e.Whentheloopterminates,controlflowsofftheendoftheprocedurewithoutproducinganothervalue.
3.StringScanning
Forcomplicatedoperations,thebookkeepinginvolvedinkeepingtrackofpositionsinstringsbecomesburdensomeanderrorprone.Iconhasastringscanningfacilitythatismanagespositionsautomatically.Attentionisfocusedonacurrentpositioninastringasitisexaminedbyasequenceofoperations.
Thestringscanningoperationhastheform
s?expr
wheresisthesubjectstringtobeexaminedandexprisanexpressionthatperformstheexamination.Apositioninthesubject,whichstartsat1,isthefocusofexamination.
Matchingfunctionschangethisposition.Onematchingfunction,move(i),movesthepositionbyiandproducesthesubstringofthesubjectbetweenthepreviousandnewpositions.Ifthepositioncannotbemovedbythespecifiedamount(becausethesubjectisnotlongenough),move(i)fails.Asimpleexampleis
line?whilewrite(move(2))
whichwritessuccessivetwo-charactersubstringsofline,stoppingwhentherearenomorecharacters.
Anothermatchingfunctionistab(i),whichsetsthepositioninthesubjecttoiandalsoreturnsthesubstringofthesubjectbetweenthepreviousandnewpositions.Forexample,
line?iftab(10)thenwrite(tab(0))
firstsetsthepositioninthesubjectto10andthentotheendofthesubject,writingtheremainingcharacters.Notethatnovalueiswrittenifthesubjectisnotlongenough.
Stringanalysisfunctionssuchasfind()canbeusedinstringscanning.Inthiscontext,thestringthattheyoperateonisnotspecifiedandistakentobethesubject.Forexample,
line?whilewrite(tab(find(\"or\")))
domove(2)
writesallthesubstringsoflinepriortooccurrencesof\"or\".Notethatfind()producesaposition,whichisthenusedbytab()tochangethepositionandproducethedesiredsubstring.Themove(2)skipsthe\"or\"thatisfound.
Anotherexampleoftheuseofstringanalysisfunctionsinscanningis
line?whiletab(upto(&letters))do
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