eMacintosh,MS-DOS,manyUNIXsystems,andVAX\/VMS.TheremainderofthisreportbrieflydescribesthehighlightsofIcon.Foracompletedescription,seeReferences1and2.
2.ExpressionEvaluation
2.1ConditionalExpressions
InIconthereareconditionalexpressionsthatmaysucceedandproducearesult,ormayfailandnotproduceanyresult.Anexampleisthecomparisonoperation
i>j
whichsucceeds(andproducesthevalueofj)providedthatthevalueofiisgreaterthanthevalueofj,butfailsotherwise.Similarly,
i>j>k
succeedsifthevalueofjisbetweeniandk.
【𝗼𝗼𝗽.𝘁𝘄 無錯章節】
ThesuccessorfailureofconditionaloperationsisusedinsteadofBooleanvaluestodrivecontrolstructuresinIcon.Anexampleis
ifi>jthenk:=ielsek:=j
whichassignsthevalueofitokifthevalueofiisgreaterthanthevalueofj,butassignsthevalueofjtokotherwise.
Theusefulnessoftheconceptsofsuccessandfailureisillustratedbyfind(s1,s2),whichfailsifs1doesnotoccurasasubstringofs2.Thus
ifi:=find(\"or\",line)thenwrite(i)
writesthepositionatwhich\"or\"occursinline,ifitoccurs,butdoesnotwriteavalueifitdoesnotoccur.
ManyexpressionsinIconareconditional.Anexampleisread(),whichproducesthenextlinefromtheinputfile,butfailswhentheendofthefileisreached.ThefollowingexpressionistypicalofprogramminginIconandillustratestheintegrationofconditionalexpressionsandconventionalcontrolstructures:
whileline:=read()do
write(line)
Thisexpressioncopiestheinputfiletotheoutputfile.
Ifanargumentofafunctionfails,thefunctionisnotcalled,andthefunctioncallfailsaswell.This\"inheritance\"offailureallowstheconciseformulationofmanyprogrammingtasks.Omittingtheoptionaldoclauseinwhile-do,thepreviousexpressioncanberewrittenas
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