TheABCsofAutoLISPbyGeorgeOmura

1Copyright2001GeorgeOmura,,Worldrightsreserved

IntroductionOneofAutoCAD'sgreatestassetsisitsadaptability.YoucancontroljustabouteveryaspectofAutoCAD'soperationsfromtheappearanceofitsdrawingeditortoitsvarietyofmenus.Akeyelementofthisadaptabilityisitsbuiltinprogramminglanguage,AutoLISP.WithAutoLISP,youcanvirtuallywriteyourowncommandsorredefineothers.

YoucanthinkofAutoLISPasaverysophisticatedmacrobuildingfacility.(Simplemacrosarelikescriptsthatautomaterepetitivekeystrokes.)Youdon'tneedtobeaprogrammertouseAutoLISP.Infact,AutoLISPisdesignedsothateverydayusersofAutoCADcanstarttouseitafteraminimumoftraining.ThisbookmakesAutoLISPaccessibletoAutoCADuserswhoarelookingforawaytoenhanceandextendtheiruseofAutoCAD.

TheABCsofAutoLISPbyGeorgeOmuraW

hoshouldreadthisbookThisbookintroducesnonprogrammerstotheuseofAutoLISP.IfyouareanintermediatelevelAutoCADuser,interestedinlearningaboutthispowerfultool,thenthisisthebookforyou.IfyouarejustbeginningtolearnAutoCAD,thenyoushouldprobablybecomeabitmorefamiliarwithAutoCADbeforeattemptingtolearnAutoLISP.ThisbookassumesthatyouhaveatleastanintermediatelevelofexpertisewithAutoCADandareacquaintedwithsimpleWindowsoperations.

HowThisBookIsOrganizedThebookisdividedinto11chapters.ThefirstthreechaptersgiveyouanintroductiontoprogramminginAutoLISP.TheChapter1introducesyoutoAutoLISPbyshowingyouhowtouseitdirectlyfromtheAutoCADcommandprompt.TheChapter2showsyouhowtocreateandsaveprogramsinafile.Chapter3discusseswaysoforganizingyourprogrammingprojectsandhowtomanageyourcomputers'memory.

ThenextfourchaptersshowyouhowtouseAutoLISPtodoavarietyofeditingtasks.Chapter4discussesthefunctionsthatallowyoutoasktheuserforinput.Chapter5explainshowtobuilddecisionmakingcapabilitiesintoyourprograms.Chapter6showsyouhowtodealwithgeometricproblemsusingAutoCAD.Chapter7discussesthemanipulationoftext.

ThelastfourchaptersshowyouhowAutoCADandAutoLISPinteract.InChapter8,youwillseehowyoucancontrolmanyfacetsofAutoCADthroughAutoLISP.Chapter9delvesintolists,afundamentalcomponentofallAutoLISPprograms.Chapter10showsyouwaysofmodifyingAutoCADobjectsbydirectlyaccessingtheAutoCADdrawingdatabase.Andfinally,Chapter11looksatwaystodigdeeperintothedrawingdatabasetogetinformationoncomplexdrawingobjectslikepolylinesandblockattributes.

Inaddition,fiveappendicesareincludedasreferencematerialtothebook.Intheoriginalversionofthisbook,theseappendicescontainedtheresourcesindicatedbytheirtitle.Inthiselectronicversion,theseappendicesofferdirectionsonhowtofindinformationintheAutoCADhelpsystem.ThefirstthreeshowyouhowtofindinformationontheAutoCADmenustructure,AutoLISPerrormessages,andAutoCADgroupcodes.ThefourthappendixdescribeshowtofindinformatononthestandardAutoCADdimensionvariablesandsystemvariables.ThefifthappendixdescribeshowtofindinformatonontheTablegroupcodes.

2Copyright2001GeorgeOmura,,Worldrightsreserved

TheABCsofAutoLISPbyGeorgeOmura

HowtoUseThisBookEachchapteroffersexercisesandsampleprogramsthatdemonstratesomegeneralconceptregardingAutoLISP.Throughtheseexercises,thebookshowsyouhowprogramsdevelopfromideasintofinished,runningprograms.Also,theinformationyoulearninonechapterwillbuildonwhatyoulearnedinthepreviouschapter.Thisway,yourknowledgeofAutoLISPwillbeintegratedandcohesive,ratherthanfragmented.Forthisreason,thebestwaytousethisbookistoreadeachchapterinorderanddoalloftheexercises.Sincethetopicsareorientedtowardaccomplishingtasksratherthansimplyfocusingonindividualfunctions,youwillhaveagoodgraspofhowtouseAutoLISPinrealworldsituationsbytheendofthisbook.

WheretoFindtheLISPProgramsAsyoureadthechaptersanddotheexercise,youwillbeaskedtoenterprogramcodeintoafile.Ifyouareinahurry,youcancutandpastethecodedirectlyfromthechapteryouarereading.Thiswillsaveagooddealoftime,butmakesureyoustudythecodethatyoucutandpaste.Thisbookwasoriginallypublishedin1990bySybexInc.IthasbeenreproducedhereinanelectronicformatbytheAuthorforthebenefitofMasteringAutoCADreaderseverywhere.Enjoy....

Copyright2001GeorgeOmura,,Worldrightsreserved.Nopartofthispublicationmaybestoredinaretievalsystem,transmitted,orreproducedinanyway,includingbutnotlimitedtophotocopy,photograph,magneticorotherrecord,withouttheprioragreementandwrittenpermissionoftheauthor.

3Copyright2001GeorgeOmura,,Worldrightsreserved

TheABCsofAutoLISPbyGeorgeOmura

4Copyright2001GeorgeOmura,,Worldrightsreserved

TheABCsofAutoLISPbyGeorgeOmuraChapter1:IntroducingAutoLISPFeaturing

UnderstandingtheAutoLISPInterpreterandEvaluation

ExpressionsandArguments

VariablesandDataTypes

ManipulatingListswithFunctions

GetFunctions

Ifyouhaveneverprogrammedacomputerbefore,youmaythinkthatlearningAutoLISPwillbedifficult.Actually,whenyouuseaprogramsuchasAutoCAD,youare,inasense,programmingyourcomputertocreateandmanipulateadatabase.AsyoubecomemorefamiliarwithAutoCAD,youmaybegintoexplorethecreationoflinetypesandhatchpatterns,forexample.Oryoumaycustomizeyourmenutoincludeyourownspecializedfunctionsandmacros.(Macrosarelikescriptsthatthecomputerfollowstoperformapredeterminedsequenceofcommands.)Atthislevel,youaredelvingdeeperintotheworkingsofAutoCADandatthesametimeprogrammingyourcomputerinamoretraditionalsense.

UsingAutoLISPisreallyjustextendingyourknowledgeanduseofAutoCAD.Infact,onceyoulearnthebasicsyntaxofAutoLISP,youneedonlytofamiliarizeyourselfwithAutoLISP'sbuiltinfunctionstostartwritingusefulprograms.(AutoLISP'ssyntaxisthestandardorderofelementsinitsexpressions.)YoumightlookatAutoLISPfunctionsasanextensiontoAutoCAD'slibraryofcommands.Themorefunctionsyouarefamiliarwith,thebetterequippedyouareforusingtheprogrameffectively.

AutoLISPcloselyresemblesCommonLISP,themostrecentversionoftheoldestartificialintelligenceprogramminglanguagestillinusetoday.AutoLISPisessentiallyapareddownversionofCommonLISPwithsomeadditionalfeaturesuniquetoAutoCAD.ManyconsiderLISPtobeoneoftheeasiestprogramminglanguagestolearn,partlybecauseofitssimplesyntax.SinceAutoLISPisasubsetofcommonLISP,itisthatmucheasiertolearn.

In this chapter, you will become familiar with some of the basic elements of AutoLISP by using AutoLISP directly from the AutoCAD command prompt to perform a few simple operations. While doing this, you will be introduced to someoftheconceptsyouwillneedtoknowtodevelopyourownAutoLISPapplications.

UnderstandingtheInterpreterandEvaluationAutoLISPisaccessedthroughtheAutoLISPinterpreter.WhenyouenterdataattheAutoCADcommandprompt,theinterpreterfirstreadsittodetermineifthedataisanAutoLISPformula.IfthedataturnsouttobeintendedforAutoLISP,thenAutoLISPevaluatesit,andreturnsananswertothescreen.Thisprocessofreadingthecommandprompt,evaluatingthedata,thenprintingtothescreen,occurswheneveranythingisenteredatthecommandprompt

5Copyright2001GeorgeOmura,,Worldrightsreserved

TheABCsofAutoLISPbyGeorgeOmura

andisanimportantpartofhowAutoLISPfunctions.

Insomeways,theinterpreterislikeahandheldcalculator.Justaswithacalculator,theinformationyouwishtohaveAutoLISPevaluatemustfollowacertainorder.Forexample,theformula0.618plus1mustbeenteredasfollows:

(+0.6181)

Tryenteringtheaboveformulaatthecommandprompt.AutoLISPevaluatestheformula(+0.6181)andreturnstheanswer,1.618,displayingitonthepromptline.

Thisstructure+0.6181enclosedbyparentheses,iscalledanexpressionanditisthebasicstructureforallAutoLISPprograms.EverythingintendedfortheAutoLISPinterpreter,fromthesimplestexpressiontothemostcomplexprogram,mustbewrittenwiththisstructure.Theresultreturnedfromevaluatinganexpressioniscalledthevalueoftheexpression.

TheComponentsofanExpressionAnAutoLISPexpressionmustincludeanoperatorofsomesortfollowedbytheitemstobeoperatedon.Anoperatorisaninstructiontotakesomespecificactionsuchasaddingtwonumberstogetherordividingonenumberbyanother.Examplesofmathematicaloperatorsincludetheplussign(+)foradditionandforwardslash(/)fordivision.

Wewilloftenrefertotheoperatorasafunctionandtheitemstobeoperatedonastheargumentstothefunctionorsimply,thearguments.So,intheexpression(+0.6181),the+isthefunctionandthe0.618and1arethearguments.AllAutoLISPexpressions,nomatterwhatsize,followthisstructureandareenclosedbyparentheses.

Parenthesesareimportantelementsofanexpression.Allparenthesesmustalsobebalanced,thatis,foreachleftparenthesis,theremustbearightparenthesis.IfyouenteranunbalancedexpressionintotheAutoLISPinterpreter,yougetthefollowingprompt:

((_>

wherethenumberofparenthesestotheleftisthenumberofparenthesesrequiredtocompletetheexpression.Ifyouseethisprompt,youmustenterthenumberofclosingparenthesesindicatedinordertoreturntothecommandprompt.Inthisexample,youwouldneedtoentertworightparenthesestocompletetheexpression.

Double quotation marks enclosing text must also be carefully balanced. If an AutoLISP expression is unbalanced, it can be quite difficult to complete it and exit AutoLISP. Figure 1.1 shows the components of the expression you just entered.

6Copyright2001GeorgeOmura,,Worldrightsreserved

TheABCsofAutoLISPbyGeorgeOmura

7Copyright2001GeorgeOmura,,Worldrightsreserved

Figure1.1:ThepartsofandAutoLISPexpression

Notethatspacesareusedtoseparatethefunctionsandargumentsoftheexpression.Spacesarenotrequiredbetweentheparenthesesandtheelementsoftheexpressionthoughyoucanaddspacestohelpimprovethereadabilityofexpressionswhentheybecomecomplex.However,itisveryimportanttomaintainspacesbetweentheelementsoftheexpression.SpaceshelpbothyouandAutoLISPkeeptrackofwhereoneelementendsandanotherbegins.

UsingArgumentsandExpressionsAutoLISPevaluateseverything,notjustexpressions,buttheargumentsinexpressionsaswell.Thismeansthatintheaboveexample,AutoLISPevaluatesthenumbers0.618and1beforeitappliesthesenumberstotheplusoperator.InAutoLISP,numbersevaluatetothemselves.ThismeansthatwhenAutoLISPevaluatesthenumber0.618,0.618isreturnedunchanged.SinceAutoLISPevaluatesallarguments,expressionscanalsobeusedasargumentstoafunction.

Forexample,enterthefollowingatthecommandprompt:

(/1(+0.6181))

Inthisexample,thedividefunction(/)isgiventwoargumentsnumber1andanexpression(+0.6181).Thistypeofexpressioniscalledacomplexornestedexpressionbecauseoneexpressioniscontainedwithinanother.Soinourexample,AutoLISPfirstevaluatestheargumentsoftheexpression,whicharetheexpression(+0.6181)andthenumber1.Itthenappliestheresultingvalueoftheexpressionandthenumber1tothedividefunctionandreturnstheanswerof0.618047(seefigure1.2).

FunctionTArguments

BalancedParenthesis

TheABCsofAutoLISPbyGeorgeOmura

8Copyright2001GeorgeOmura,,Worldrightsreserved

Figure1.2:Evaluationofanestedexpression

UsingVariablesAnothercalculatorlikecapabilityoftheinterpreterisitsabilitytoremembervalues.Youprobablyhaveacalculatorthathassomememory.Thiscapabilityallowsyoutostorethevalueofanequationforfutureuse.Inasimilarway,youcanstorevaluesusingvariables.

Avariableislikeacontainerthatholdsavalue.Thatvaluecanchangeinthecourseofaprogram'soperation.Asimpleanalogytothisisthetitleofagovernmentposition.Thepositionofpresidentcouldbethoughtofasavariable.Thisvariablecanbeassignedavalue,suchasRonaldReaganorBillClinton.

UnderstandingDataTypesVariablescantakeonseveraltypesofvaluesordatatypes.HereiswhatsomeofthesedatatypeslooklikeinAutoLISP.

Function

lII:06181))LflArgumentsareevaluatedU11.618)I*IThenappliedtuthefunction

TheABCsofAutoLISPbyGeorgeOmura

DATATYPEEXAMPLE

Integer24

RealNumber0.618

String``20Feet6Inches''

List(4.50216.30110.0)

FileDescriptor

ObjectName

SelectionSet

SymbolsPoint1

SubrsSetq

Byseparatingdataintotypes,theinterpreterisbetterabletodeterminepreciselyhowtoevaluatethedataandkeepprogramsrunningquickly.Also,acomputerstoresdifferenttypesofdatadifferently,andsodatatypeshelpAutoLISPtomanageitsmemorymoreefficiently.Finally,datatypeshelpkeepyourprogrammingeffortsclearbyforcingyoutothinkofdataashavingcertaincharacteristics.Thefollowingdescriptionsgiveyouanideaofwhateachofthesedatatypesare.

IntegersandRealNumbersIntegersarewholenumbersfrom32768to+32767.Thevalueofanexpressioncontainingonlyintegersisalwaysaninteger.Forexample,thevalueoftheexpression(/252)is12.Thedecimalvalueisdroppedfromtheresultingvalue.

Realnumbersarenumbersthatincludeadecimalvalue.Ifthesameexpressionaboveiswrittenusingrealnumbers,(/25.02.0),itsvaluewillbeexpressedastherealnumber12.5.Integershaveablackandwhitequalityaboutthem.24willalwaysequal24.Realnumbers(sometimesreferredtoasreals),ontheotherhandcanbeabitlessdefinite.Forexample,tworealvalues,24.001245781and24.001245782arenearlyidenticalbutarenotequal.Ifyouweretodropthelastdecimalplaceinboththesenumbers,thentheywouldbeequalvalues.Thisdefinitivequalityofintegersmakesthemmoresuitedtocertaintypesofuses,likecounting,whilerealnumbersarebettersuitedtosituationsthatrequireexactingvaluessuchascoordinatevaluesandangles.Also,computationsperformedonintegersarefasterthanthoseperformedonreals.

Youmayhavenoticedthatinourpreviousexamples,therealnumber0.618isprecededbyazeroandnotwrittenas.618.InAutoLISP,realnumberswithvaluesbetween1.0and0.0mustbeginwithzero.Ifyoudonotfollowthisrule,youwillgetanerrormessage.Enterthefollowingatthecommandprompt:

(+.6181)

9Copyright2001GeorgeOmura,,Worldrightsreserved

TheABCsofAutoLISPbyGeorgeOmura

Thoughtheaboveexpressionlooksperfectlynormal,thefollowingerrormessageappears:

error:invaliddottedpair

MostbeginnersandevensomeexperiencedAutoLISPusersmightbecompletelybaffledbytheerrormessage.Wewilllookatwhatdottedpairsarelaterinthisbookbutfornow,justkeepinmindthatrealvaluesbetween1.0and0.0mustbeenteredwitha0precedingthedecimalpoint.

StringsThetermstringreferstotext.StringsareoftenusedaspromptsinAutoLISPexpressionsbuttheycanalsobemanipulatedusingAutoLISP.Forexample,usingtheStrcatAutoLISPfunction,youcouldcombinetwostrings,"thirtysevenfeet"and"sixinches",intoonestring"thirtysevenfeetsixinches".Tryenteringthis:

(strcat"thirtysevenfeet""sixinches")

Thefollowingisreturned:

"thirtysevenfeetsixinches"

ListsListsaredataelementsenclosedinparentheses.TheyarethebasicdatastructureinAutoLISP.Alistcanbemadeupofanynumberofintegers,realnumbers,strings,andevenotherlists.

Therearetwotypesoflists.Thoseintendedforevaluationandthoseintendedasrepositoriesfordata.Whenalistcontainsafunctionasitsfirstelement,wecangenerallyassumethatitisanexpressionintendedforevaluation.Suchalistisoftenreferredtoasaform.Anexampleofalistasarepositoryofdataisalistthatrepresentsacoordinatelocation.Forexample,thelist

(1.22.34.4)

containsthreeelements,anX,Y,andZcoordinate.Thefirstelement,1.2,isthexcoordinate,thesecondelement,2.3istheycoordinate,andthethirdelement,4.4,isthezcoordinate.

FileDescriptorsAutoLISPallowsyoutoreadandwritetextfilestodisk.Filedescriptorsareusedinaprogramtoaccessfilesthathavebeenopenedforprocessing.Youmightthinkofafiledescriptorasavariablerepresentingthefileinquestion.WewilldiscussthisdatatypeinmoredetailinChapter7.

ObjectNamesEveryobjectinanAutoCADdrawinghasaname.Thenameisanalphanumericcodeuniquetothatobject.ThisnamecanbeaccessedbyAutoLISPandusedasameansofselectingindividualobjectsforprocessing.ObjectnamesareprovidedbyAutoCADandarenotuserdefinable.AlsoObjectnamescanchangefromonedrawingsessiontoanother.

10Copyright2001GeorgeOmura,,Worldrightsreserved