinputitemorduringthenextcallofscanf.

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HowscanfWorks (4/4)• Sampleinput:

1-20.3-4.0e3¤

• Thecallofscanf isthesameasbefore:

scanf("%d%d%f%f", &i, &j, &x, &y);

• Here’showscanf wouldprocessthenewinput:• %d :Stores1intoi andputsthe- characterback.• %d :Stores–20intoj andputsthe. characterback.• %f :Stores0.3intox andputsthe- characterback.• %f :Stores–4.0× 103intoy andputsthenew-linecharacterback.

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OrdinaryCharactersinFormatStrings• Whenitencountersoneormorewhite-spacecharactersinaformatstring,scanf readswhite-spacecharactersfromtheinputuntilitreachesanon-white-spacecharacter(whichis“putback”).

• Whenitencountersanon-white-spacecharacterinaformatstring,scanfcomparesitwiththenextinputcharacter.• Iftheymatch,scanf discardstheinputcharacterandcontinuesprocessingtheformatstring.

• Iftheydon’tmatch,scanf putstheoffendingcharacterbackintotheinput,thenaborts.

• Examples:• Iftheformatstringis"%d/%d" andtheinputis•5/•96,scanf succeeds.• Iftheinputis•5•/•96 ,scanf fails,becausethe/ intheformatstringdoesn’tmatchthespaceintheinput.

• Toallowspacesafterthefirstnumber,usetheformatstring"%d /%d"instead.

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Confusingprintf withscanf (1/2)• Althoughcallsofscanf andprintfmayappearsimilar,therearesignificantdifferencesbetweenthetwo.

• Onecommonmistakeistoput& infrontofvariablesinacallofprintf:

• Incorrectlyassumingthatscanf formatstringsshouldresembleprintfformatstringsisanothercommonerror.

• Considerthefollowingcallofscanf:

• scanf willfirstlookforanintegerintheinput,whichitstoresinthevariablei.

• scanf willthentrytomatchacommawiththenextinputcharacter.• Ifthenextinputcharacterisaspace,notacomma,scanf willterminatewithoutreadingavalueforj.

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printf("%d %d\n", &i, &j); /*** WRONG ***/

scanf("%d, %d", &i, &j);

Confusingprintf withscanf (2/2)• Puttinganew-linecharacterattheendofascanf formatstringisusuallyabadidea.

• Toscanf,anew-linecharacterinaformatstringisequivalenttoaspace;bothcausescanf toadvancetothenextnon-white-spacecharacter.

• Iftheformatstringis"%d\n",scanf willskipwhitespace,readaninteger,thenskiptothenextnon-white-spacecharacter.

• Aformatstringlikethiscancauseaninteractiveprogramto“hang.”

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