KNK C 04 array functions - gnu.ac.kropen.gnu.ac.kr/lecslides/2017-2-introProg/KNK_C_04_array... · 2017-11-13 · •A common mistake: forgetting that an array with nelements is indexed

Arrays,FunctionsadoptedfromKNKCProgramming:AModernApproach

요약

Copyright©2008W.W.Norton&Company.Allrightsreserved. 2

Array,배열• 하나이상의값을하나의변수에저장하는데이터타입• 선언• 값초기화와할당 (다양한방법이있음,자료참고)

• 인덱스• 활용• 배열의차원(dimension)


Array,배열:선언• 구성요소:• 타입: int,float,char,double,long,등• 변수명:변수이름짖는제약조건을따름• 배열크기:임의의양수,저장하고자하는값의개수만큼


int scores[100];타입 변수명 배열크기

Array,배열:초기화와할당 (상세내용은자료참고)

• 하나의값할당(변수선언후가능)

• 일부초기화(c99)

• 모든값초기화

• 배열크기미지정초기화


int scores[5];int scores[5];scores[2] = 80;

int a[5] = {99, 80, 70, 92, 100};

int a[5] = {[1] = 29, [3] = 7};

int a[] = {99, 80, 70, 92, 100};

5개의값을저장할수있는scores라는이름의정수형배열

Array,배열:인덱스• 배열의물리적표현


10개의값을저장할수있는a라는이름의배열

중요:0에서인덱스시작

int a[10];배열의물리적표현

값이저장된위치의주소

Array,배열:활용• 바코드계산프로그램


int i1, i2, i3, i4, i5;scanf("%1d%1d%1d%1d%1d", &i1, &i2, &i3, &i4, &i5);first_sum = d + i2 + i4 + j1 + j3 + j5;

int i[5];scanf("%1d%1d%1d%1d%1d", &i[0], &i[1], &i[2], &i[3], &i[4]);first_sum = d + i[1] +i[2];

int i[5];for(cnt = 0; cnt < 5; cnt++)

scanf(“%1d”, &i[cnt]);first_sum = d + i[1] +i[2];

Array,배열:차원• 1차원

• 2차원


int a[10];

int a[5][9];3차원이상도표현가능단,시각화는못함

Function,함수• 수행하고자하는일련의동작들에붙여진이름

• 프로그램을이해하고수정하는데도움이됨• Definition,정의• Calling,호출• Arguments,인자• return,리턴• recursion,재귀


Function,함수: Definition,정의• 호출하려는함수보다먼저함수의정의가작성되야함• 함수선언:• 함수의내용은없이앞으로사용할함수의이름과인자값을프로그램에등록함.• 작성스타일에따라항상필요하진않음.

• 함수정의:• 함수의실제내용이기록됨


Function,함수: Declaration,선언


#include <stdio.h>

int sum(int a, int b);int main(){

…}int sum(int a, int b)

{함수 내용

}

함수선언

함수정의

Function,함수: Definition,정의


return-type function-name ( parameters ){

declarationsstatements

}int sum(int a, int b){

int result=0;for( ; a <= b; a++)

sum += a;return result;

}

리턴데이터타입 함수이름 함수 내에서사용될인자들

함수내에서사용될변수선언

함수내에서실행할문장들

타입일치

Function,함수: Calling,호출• 코드내에함수이름을작성하여호출


int main(){…answer = sum(val1, val2);…

}

int sum(int a, int b){

…return result;

}

기존코드의흐름은위에서아래로실행

함수호출시함수정의위치로실행의흐름이이동함

Function,함수: Arguments,인자• 함수밖에서정의된값으로함수내에서활용할변수이름


int main(){…answer = sum(val1, val2);…

}


…return a + b;

}

메인코드에서선언된 val1과 val2를sum이라는함수에활용할수있도록인자로넣었음

함수에서활용할변수이름은메인코드의변수이름과달라도됨

Function,함수:종합예제


#include <stdio.h>int sum(int a, int b);int main(){

int val1, val2, answer;printf(“Input two numbers in increasing order: “);scanf(“%d %d”, &val1, &val2);answer = sum(val1, val2);printf(“The sum is %d\n”, answer);

}


int result=0;for( ; a <= b; a++)

sum += a;return result;

}

호출

결과값반환

Function,함수: Recursion,재귀• 함수가호출되면해당코드를실행하고결과를리턴함

• 함수가자기스스로를다시호출하는것이가능함• 팩토리얼을기억해보기바람


int fact(int n){

if (n <= 1)return 1;

else

return n * fact(n - 1);}

다시호출

WorkshopUsetheworksheet


ArraysadoptedfromKNKCProgramming:AModernApproach

ScalarVariablesversusAggregateVariables• Sofar,theonlyvariableswe’veseenarescalar: capableofholdingasingledataitem.스칼라만다룸,한번에한값만저장가능

• Calsosupportsaggregate variables,whichcanstorecollectionsofvalues.집합변수지원,하나이상의다른값저장가능

• TherearetwokindsofaggregatesinC:arraysandstructures.두종류가있음,배열(array),구조체(structure)


One-DimensionalArrays• Anarray isadatastructurecontaininganumberofdatavalues,allofwhichhavethesametype.같은타입의값이하나의배열에저장

• Thesevalues,knownaselements, canbeindividuallyselectedbytheirpositionwithinthearray. 각값을요소라부름

• Thesimplestkindofarrayhasjustonedimension.1차원배열이가장간단함

• Theelementsofaone-dimensionalarraya areconceptuallyarrangedoneafteranotherinasinglerow(orcolumn):개념적으로 1줄짜리바둑판으로표현됨


One-DimensionalArrays• Todeclareanarray,wemustspecifythetype ofthearray’selementsandthenumber ofelements:배열선언시배열크기명시

int a[10];

• Theelementsmaybeofanytype;thelengthofthearraycanbeany(integer)constantexpression.어떤타입도사용가능,고정된크기를갖음

• Usingamacrotodefinethelengthofanarrayisanexcellentpractice:배열의크기는매크로로정의하는것이바람직함

#define N 10…int a[N];


ArraySubscripting• Toaccessanarrayelement,writethearraynamefollowedbyanintegervalueinsquarebrackets.참조법:변수이름과대괄호에수를넣음

• Thisisreferredtoassubscripting orindexing thearray.참조할배열의위치를인덱스라고부름

• Theelementsofanarrayoflengthn areindexedfrom0ton – 1.배열의크기가 n일때인덱스시작과끝값은 0과 n-1임

• Ifa isanarrayoflength10,itselementsaredesignatedbya[0],a[1],…,a[9]:크기 10일때시작은 0끝은 9


ArraySubscripting• Expressionsoftheforma[i] arelvalues,sotheycanbeusedinthesamewayasordinaryvariables:a[i]은 lvalue이기때문에값을할당받을수있음

a[0] = 1;printf("%d\n", a[5]);++a[i];


인덱스가실제저장된값이아님!!

ArraySubscripting• Manyprogramscontainfor loopswhosejobistoperformsomeoperationoneveryelementinanarray.for 루프의사용예

• Examplesoftypicaloperationsonanarraya oflengthN:for (i = 0; i < N; i++)

a[i] = 0; /* clears a */

for (i = 0; i < N; i++)scanf("%d", &a[i]); /* reads data into a */

for (i = 0; i < N; i++)sum += a[i]; /* sums the elements of a */


ArraySubscripting• Cdoesn’trequirethatsubscriptboundsbechecked;ifasubscriptgoesoutofrange,theprogram’sbehaviorisundefined.인덱스경계를검사하지않음;범위를넘으면오동작을함

• Acommonmistake:forgettingthatanarraywithn elementsisindexedfrom0ton – 1,not1ton:흔한실수:인덱스를 1부터시작

int a[10], i;

for (i = 1; i <= 10; i++)a[i] = 0;

Withsomecompilers,thisinnocent-lookingfor statementcausesaninfiniteloop.어떤컴파일러의경우무한루프에빠지게됨.


ArraySubscripting• Anarraysubscriptmaybeanyintegerexpression:인덱스는정수표현식으로사용가능

a[i+j*10] = 0;

• Theexpressioncanevenhavesideeffects:수식에사이드이펙트가있어도됨

i = 0;while (i < N)a[i++] = 0;


ArraySubscripting• Becarefulwhenanarraysubscripthasasideeffect:인덱스값에사이드이펙트가있는경우주의해야함

i = 0;while (i < N)a[i] = b[i++];

• Theexpressiona[i] = b[i++] accessesthevalueofi andalsomodifiesi,causingundefinedbehavior.정의되지않은동작

• Theproblemcanbeavoidedbyremovingtheincrementfromthesubscript:인덱스에서증가값을분리해서문제해결가능

for (i = 0; i < N; i++)a[i] = b[i];


ArrayInitialization• Anarray,likeanyothervariable,canbegivenaninitialvalueatthetimeit’sdeclared.변수선언시값을할당하여초기화할수있음

• Themostcommonformofarrayinitializerisalistofconstantexpressionsenclosedinbracesandseparatedbycommas:흔한초기화방법:중괄호와값그리고쉼표로구분

int a[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};


ArrayInitialization• Iftheinitializerisshorterthanthearray,theremainingelementsofthearrayaregiventhevalue0:int a[10] = {1, 2, 3, 4, 5, 6};/* initial value of a is {1, 2, 3, 4, 5, 6, 0, 0, 0, 0} */

• Usingthisfeature,wecaneasilyinitializeanarraytoallzeros:int a[10] = {0};/* initial value of a is {0, 0, 0, 0, 0, 0, 0, 0, 0, 0} */

There’sasingle0insidethebracesbecauseit’sillegalforaninitializertobecompletelyempty.

• It’salsoillegalforaninitializertobelongerthanthearrayitinitializes.


ArrayInitialization• Ifaninitializerispresent,thelengthofthearraymaybeomitted:int a[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

• Thecompilerusesthelengthoftheinitializertodeterminehowlongthearrayis.


DesignatedInitializers(C99)• It’softenthecasethatrelativelyfewelementsofanarrayneedtobeinitializedexplicitly;theotherelementscanbegivendefaultvalues.• Anexample:int a[15] ={0, 0, 29, 0, 0, 0, 0, 0, 0, 7, 0, 0, 0, 0, 48};

• Foralargearray,writinganinitializerinthisfashionistediousanderror-prone.


DesignatedInitializers(C99)• C99’sdesignatedinitializerscanbeusedtosolvethisproblem.

• Here’showwecouldredothepreviousexampleusingadesignatedinitializer:int a[15] = {[2] = 29, [9] = 7, [14] = 48};

• Eachnumberinbracketsissaidtobeadesignator.


DesignatedInitializers(C99)• Designatedinitializersareshorterandeasiertoread(atleastforsomearrays).

• Also,theorderinwhichtheelementsarelistednolongermatters.

• Anotherwaytowritethepreviousexample:int a[15] = {[14] = 48, [9] = 7, [2] = 29};


DesignatedInitializers(C99)• Designatorsmustbeintegerconstantexpressions.

• Ifthearraybeinginitializedhaslengthn,eachdesignatormustbebetween0andn – 1.

• Ifthelengthofthearrayisomitted,adesignatorcanbeanynonnegativeinteger.• Thecompilerwilldeducethelengthofthearrayfromthelargestdesignator.

• Thefollowingarraywillhave24elements:int b[] = {[5] = 10, [23] = 13, [11] = 36, [15] = 29};


DesignatedInitializers(C99)• Aninitializermayuseboththeolder(element-by-element)techniqueandthenewer(designated)technique:int c[10] = {5, 1, 9, [4] = 3, 7, 2, [8] = 6};


Usingthesizeof OperatorwithArrays• Thesizeof operatorcandeterminethesizeofanarray(inbytes).• Ifa isanarrayof10integers,thensizeof(a) istypically40(assumingthateachintegerrequiresfourbytes).• Wecanalsousesizeof tomeasurethesizeofanarrayelement,suchasa[0].

• Dividingthearraysizebytheelementsizegivesthelengthofthearray:sizeof(a) / sizeof(a[0])


Usingthesizeof OperatorwithArrays• Someprogrammersusethisexpressionwhenthelengthofthearrayisneeded.• Aloopthatclearsthearraya:for (i = 0; i < sizeof(a) / sizeof(a[0]); i++)a[i] = 0;

Notethattheloopdoesn’thavetobemodifiedifthearraylengthshouldchangeatalaterdate.


Usingthesizeof OperatorwithArrays• Somecompilersproduceawarningmessagefortheexpressioni< sizeof(a) / sizeof(a[0]).

• Thevariablei probablyhastypeint (asignedtype),whereassizeof producesavalueoftypesize_t (anunsignedtype).

• Comparingasignedintegerwithanunsignedintegercanbedangerous,butinthiscaseit’ssafe.


Usingthesizeof OperatorwithArrays• Toavoidawarning,wecanaddacastthatconvertssizeof(a)/ sizeof(a[0]) toasignedinteger:for (i = 0; i < (int) (sizeof(a) / sizeof(a[0])); i++)

a[i] = 0;

• Definingamacroforthesizecalculationisoftenhelpful:#define SIZE ((int) (sizeof(a) / sizeof(a[0])))

for (i = 0; i < SIZE; i++)a[i] = 0;


MultidimensionalArrays• Anarraymayhaveanynumberofdimensions.

• Thefollowingdeclarationcreatesatwo-dimensionalarray(amatrix, inmathematicalterminology):int m[5][9];

• m has5rowsand9columns.Bothrowsandcolumnsareindexedfrom0:


MultidimensionalArrays• Toaccesstheelementofm inrowi,columnj,wemustwritem[i][j].

• Theexpressionm[i] designatesrowi ofm,andm[i][j] thenselectselementj inthisrow.

• Resistthetemptationtowritem[i,j] insteadofm[i][j].

• Ctreatsthecommaasanoperatorinthiscontext,som[i,j] isthesameasm[j].


MultidimensionalArrays• Althoughwevisualizetwo-dimensionalarraysastables,that’snotthewaythey’reactuallystoredincomputermemory.

• Cstoresarraysinrow-majororder, withrow0first,thenrow1,andsoforth.• Howthem arrayisstored:


MultidimensionalArrays• Nestedfor loopsareidealforprocessingmultidimensionalarrays.

• Considertheproblemofinitializinganarrayforuseasanidentitymatrix.Apairofnestedfor loopsisperfect:#define N 10

double ident[N][N];int row, col;

for (row = 0; row < N; row++)for (col = 0; col < N; col++)

if (row == col)ident[row][col] = 1.0;

elseident[row][col] = 0.0;


InitializingaMultidimensionalArray• Wecancreateaninitializerforatwo-dimensionalarraybynestingone-dimensionalinitializers:int m[5][9] = {{1, 1, 1, 1, 1, 0, 1, 1, 1},

{0, 1, 0, 1, 0, 1, 0, 1, 0},{0, 1, 0, 1, 1, 0, 0, 1, 0},{1, 1, 0, 1, 0, 0, 0, 1, 0},{1, 1, 0, 1, 0, 0, 1, 1, 1}};

• Initializersforhigher-dimensionalarraysareconstructedinasimilarfashion.• Cprovidesavarietyofwaystoabbreviateinitializersformultidimensionalarrays


InitializingaMultidimensionalArray• Ifaninitializerisn’tlargeenoughtofillamultidimensionalarray,theremainingelementsaregiventhevalue0.• Thefollowinginitializerfillsonlythefirstthreerowsofm;thelasttworowswillcontainzeros:int m[5][9] = {{1, 1, 1, 1, 1, 0, 1, 1, 1},

{0, 1, 0, 1, 0, 1, 0, 1, 0},{0, 1, 0, 1, 1, 0, 0, 1, 0}};


InitializingaMultidimensionalArray• Ifaninnerlistisn’tlongenoughtofillarow,theremainingelementsintherowareinitializedto0:int m[5][9] = {{1, 1, 1, 1, 1, 0, 1, 1, 1},

{0, 1, 0, 1, 0, 1, 0, 1},{0, 1, 0, 1, 1, 0, 0, 1},{1, 1, 0, 1, 0, 0, 0, 1},{1, 1, 0, 1, 0, 0, 1, 1, 1}};


InitializingaMultidimensionalArray• Wecanevenomittheinnerbraces:int m[5][9] = {1, 1, 1, 1, 1, 0, 1, 1, 1,

0, 1, 0, 1, 0, 1, 0, 1, 0,0, 1, 0, 1, 1, 0, 0, 1, 0,1, 1, 0, 1, 0, 0, 0, 1, 0,1, 1, 0, 1, 0, 0, 1, 1, 1};

Oncethecompilerhasseenenoughvaluestofillonerow,itbeginsfillingthenext.

• Omittingtheinnerbracescanberisky,sinceanextraelement(orevenworse,amissingelement)willaffecttherestoftheinitializer.


InitializingaMultidimensionalArray• C99’sdesignatedinitializersworkwithmultidimensionalarrays.

• Howtocreate2× 2identitymatrix:double ident[2][2] = {[0][0] = 1.0, [1][1] = 1.0};

Asusual,allelementsforwhichnovalueisspecifiedwilldefaulttozero.


ConstantArrays• Anarraycanbemade“constant”bystartingitsdeclarationwiththewordconst:const char hex_chars[] ={'0', '1', '2', '3', '4', '5', '6', '7', '8', '9','A', 'B', 'C', 'D', 'E', 'F'};

• Anarraythat’sbeendeclaredconst shouldnotbemodifiedbytheprogram.


ConstantArrays• Advantagesofdeclaringanarraytobeconst:• Documentsthattheprogramwon’tchangethearray.• Helpsthecompilercatcherrors.

• const isn’tlimitedtoarrays,butit’sparticularlyusefulinarraydeclarations.


Variable-LengthArrays(C99)• InC89,thelengthofanarrayvariablemustbespecifiedbyaconstantexpression.

• InC99,however,it’ssometimespossibletouseanexpressionthat’snot constant.• Thereverse2.c program—amodificationofreverse.c—illustratesthisability.


Variable-LengthArrays(C99)• Thearraya inthereverse2.c programisanexampleofavariable-lengtharray(orVLA).

• ThelengthofaVLAiscomputedwhentheprogramisexecuted.

• ThechiefadvantageofaVLAisthataprogramcancalculateexactlyhowmanyelementsareneeded.

• Iftheprogrammermakesthechoice,it’slikelythatthearraywillbetoolong(wastingmemory)ortooshort(causingtheprogramtofail).


Variable-LengthArrays(C99)• ThelengthofaVLAdoesn’thavetobespecifiedbyasinglevariable.Arbitraryexpressionsarelegal:int a[3*i+5];int b[j+k];

• Likeotherarrays,VLAscanbemultidimensional:int c[m][n];

• RestrictionsonVLAs:• Can’thavestaticstorageduration(discussedinChapter18).• Can’thaveaninitializer.


FunctionsadoptedfromKNKCProgramming:AModernApproach

Introduction• Afunctionisaseriesofstatementsthathavebeengroupedtogetherandgivenaname.

• Eachfunctionisessentiallyasmallprogram,withitsowndeclarationsandstatements.

• Advantagesoffunctions:• Aprogramcanbedividedintosmallpiecesthatareeasiertounderstandandmodify.

• Wecanavoidduplicatingcodethat’susedmorethanonce.• Afunctionthatwasoriginallypartofoneprogramcanbereusedinotherprograms.


DefiningandCallingFunctions• Beforewegoovertheformalrulesfordefiningafunction,let’slookatthreesimpleprogramsthatdefinefunctions.


Program:ComputingAverages• Afunctionnamedaverage thatcomputestheaverageoftwodouble values:double average(double a, double b){return (a + b) / 2;

}

• Theworddouble atthebeginningisthereturntype ofaverage.

• Theidentifiersa andb (thefunction’sparameters)representthenumbersthatwillbesuppliedwhenaverage iscalled.


Program:ComputingAverages• Everyfunctionhasanexecutablepart,calledthebody, whichisenclosedinbraces.• Thebodyofaverage consistsofasinglereturn statement.

• Executingthisstatementcausesthefunctionto“return”totheplacefromwhichitwascalled;thevalueof(a + b) / 2 willbethevaluereturnedbythefunction.


Program:ComputingAverages• Afunctioncallconsistsofafunctionnamefollowedbyalistofarguments.• average(x, y) isacalloftheaverage function.

• Argumentsareusedtosupplyinformationtoafunction.• Thecallaverage(x, y) causesthevaluesofx andy tobecopiedintotheparametersa andb.

• Anargumentdoesn’thavetobeavariable;anyexpressionofacompatibletypewilldo.• average(5.1, 8.9) andaverage(x/2, y/3) arelegal.


Program:ComputingAverages• We’llputthecallofaverage intheplacewhereweneedtousethereturnvalue.• Astatementthatprintstheaverageofx andy:printf("Average: %g\n", average(x, y));

Thereturnvalueofaverage isn’tsaved;theprogramprintsitandthendiscardsit.

• Ifwehadneededthereturnvaluelaterintheprogram,wecouldhavecaptureditinavariable:avg = average(x, y);


Program:ComputingAverages• Theaverage.c programreadsthreenumbersandusestheaverage functiontocomputetheiraverages,onepairatatime:Enter three numbers: 3.5 9.6 10.2Average of 3.5 and 9.6: 6.55Average of 9.6 and 10.2: 9.9Average of 3.5 and 10.2: 6.85


average.c/* Computes pairwise averages of three numbers */

#include <stdio.h>

double average(double a, double b){return (a + b) / 2;

}

int main(void){double x, y, z;

printf("Enter three numbers: ");scanf("%lf%lf%lf", &x, &y, &z);printf("Average of %g and %g: %g\n", x, y, average(x, y));printf("Average of %g and %g: %g\n", y, z, average(y, z));printf("Average of %g and %g: %g\n", x, z, average(x, z));

return 0;}


Program:PrintingaCountdown• Toindicatethatafunctionhasnoreturnvalue,wespecifythatitsreturntypeisvoid:void print_count(int n){printf("T minus %d and counting\n", n);

}

• void isatypewithnovalues.• Acallofprint_countmustappearinastatementbyitself:print_count(i);

• Thecountdown.c programcallsprint_count 10timesinsidealoop.


countdown.c

/* Prints a countdown */

#include <stdio.h>

void print_count(int n){printf("T minus %d and counting\n", n);

}

int main(void){int i;

for (i = 10; i > 0; --i)print_count(i);

return 0;}


Program:PrintingaPun(Revisited)• Whenafunctionhasnoparameters,thewordvoid isplacedinparenthesesafterthefunction’sname:void print_pun(void){printf("To C, or not to C: that is the question.\n");

}

• Tocallafunctionwithnoarguments,wewritethefunction’sname,followedbyparentheses:print_pun();

Theparenthesesmust bepresent.• Thepun2.c programteststheprint_pun function.


pun2.c

/* Prints a bad pun */

#include <stdio.h>

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

}

int main(void){print_pun();return 0;

}


FunctionDefinitions• Generalformofafunctiondefinition:return-type function-name ( parameters ){

declarationsstatements

}


FunctionDefinitions• Thereturntypeofafunctionisthetypeofvaluethatthefunctionreturns.

• Rulesgoverningthereturntype:• Functionsmaynotreturnarrays.• Specifyingthatthereturntypeisvoid indicatesthatthefunctiondoesn’treturnavalue.

• IfthereturntypeisomittedinC89,thefunctionispresumedtoreturnavalueoftypeint.

• InC99,omittingthereturntypeisillegal.


FunctionDefinitions• Asamatterofstyle,someprogrammersputthereturntypeabove thefunctionname:doubleaverage(double a, double b){return (a + b) / 2;

}

• Puttingthereturntypeonaseparatelineisespeciallyusefulifthereturntypeislengthy,likeunsigned long int.


FunctionDefinitions• Afterthefunctionnamecomesalistofparameters.

• Eachparameterisprecededbyaspecificationofitstype;parametersareseparatedbycommas.• Ifthefunctionhasnoparameters,thewordvoid shouldappearbetweentheparentheses.


FunctionDefinitions• Thebodyofafunctionmayincludebothdeclarationsandstatements.• Analternativeversionoftheaverage function:double average(double a, double b){double sum; /* declaration */

sum = a + b; /* statement */return sum / 2; /* statement */

}


FunctionDefinitions• Variablesdeclaredinthebodyofafunctioncan’tbeexaminedormodifiedbyotherfunctions.

• InC89,variabledeclarationsmustcomefirst,beforeallstatementsinthebodyofafunction.

• InC99,variabledeclarationsandstatementscanbemixed,aslongaseachvariableisdeclaredpriortothefirststatementthatusesthevariable.


FunctionDefinitions• Thebodyofafunctionwhosereturntypeisvoid (a“voidfunction”)canbeempty:void print_pun(void){}

• Leavingthebodyemptymaymakesenseasatemporarystepduringprogramdevelopment.


FunctionCalls• Afunctioncallconsistsofafunctionnamefollowedbyalistofarguments,enclosedinparentheses:average(x, y)print_count(i)print_pun()

• Iftheparenthesesaremissing,thefunctionwon’tbecalled:print_pun; /*** WRONG ***/

Thisstatementislegalbuthasnoeffect.


FunctionCalls• Acallofavoid functionisalwaysfollowedbyasemicolontoturnitintoastatement:print_count(i);print_pun();

• Acallofanon-void functionproducesavaluethatcanbestoredinavariable,tested,printed,orusedinsomeotherway:avg = average(x, y);if (average(x, y) > 0)printf("Average is positive\n");

printf("The average is %g\n", average(x, y));


FunctionCalls• Thevaluereturnedbyanon-void functioncanalwaysbediscardedifit’snotneeded:average(x, y); /* discards return value */

Thiscallisanexampleofanexpressionstatement:astatementthatevaluatesanexpressionbutthendiscardstheresult.


FunctionCalls• Ignoringthereturnvalueofaverage isanoddthingtodo,butforsomefunctionsitmakessense.• printf returnsthenumberofcharactersthatitprints.

• Afterthefollowingcall,num_chars willhavethevalue9:num_chars = printf("Hi, Mom!\n");

• We’llnormallydiscardprintf’sreturnvalue:printf("Hi, Mom!\n");/* discards return value */


FunctionCalls• Tomakeitclearthatwe’redeliberatelydiscardingthereturnvalueofafunction,Callowsustoput(void) beforethecall:(void) printf("Hi, Mom!\n");

• Using(void)makesitcleartoothersthatyoudeliberatelydiscardedthereturnvalue,notjustforgotthattherewasone.


Program:TestingWhetheraNumberIsPrime• Theprime.c programtestswhetheranumberisprime:Enter a number: 34Not prime

• Theprogramusesafunctionnamedis_prime thatreturnstrue ifitsparameterisaprimenumberandfalse ifitisn’t.

• is_prime dividesitsparametern byeachofthenumbersbetween2andthesquarerootofn;iftheremainderisever0,nisn’tprime.


prime.c

/* Tests whether a number is prime */

#include <stdbool.h> /* C99 only */#include <stdio.h>

bool is_prime(int n){int divisor;

if (n <= 1)return false;

for (divisor = 2; divisor * divisor <= n; divisor++)if (n % divisor == 0)return false;

return true;}


int main(void){int n;

printf("Enter a number: ");scanf("%d", &n);if (is_prime(n))printf("Prime\n");

elseprintf("Not prime\n");

return 0;}


FunctionDeclarations• Cdoesn’trequirethatthedefinitionofafunctionprecedeitscalls.• Supposethatwerearrangetheaverage.c programbyputtingthedefinitionofaverage after thedefinitionofmain.


FunctionDeclarations#include <stdio.h>



return 0;}

double average(double a, double b){return (a + b) / 2;

}


FunctionDeclarations• Whenthecompilerencountersthefirstcallofaverage inmain,ithasnoinformationaboutthefunction.

• Insteadofproducinganerrormessage,thecompilerassumesthataverage returnsanint value.

• Wesaythatthecompilerhascreatedanimplicitdeclaration ofthefunction.


FunctionDeclarations• Thecompilerisunabletocheckthatwe’repassingaverage therightnumberofargumentsandthattheargumentshavethepropertype.• Instead,itperformsthedefaultargumentpromotionsandhopesforthebest.• Whenitencountersthedefinitionofaverage laterintheprogram,thecompilernoticesthatthefunction’sreturntypeisactuallydouble,notint,andsowegetanerrormessage.


FunctionDeclarations• Onewaytoavoidtheproblemofcall-before-definitionistoarrangetheprogramsothatthedefinitionofeachfunctionprecedesallitscalls.• Unfortunately,suchanarrangementdoesn’talwaysexist.

• Evenwhenitdoes,itmaymaketheprogramhardertounderstandbyputtingitsfunctiondefinitionsinanunnaturalorder.


FunctionDeclarations• Fortunately,Coffersabettersolution:declareeachfunctionbeforecallingit.• Afunctiondeclaration providesthecompilerwithabriefglimpseatafunctionwhosefulldefinitionwillappearlater.• Generalformofafunctiondeclaration:return-type function-name ( parameters ) ;

• Thedeclarationofafunctionmustbeconsistentwiththefunction’sdefinition.• Here’stheaverage.c programwithadeclarationofaverage added.


FunctionDeclarations#include <stdio.h>

double average(double a, double b); /* DECLARATION */



return 0;}

double average(double a, double b) /* DEFINITION */{return (a + b) / 2;

}


FunctionDeclarations• Functiondeclarationsofthekindwe’rediscussingareknownasfunctionprototypes.

• Calsohasanolderstyleoffunctiondeclarationinwhichtheparenthesesareleftempty.

• Afunctionprototypedoesn’thavetospecifythenamesofthefunction’sparameters,aslongastheirtypesarepresent:double average(double, double);

• It’susuallybestnottoomitparameternames.


FunctionDeclarations• C99hasadoptedtherulethateitheradeclarationoradefinitionofafunctionmustbepresentpriortoanycallofthefunction.

• Callingafunctionforwhichthecompilerhasnotyetseenadeclarationordefinitionisanerror.


Arguments• InC,argumentsarepassedbyvalue: whenafunctioniscalled,eachargumentisevaluatedanditsvalueassignedtothecorrespondingparameter.• Sincetheparametercontainsacopyoftheargument’svalue,anychangesmadetotheparameterduringtheexecutionofthefunctiondon’taffecttheargument.


Arguments• Thefactthatargumentsarepassedbyvaluehasbothadvantagesanddisadvantages.

• Sinceaparametercanbemodifiedwithoutaffectingthecorrespondingargument,wecanuseparametersasvariableswithinthefunction,reducingthenumberofgenuinevariablesneeded.


Arguments• Considerthefollowingfunction,whichraisesanumberx toapowern:int power(int x, int n){int i, result = 1;

for (i = 1; i <= n; i++)result = result * x;

return result;}


Arguments• Sincen isacopy oftheoriginalexponent,thefunctioncansafelymodifyit,removingtheneedfori:int power(int x, int n){int result = 1;

while (n-- > 0)result = result * x;

return result;}


Arguments• C’srequirementthatargumentsbepassedbyvaluemakesitdifficulttowritecertainkindsoffunctions.• Supposethatweneedafunctionthatwilldecomposeadouble valueintoanintegerpartandafractionalpart.

• Sinceafunctioncan’treturn twonumbers,wemighttrypassingapairofvariablestothefunctionandhavingitmodifythem:void decompose(double x, long int_part,

double frac_part){

int_part = (long) x;frac_part = x - int_part;

}


Arguments• Acallofthefunction:decompose(3.14159, i, d);

• Unfortunately,i andd won’tbeaffectedbytheassignmentstoint_part andfrac_part.

• Chapter11showshowtomakedecompose workcorrectly.


ArgumentConversions• Callowsfunctioncallsinwhichthetypesoftheargumentsdon’tmatchthetypesoftheparameters.

• Therulesgoverninghowtheargumentsareconverteddependonwhetherornotthecompilerhasseenaprototypeforthefunction(orthefunction’sfulldefinition)priortothecall.


ArgumentConversions• Thecompilerhasencounteredaprototypepriortothecall.

• Thevalueofeachargumentisimplicitlyconvertedtothetypeofthecorrespondingparameterasifbyassignment.• Example:Ifanint argumentispassedtoafunctionthatwasexpectingadouble,theargumentisconvertedtodoubleautomatically.


ArgumentConversions• Thecompilerhasnotencounteredaprototypepriortothecall.

• Thecompilerperformsthedefaultargumentpromotions:• float argumentsareconvertedtodouble.• Theintegralpromotionsareperformed,causingchar andshort argumentstobeconvertedtoint.(InC99,theintegerpromotionsareperformed.)


ArgumentConversions• Relyingonthedefaultargumentpromotionsisdangerous.

• Example:#include <stdio.h>

int main(void){

double x = 3.0;printf("Square: %d\n", square(x));

return 0;}

int square(int n){

return n * n;}

• Atthetimesquare iscalled,thecompilerdoesn’tknowthatitexpectsanargumentoftypeint.


ArgumentConversions• Instead,thecompilerperformsthedefaultargumentpromotionsonx,withnoeffect.

• Sinceit’sexpectinganargumentoftypeint buthasbeengivenadouble valueinstead,theeffectofcallingsquare isundefined.

• Theproblemcanbefixedbycastingsquare’sargumenttothepropertype:printf("Square: %d\n", square((int) x));

• Amuchbettersolutionistoprovideaprototypeforsquare beforecallingit.

• InC99,callingsquare withoutfirstprovidingadeclarationordefinitionofthefunctionisanerror.


ArrayArguments• Whenafunctionparameterisaone-dimensionalarray,thelengthofthearraycanbeleftunspecified:int f(int a[]) /* no length specified */{…

}

• Cdoesn’tprovideanyeasywayforafunctiontodeterminethelengthofanarraypassedtoit.• Instead,we’llhavetosupplythelength—ifthefunctionneedsit—asanadditionalargument.


ArrayArguments• Example:int sum_array(int a[], int n){int i, sum = 0;

for (i = 0; i < n; i++)sum += a[i];

return sum;}

• Sincesum_array needstoknowthelengthofa,wemustsupplyitasasecondargument.


ArrayArguments• Theprototypeforsum_array hasthefollowingappearance:int sum_array(int a[], int n);

• Asusual,wecanomittheparameternamesifwewish:int sum_array(int [], int);


ArrayArguments• Whensum_array iscalled,thefirstargumentwillbethenameofanarray,andthesecondwillbeitslength:#define LEN 100

int main(void){

int b[LEN], total;…total = sum_array(b, LEN);…

}

• Noticethatwedon’tputbracketsafteranarraynamewhenpassingittoafunction:total = sum_array(b[], LEN); /*** WRONG ***/


ArrayArguments• Afunctionhasnowaytocheckthatwe’vepasseditthecorrectarraylength.

• Wecanexploitthisfactbytellingthefunctionthatthearrayissmallerthanitreallyis.• Supposethatwe’veonlystored50numbersintheb array,eventhoughitcanhold100.

• Wecansumjustthefirst50elementsbywritingtotal = sum_array(b, 50);


ArrayArguments• Becarefulnottotellafunctionthatanarrayargumentislargerthanitreallyis:total = sum_array(b, 150); /*** WRONG ***/

sum_array willgopasttheendofthearray,causingundefinedbehavior.


ArrayArguments• Afunctionisallowedtochangetheelementsofanarrayparameter,andthechangeisreflectedinthecorrespondingargument.• Afunctionthatmodifiesanarraybystoringzerointoeachofitselements:void store_zeros(int a[], int n){int i;

for (i = 0; i < n; i++) a[i] = 0;

}


ArrayArguments• Acallofstore_zeros:store_zeros(b, 100);

• TheabilitytomodifytheelementsofanarrayargumentmayseemtocontradictthefactthatCpassesargumentsbyvalue.

• Chapter12explainswhythere’sactuallynocontradiction.


ArrayArguments• Ifaparameterisamultidimensionalarray,onlythelengthofthefirstdimensionmaybeomitted.• Ifwerevisesum_array sothata isatwo-dimensionalarray,wemustspecifythenumberofcolumnsina:#define LEN 10

int sum_two_dimensional_array(int a[][LEN], int n){

int i, j, sum = 0;

for (i = 0; i < n; i++)for (j = 0; j < LEN; j++)

sum += a[i][j];

return sum;}


ArrayArguments• Notbeingabletopassmultidimensionalarrayswithanarbitrarynumberofcolumnscanbeanuisance.

• Wecanoftenworkaroundthisdifficultybyusingarraysofpointers.

• C99’svariable-lengtharrayparametersprovideanevenbettersolution.


Variable-LengthArrayParameters(C99)• C99allowstheuseofvariable-lengtharraysasparameters.• Considerthesum_array function:int sum_array(int a[], int n){…

}

Asitstandsnow,there’snodirectlinkbetweenn andthelengthofthearraya.• Althoughthefunctionbodytreatsn asa’slength,theactuallengthofthearraycouldbelargerorsmallerthann.


Variable-LengthArrayParameters(C99)• Usingavariable-lengtharrayparameter,wecanexplicitlystatethata’slengthisn:int sum_array(int n, int a[n]){…

}

• Thevalueofthefirstparameter(n)specifiesthelengthofthesecondparameter(a).• Notethattheorderoftheparametershasbeenswitched;orderisimportantwhenvariable-lengtharrayparametersareused.


Variable-LengthArrayParameters(C99)• Thereareseveralwaystowritetheprototypeforthenewversionofsum_array.

• Onepossibilityistomakeitlookexactlylikethefunctiondefinition:int sum_array(int n, int a[n]); /* Version 1 */

• Anotherpossibilityistoreplacethearraylengthbyanasterisk(*):int sum_array(int n, int a[*]); /* Version 2a */


Variable-LengthArrayParameters(C99)• Thereasonforusingthe* notationisthatparameternamesareoptionalinfunctiondeclarations.

• Ifthenameofthefirstparameterisomitted,itwouldn’tbepossibletospecifythatthelengthofthearrayisn,butthe*providesacluethatthelengthofthearrayisrelatedtoparametersthatcomeearlierinthelist:int sum_array(int, int [*]); /* Version 2b */


Variable-LengthArrayParameters(C99)• It’salsolegaltoleavethebracketsempty,aswenormallydowhendeclaringanarrayparameter:int sum_array(int n, int a[]); /* Version 3a */int sum_array(int, int []); /* Version 3b */

• Leavingthebracketsemptyisn’tagoodchoice,becauseitdoesn’texposetherelationshipbetweenn anda.


Variable-LengthArrayParameters(C99)• Ingeneral,thelengthofavariable-lengtharrayparametercanbeanyexpression.• Afunctionthatconcatenatestwoarraysa andb,storingtheresultintoathirdarraynamedc:int concatenate(int m, int n, int a[m], int b[n],

int c[m+n]){

…}

• Theexpressionusedtospecifythelengthofc involvestwootherparameters,butingeneralitcouldrefertovariablesoutsidethefunctionorevencallotherfunctions.


Variable-LengthArrayParameters(C99)• Variable-lengtharrayparameterswithasingledimensionhavelimitedusefulness.

• Theymakeafunctiondeclarationordefinitionmoredescriptivebystatingthedesiredlengthofanarrayargument.

• However,noadditionalerror-checkingisperformed;it’sstillpossibleforanarrayargumenttobetoolongortooshort.


Variable-LengthArrayParameters(C99)• Variable-lengtharrayparametersaremostusefulformultidimensionalarrays.

• Byusingavariable-lengtharrayparameter,wecangeneralizethesum_two_dimensional_array functiontoanynumberofcolumns:int sum_two_dimensional_array(int n, int m, int a[n][m]){int i, j, sum = 0;

for (i = 0; i < n; i++)for (j = 0; j < m; j++)sum += a[i][j];

return sum;}


Variable-LengthArrayParameters(C99)• Prototypesforthisfunctioninclude:int sum_two_dimensional_array(int n, int m, int a[n][m]);int sum_two_dimensional_array(int n, int m, int a[*][*]);int sum_two_dimensional_array(int n, int m, int a[][m]);int sum_two_dimensional_array(int n, int m, int a[][*]);


Usingstatic inArrayParameterDeclarations(C99)• C99allowstheuseofthekeywordstatic inthedeclarationofarrayparameters.• Thefollowingexampleusesstatic toindicatethatthelengthofa isguaranteedtobeatleast3:int sum_array(int a[static 3], int n){…

}


Usingstatic inArrayParameterDeclarations(C99)

• Usingstatic hasnoeffectonprogrambehavior.

• Thepresenceofstatic ismerelya“hint”thatmayallowaCcompilertogeneratefasterinstructionsforaccessingthearray.• Ifanarrayparameterhasmorethanonedimension,static canbeusedonlyinthefirstdimension.


CompoundLiterals(C99)• Let’sreturntotheoriginalsum_array function.• Whensum_array iscalled,thefirstargumentisusuallythenameofanarray.• Example:int b[] = {3, 0, 3, 4, 1};total = sum_array(b, 5);

• bmustbedeclaredasavariableandtheninitializedpriortothecall.• Ifb isn’tneededforanyotherpurpose,itcanbeannoyingtocreateitsolelyforthepurposeofcallingsum_array.


CompoundLiterals(C99)• InC99,wecanavoidthisannoyancebyusingacompoundliteral:anunnamedarraythat’screated“onthefly”bysimplyspecifyingwhichelementsitcontains.• Acallofsum_array withacompoundliteral(showninbold)asitsfirstargument:total = sum_array((int []){3, 0, 3, 4, 1}, 5);

• Wedidn’tspecifythelengthofthearray,soit’sdeterminedbythenumberofelementsintheliteral.

• Wealsohavetheoptionofspecifyingalengthexplicitly:(int [4]){1, 9, 2, 1}

isequivalentto(int []){1, 9, 2, 1}


CompoundLiterals(C99)• Acompoundliteralresemblesacastappliedtoaninitializer.• Infact,compoundliteralsandinitializersobeythesamerules.• Acompoundliteralmaycontaindesignators,justlikeadesignatedinitializer,anditmayfailtoprovidefullinitialization(inwhichcaseanyuninitializedelementsdefaulttozero).• Forexample,theliteral(int [10]){8, 6} has10elements;thefirsttwohavethevalues8and6,andtheremainingelementshavethevalue0.


CompoundLiterals(C99)• Compoundliteralscreatedinsideafunctionmaycontainarbitraryexpressions,notjustconstants:total = sum_array((int []){2 * i, i + j, j * k}, 3);

• Acompoundliteralisanlvalue,sothevaluesofitselementscanbechanged.

• Ifdesired,acompoundliteralcanbemade“read-only”byaddingthewordconst toitstype:(const int []){5, 4}


Thereturn Statement• Anon-void functionmustusethereturn statementtospecifywhatvalueitwillreturn.• Thereturn statementhastheformreturn expression ;

• Theexpressionisoftenjustaconstantorvariable:return 0;return status;

• Morecomplexexpressionsarepossible:return n >= 0 ? n : 0;


Thereturn Statement• Ifthetypeoftheexpressioninareturn statementdoesn’tmatchthefunction’sreturntype,theexpressionwillbeimplicitlyconvertedtothereturntype.• Ifafunctionreturnsanint,butthereturn statementcontainsadouble expression,thevalueoftheexpressionisconvertedtoint.


Thereturn Statement• return statementsmayappearinfunctionswhosereturntypeisvoid,providedthatnoexpressionisgiven:return; /* return in a void function */

• Example:void print_int(int i){if (i < 0)return;

printf("%d", i);}


Thereturn Statement• Areturn statementmayappearattheendofavoid function:void print_pun(void){printf("To C, or not to C: that is the question.\n");return; /* OK, but not needed */

}

Usingreturn hereisunnecessary.

• Ifanon-void functionfailstoexecuteareturn statement,thebehavioroftheprogramisundefinedifitattemptstousethefunction’sreturnvalue.


ProgramTermination• Normally,thereturntypeofmain isint:int main(void){…

}

• OlderCprogramsoftenomitmain’sreturntype,takingadvantageofthefactthatittraditionallydefaultstoint:main(){…

}


ProgramTermination• Omittingthereturntypeofafunctionisn’tlegalinC99,soit’sbesttoavoidthispractice.• Omittingthewordvoid inmain’sparameterlistremainslegal,but—asamatterofstyle—it’sbesttoincludeit.


ProgramTermination• Thevaluereturnedbymain isastatuscodethatcanbetestedwhentheprogramterminates.• main shouldreturn0iftheprogramterminatesnormally.

• Toindicateabnormaltermination,main shouldreturnavalueotherthan0.

• It’sgoodpracticetomakesurethateveryCprogramreturnsastatuscode.


Theexit Function• Executingareturn statementinmain isonewaytoterminateaprogram.• Anotheriscallingtheexit function,whichbelongsto<stdlib.h>.

• Theargumentpassedtoexit hasthesamemeaningasmain’sreturnvalue:bothindicatetheprogram’sstatusattermination.

• Toindicatenormaltermination,we’dpass0:exit(0); /* normal termination */


Theexit Function• Since0isabitcryptic,CallowsustopassEXIT_SUCCESSinstead(theeffectisthesame):exit(EXIT_SUCCESS);

• PassingEXIT_FAILURE indicatesabnormaltermination:exit(EXIT_FAILURE);

• EXIT_SUCCESS andEXIT_FAILURE aremacrosdefinedin<stdlib.h>.• ThevaluesofEXIT_SUCCESS andEXIT_FAILURE areimplementation-defined;typicalvaluesare0and1,respectively.


Theexit Function• Thestatementreturn expression;inmain isequivalenttoexit(expression);• Thedifferencebetweenreturn andexit isthatexit causesprogramterminationregardlessofwhichfunctioncallsit.• Thereturn statementcausesprogramterminationonlywhenitappearsinthemain function.


Recursion• Afunctionisrecursive ifitcallsitself.• Thefollowingfunctioncomputesn!recursively,usingtheformulan!=n × (n – 1)!:int fact(int n){if (n <= 1) return 1;

elsereturn n * fact(n - 1);

}


Recursion• Toseehowrecursionworks,let’stracetheexecutionofthestatementi = fact(3);

fact(3) findsthat3isnotlessthanorequalto1,soitcalls

fact(2),whichfindsthat2isnotlessthanorequalto1,soitcalls

fact(1),whichfindsthat1islessthanorequalto1,soitreturns1,causing

fact(2) toreturn2× 1=2,causing

fact(3) toreturn3× 2=6.


Recursion• Thefollowingrecursivefunctioncomputesxn,usingtheformulaxn =x × xn–1.int power(int x, int n){if (n == 0)return 1;

elsereturn x * power(x, n - 1);

}


Recursion• Wecancondensethepower functionbyputtingaconditionalexpressioninthereturn statement:int power(int x, int n){return n == 0 ? 1 : x * power(x, n - 1);

}

• Bothfact andpower arecarefultotesta“terminationcondition”assoonasthey’recalled.

• Allrecursivefunctionsneedsomekindofterminationconditioninordertopreventinfiniterecursion.


TheQuicksortAlgorithm• Recursionismosthelpfulforsophisticatedalgorithmsthatrequireafunctiontocallitselftwoormoretimes.

• Recursionoftenarisesasaresultofanalgorithmdesigntechniqueknownasdivide-and-conquer, inwhichalargeproblemisdividedintosmallerpiecesthatarethentackledbythesamealgorithm.


TheQuicksortAlgorithm• Aclassicexampleofdivide-and-conquercanbefoundinthepopularQuicksort algorithm.

• Assumethatthearraytobesortedisindexedfrom1ton.Quicksortalgorithm

1.Chooseanarrayelemente (the“partitioningelement”),thenrearrangethearraysothatelements1,…,i – 1arelessthanorequaltoe,elementi containse,andelementsi +1,…,n aregreaterthanorequaltoe.

2.Sortelements1,…,i – 1byusingQuicksortrecursively.3.Sortelementsi +1,…,n byusingQuicksortrecursively.


TheQuicksortAlgorithm• Step1oftheQuicksortalgorithmisobviouslycritical.

• Therearevariousmethodstopartitionanarray.

• We’lluseatechniquethat’seasytounderstandbutnotparticularlyefficient.

• Thealgorithmreliesontwo“markers”namedlow andhigh,whichkeeptrackofpositionswithinthearray.


TheQuicksortAlgorithm• Initially,low pointstothefirstelement;high pointstothelast.

• Wecopythefirstelement(thepartitioningelement)intoatemporarylocation,leavinga“hole”inthearray.

• Next,wemovehigh acrossthearrayfromrighttoleftuntilitpointstoanelementthat’ssmallerthanthepartitioningelement.

• Wethencopytheelementintotheholethatlow pointsto,whichcreatesanewhole(pointedtobyhigh).

• Wenowmovelow fromlefttoright,lookingforanelementthat’slargerthanthepartitioningelement.Whenwefindone,wecopyitintotheholethathigh pointsto.

• Theprocessrepeatsuntillow andhighmeetatahole.

• Finally,wecopythepartitioningelementintothehole.


TheQuicksortAlgorithm• Exampleofpartitioninganarray:


TheQuicksortAlgorithm• Bythefinalfigure,allelementstotheleftofthepartitioningelementarelessthanorequalto12,andallelementstotherightaregreaterthanorequalto12.• Nowthatthearrayhasbeenpartitioned,wecanuseQuicksortrecursivelytosortthefirstfourelementsofthearray(10,3,6,and7)andthelasttwo(15and18).


Program:Quicksort• Let’sdeveloparecursivefunctionnamedquicksortthatusestheQuicksortalgorithmtosortanarrayofintegers.• Theqsort.c programreads10numbersintoanarray,callsquicksort tosortthearray,thenprintstheelementsinthearray:Enter 10 numbers to be sorted: 9 16 47 82 4 66 12 3 25 51In sorted order: 3 4 9 12 16 25 47 51 66 82

• Thecodeforpartitioningthearrayisinaseparatefunctionnamedsplit.


qsort.c/* Sorts an array of integers using Quicksort algorithm */

#include <stdio.h>

#define N 10

void quicksort(int a[], int low, int high);int split(int a[], int low, int high);

int main(void){int a[N], i;

printf("Enter %d numbers to be sorted: ", N);for (i = 0; i < N; i++)scanf("%d", &a[i]);

quicksort(a, 0, N - 1);

printf("In sorted order: ");for (i = 0; i < N; i++)printf("%d ", a[i]);

printf("\n");return 0;

}


void quicksort(int a[], int low, int high){int middle;

if (low >= high) return;middle = split(a, low, high);quicksort(a, low, middle - 1);quicksort(a, middle + 1, high);

}


int split(int a[], int low, int high){int part_element = a[low];

for (;;) {while (low < high && part_element <= a[high])high--;

if (low >= high) break;a[low++] = a[high];

while (low < high && a[low] <= part_element)low++;

if (low >= high) break;a[high--] = a[low];

}

a[high] = part_element;return high;

}


Program:Quicksort• Waystoimprovetheprogram’sperformance:• Improvethepartitioningalgorithm.• Useadifferentmethodtosortsmallarrays.• MakeQuicksort nonrecursive.
