loop statement

Languageloop

InClanguage,therearethreetypesofloopstatements:forstatement,whilestatementanddoWhilestatement.Therespectiveintroductionsareasfollows:

for

• Forisacurrenttypeloopstatement,whichwellreflectsthethreeproblemsthatshouldbepaidattentiontointhecorrectexpressionofloopstructure:

⑴Initializationofcontrolvariables.

⑵Circulationconditions.

(3)Theupdateofloopcontrolvariables.

• forexpression

Expression1:Generallyanassignmentexpression,whichassignsaninitialvaluetothecontrolvariable;

Expression2:Relationalexpressionorlogicalexpression,loopcontrolcondition;

Expression3:Generallyanassignmentexpression,incrementordecrementthecontrolvariable.

Statement:loopbody,whentherearemultiplesentences,compoundsentencesmustbeused.

• Theformatoftheforstatementis:

For(expression1;expression2;expression3)statement:/*Loopbody*/

• Theexecutionprocessoftheforstatementisasfollows:

①Firstassignaninitialvaluetoexpression1;

②Judgingwhetherexpression2meetsthegivencondition,ifitsvalueisnot0,andtheloopconditionismet,thestatementintheloopisexecuted,thenexpression3isexecuted,andthesecondloopisentered,andthenexpression2isjudged...;Otherwise,itisjudgedthatthevalueofexpression2is0,andtheconditionisnotmet,theforloopisterminated,andthestatementaftertheloopbodyisexecuted.ThesyntaxflowofforstatementisshowninFigure1below:

• Examplequestion:

#includeintmain(){intn,nu;Nu=0;for(n=1;n<=200;n++)nu+=n;printf("nu=%d\n",nu);return0;}

CompileandexecuteAfterthat,thescreendisplays:

nu=20100

Intheprogram,thethreeexpressionsintheparenthesesoftheforstatementare:n=1;n<=200;n++.Expression1,n=1istoassignaninitialvalueton,andexpression2isarelationalexpression.Whennislessthanorequalto200,theexpressionsarealltrue,andthestatementnu+=nintheloopbodyisexecuted;(ienu=nu+n;),andthenexecuteexpression3(n++)toenterthenextroundofloop;ifnisgreaterthan200,expression2isfalse,terminatetheloop,executetheprintf()statement,andprintoutonthescreen:nu=20100.

• Theusageofseveralspecialforstatements:

Thethreeexpressionsintheforbracketscanbeomitted,andthemiddleisomitted.Theexpressionmeansunconditionalloop

while

►whilestructureloopiswhentypeloop,whichisgenerallyusedwhenthenumberofloopsisnotknown.Whatmaintainstheloopisaconditionalexpression,theloopbodyisexecutediftheconditionissatisfied,andtheloopisexitediftheconditionisnotsatisfied.

• Theformatofthewhilestatementis:

►while(conditionalexpression)

►Loopbody

►Theconditionalexpressionmustbejudgedeverytimebeforetheloopbodyisexecuted.

Theexpressionhereistheconditionofwhethertheloopcancontinue,andthestatementisthebodyoftheloop.Aslongastheexpressionistrue,thestatementinsidetheloopisexecuted.Otherwise,theloopisterminatedandthestatementoutsidetheloopisexecuted.

• Examplequestion:f7_2.c

#ncludeintmain(){intu,v,temp;puts("pleaseEntertwonon-negativeintegers:");scanf("%d%d",&u,&v);while(v!=0)temp=u%v;/*Loopinsidesentence*/u=v;/*In-loopstatement*/v=temp;/*In-loopstatement*/Printf("Thegreatestcommondivisoris:%d\n",u);return0;}

Compileandexecutetheresult,thescreendisplays:

Pleaseentertwonon-negativeintegers:

Ifyouenterfromthekeyboard:15035andpressEnter,thescreendisplays:

Thegreatestcommondivisoris:5

Intheprogram,therearethreestatementsinthewhileloop,whichshouldbeenclosedinbracestoindicateablockofstatements.Whentheexpressionv!=0istrue,theprogramloopsthroughthethreestatementsintheloop,untilv!=0isfalse,terminatetheloop,executetheprintf()statement,anddisplaythegreatestcommondivisoronthescreen.

dowhile

►Thestructureofthedo...whilestatementisanuntiltypeloop,whichisalsousedwhenthenumberofloopsisunknown.Thedifferencebetweendo...whileandwhileisthatthedo...whilestructureisexecutedoncetheloopbodyisexecutedandthentheconditionisjudged.

• Theformatofthedowhilestatementis:

►do

►Loopbody

►while(conditionalexpression);

►Eachtimetheloopbodyisexecuted,thedo...whilestructuremustjudgetheconditionalexpression.

Aftertheprogramentersthedowhileloop,executethestatementintheloopfirst,andthendeterminethetrueorfalseoftheexpression,ifitistrue,proceedtothenextloop,otherwiseitwillterminatetheloopifitisfalse.Thecharacteristicoftheloopstatementisthatthestatementintheloopbodyisexecutedoncewhentheexpressionisfalse.

• Thesyntaxflowofthedowhilestatementisshowninthefollowingfigure:

Ingeneral,theforandwhileloopsareatthetopTheuppertestloopterminationcondition,andthedowhileloopisafter,thetestisperformedatthebottom,sotheloopbodymustbeexecutedatleastonce.

Usingthecharacteristicsofthedowhileloop,youcanwriteamenuprogram.

• Examplequestion:

#includeintmain(){intch;printf("1.Inputrecord\n");Printf("2.deleterecord\n");printf("3.displayrecord\n");printf("pleaseselect:\n");doch=getchar();case{switch(ch)'1':printf("enterrecord\n");break;case'2':printf("deleterecords\n");break;case'3':printf("displayrecord\n");break;default:printf("Wrongselection!\n");}while(ch!='1'&&ch!='2'&&ch!='3');}

Thedifferencebetweenwhileanddowhile

►Assumethattheconditionalexpressionisnotestablishedatthebeginning.

►Theloopbodyofthewhilestructurewillnotrunagain.

►Theloopbodyofthedo...whilestructurerunsagain.

Theexecutionprocessofthisprogramistoreceivetheuser'schoicethroughthefirststatementch=getchar()inthedowhileloop,andthesecondstatementswitchtodeterminewhethertheuserinputis'1','2','3',ifitisnot,itwilldisplay"Wrongselection!".Theprogramtakesadvantageofthecharacteristicsofthedowhilestatement,firstexecutesthestatementintheloopbodyonce,andthenusestheexpressionch!='1'&&ch!='2'&&ch!='3'tocyclethroughthejudgmentoftheuserKeyboardinput,aslongasitisnot'1','2',or'3',theprogramwillalwaysloophere.

Thisprogramexampleisjusttoillustratetheusageofthedowhilestatement.Intheactualapplicationofthemenuprogram,youmustwritethecorrespondingfunctiontobecalledafterthecolonofeachcasestatement.

Loopstatement

Theloopstatementistoexecutethesamestatementrepeatedlyaccordingtotheloopconditionuntiltheloopconditionisnotestablished.

Basic

ThefollowingloopstatementscanbeusedinVisualBasic\VBScript\VBA:

1. Do...Loop:When(orLoopuntil)theconditionisTrue.

2. While...Wend:LoopwhentheconditionisTrue.

3. ..Next:Specifythenumberofloopsandusethecountertorunthestatementrepeatedly.

ForEach...Next:Repeatasetofstatementsforeachiteminthecollectionoreachelementinthearray.

Do

YoucanusetheDo...Loopstatementtorunthestatementblockmultipletimes(indefinitenumberoftimes).WhentheconditionisTrueorbeforetheconditionbecomesTrue,thestatementblockisexecutedrepeatedly.

RepeatthestatementwhentheconditionisTrue

TheWhilekeywordisusedtochecktheconditionintheDo...Loopstatement.Therearetwowaystocheckthecondition:checktheconditionbeforeenteringtheloop(asintheChkFirstWhileexamplebelow);orchecktheconditionaftertheloophasrunatleastonce(asintheChkLastWhileexamplebelow).IntheChkFirstWhileprocess,iftheinitialvalueofmyNumissetto9insteadof20,thestatementintheloopbodywillneverbeexecuted.IntheChkLastWhileprocess,thestatementintheloopbodywillonlybeexecutedonce,becausetheconditionisalreadyFalsewhenchecked.

SubChkFirstWhile()

Dimcounter,myNum

counter=0

myNum=20

DoWhilemyNum>10

myNum=myNum-1

counter=counter+1

Loop

MsgBoxlooprepeated&counter&times.

EndSub

SubChkLastWhile()

Dimcounter,myNum

counter=0

myNum=9

Do

myNum=myNum-1

counter=counter+1

LoopWhilemyNum>10

MsgBoxlooprepeated&counter&times.

EndSub

ExecutethestatementrepeatedlyuntiltheconditionbecomesTrue

TheUntilkeywordisusedtochecktheconditionintheDo...Loopstatement.Therearetwowaystocheckthecondition:checktheconditionbeforeenteringtheloop(asintheChkFirstUntilexamplebelow);orchecktheconditionaftertheloophasrunatleastonce(asintheChkLastUntilexamplebelow).AslongastheconditionisFalse,theloopwillbeperformed.

SubChkFirstUntil()

Dimcounter,myNum

counter=0

myNum=20

DoUntilmyNum=10

myNum=myNum-1

counter=counter+1

Loop

MsgBoxlooprepeats&counter&times.

EndSub

SubChkLastUntil()

Dimcounter,myNum

counter=0

myNum=1

Do

myNum=myNum+1

counter=counter+1

LoopUntilmyNum=10

MsgBoxlooprepeated&counter&times.

EndSub

Exittheloop

ExitDostatementisusedtoexittheDo...Looploop.Becauseitisusuallyonlynecessarytoexittheloopinsomespecialcases(forexample,toavoidinfiniteloops),youcanusetheExitDostatementintheTrueblockoftheIf...Then...Elsestatement.IftheconditionisFalse,theloopwillrunasusual.

Inthefollowingexample,theinitialvalueofmyNumwillcauseanendlessloop.TheIf...Then...Elsestatementchecksthisconditiontopreventanendlessloop.

SubExitExample()

Dimcounter,myNum

counter=0

myNum=9

DoUntilmyNum=10

myNum=myNum-1

counter=counter+1

IfmyNum<10ThenExitDo

Loop

MsgBoxlooprepeated&counter&times.

EndSub

While..Wend

1. ..Wendstatementisprovidedforthosewhoarefamiliarwithitsusage.However,duetothelackofflexibilityofWhile...Wend,itisrecommendedtousetheDo...Loopstatement.

Next

1. ..Nextstatementisusedtorunthestatementblockaspecifiednumberoftimes.Thecountervariableisusedintheloop,andthevalueofthevariableincreasesordecreaseswitheachloop.

Forexample,thefollowingexamplerepeatstheprocedureMyProc50times.TheForstatementspecifiesthecountervariablexanditsstartvalueandendvalue.TheNextstatementincrementsthecountervariablebyoneeachtime.

SubDoMyProc50Times()

DimxForx=1To50

MyProc

Next

EndSub

ThekeywordStepisusedtospecifythevalueofeachincreaseordecreaseofthecountervariable.Intheexamplebelow,thecountervariablejisincrementedby2eachtime.Aftertheloopisover,thevalueoftotalisthesumof2,4,6,8,and10.

SubTwosTotal()

Dimj,total

Forj=2To10Step2

total=total+j

Next

ThesumofMsgBoxis&total&;.

EndSub

Todecrementthecountervariable,setSteptoanegativevalue.Atthistime,theendvalueofthecountervariablemustbelessthanthestartvalue.Inthefollowingexample,thecountervariablemyNumisdecrementedby2eachtime.Aftertheloopends,thevalueoftotalisthesumof16,14,12,10,8,6,4,and2.

SubNewTotal()

DimmyNum,total

FormyNum=16To2Step-2

total=total+myNum

Next

ThesumofMsgBoxis&total&;.

EndSub

TheExitForstatementisusedtoexittheFor...Nextstatementbeforethecounterreachesitsendvalue.Becauseitisusuallyonlynecessarytoexittheloopinsomespecialcircumstances(forexample,whenanerroroccurs),youcanusetheExitForstatementintheTrueblockoftheIf...Then...Elsestatement.IftheconditionisFalse,theloopwillrunasusual.

UsingForEach...Next

ForEach...NextloopissimilartoFor...Nextloop.ForEach...Nextdoesnotrunthestatementaspecifiednumberoftimes,butrepeatsasetofstatementsforeachelementinthearrayoreachiteminthecollectionofobjects.Thisisveryusefulwhenyoudon'tknowthenumberofelementsintheset.

Inthefollowingexample,thecontentoftheDictionaryobjectisusedtoplacetextinmultipletextboxes:

;formsandelements

EasyLanguage

Easylanguageloopstatementsaredividedintothreetypesofcountingloops,judgmentloopsandloopjudgments

Countingloops

Thecommandsinthelooparerepeatedlyexecutedforthespecifiednumberoftimes.Theintegerparametervalueprovideddetermineshowmanytimestheexecutionwillberepeated.

Countingcyclefirst(cycletimes,cycletimesvariable)

Countingcycleend()

Judgingcycle

AccordingtoThevalueoftheprovidedlogicparameterdetermineswhethertoentertheloop.Ifthevalueoftheprovidedlogicparameteristrue,theprogramexecutesthenextcommandinsequenceandenterstheloop;otherwise,itjumpstothenextcommandofthe"JudgingLoopEnd"commandcorrespondingtothiscommandtojumpoutoftheloop.

Judgingthebeginningoftheloop(condition)

Judgingtheendoftheloop()

Judgingtheloop

Theendoftheloopjudgmentistheconditionoftheloop,Accordingtothevalueoftheprovidedlogicparameter,itisdecidedwhethertoreturntotheloopheadertocontinuetheloop.Iftheprovidedlogicparametervalueistrue,theprogramreturnstothecorresponding"loopjudgmentfirst"commandtocontinuetheloop,otherwise,executethenextcommandinordertoexittheprogramloop.

Firstofloopjudgment()

Endofloopjudgment(condition)