ComputerProgram1IntroductionComputerProgram,setofinstructionsthatdirectsacomputertoperformsomeprocessingfunctionorcombinationoffunctions.Fortheinstructionstobecarriedout,acomputermustexecuteaprogram,thatis,thecomputerreadstheprogram,andthenfollowthestepsencodedintheprograminapreciseorderuntilcompletion.Aprogramcanbeexecutedmanydifferenttimes,witheachexecutionyieldingapotentiallydifferentresultdependingupontheoptionsanddatathattheusergivesthecomputer.Programsfallintotwomajorclasses:applicationprogramsandoperatingsystems.Anapplicationprogramisonethatcarriesoutsomefunctiondirectlyforauser,suchaswordprocessingorgame-playing.Anoperatingsystemisaprogramthatmanagesthecomputerandthevariousresourcesanddevicesconnectedtoit,suchasRAM,harddrives,monitors,keyboards,printers,andmodems,sothattheymaybeusedbyotherprograms.ExamplesofoperatingsystemsareDOS,Windows95,OS2,andUNIX.2ProgramDevelopmentSoftwaredesignerscreatenewprogramsbyusingspecialapplicationsprograms,oftencalledutilityprogramsordevelopmentprograms.Aprogrammerusesanothertypeofprogramcalledatexteditortowritethenewprograminaspecialnotationcalledaprogramminglanguage.Withthetexteditor,theprogrammercreatesatextfile,whichisanorderedlistofinstructions,alsocalledtheprogramsourcefile.Theindividualinstructionsthatmakeuptheprogramsourcefilearecalledsourcecode.Atthispoint,aspecialapplicationsprogramtranslatesthesourcecodeintomachinelanguage,orobjectcodeaformatthattheoperatingsystemwillrecognizeasaproperprogramandbeabletoexecute.Threetypesofapplicationsprogramstranslatefromsourcecodetoobjectcode:compilers,interpreters,andassemblers.Thethreeoperatedifferentlyandondifferenttypesofprogramminglanguages,buttheyservethesamepurposeoftranslatingfromaprogramminglanguageintomachinelanguage.Acompilertranslatestextfileswritteninahigh-levelprogramminglanguage-suchasFORTRAN,C,orPascalfromthesourcecodetotheobjectcodeallatonce.ThisdiffersfromtheapproachtakenbyinterpretedlanguagessuchasBASIC,APLandLISP,inwhichaprogramistranslatedintoobjectcodestatementbystatementaseachinstructionisexecuted.Theadvantagetointerpretedlanguagesisthattheycanbeginexecutingtheprogramimmediatelyinsteadofhavingtowaitforallofthesourcecodetobecompiled.Changescanalsobemadetotheprogramfairlyquicklywithouthavingtowaitforittobecompiledagain.Thedisadvantageofinterpretedlanguagesisthattheyareslowtoexecute,sincetheentireprogrammustbetranslatedoneinstructionatatime,eachtimetheprogramisrun.Ontheotherhand,compiledlanguagesarecompiledonlyonceandthuscanbeexecutedbythecomputermuchmorequicklythaninterpretedlanguages.Forthisreason,compiledlanguagesaremorecommonandarealmostalwaysusedinprofessionalandscientificapplications.Anothertypeoftranslatoristheassembler,whichisusedforprogramsorpartsofprogramswritteninassemblylanguage.Assemblylanguageisanotherprogramminglanguage,butitismuchmoresimilartomachinelanguagethanothertypesofhigh-levellanguages.Inassemblylanguage,asinglestatementcanusuallybetranslatedintoasingleinstructionofmachinelanguage.Today,assemblylanguageisrarelyusedtowriteanentireprogram,butisinsteadmostoftenusedwhentheprogrammerneedstodirectlycontrolsomeaspectofthecomputersfunction.Programsareoftenwrittenasasetofsmallerpieces,witheachpiecerepresentingsomeaspectoftheoverallapplicationprogram.Aftereachpiecehasbeencompiledseparately,aprogramcalledalinkercombinesallofthetranslatedpiecesintoasingleexecutableprogram.Programsseldomworkcorrectlythefirsttime,soaprogramcalledadebuggerisoftenusedtohelpfindproblemscalledbugs.Debuggingprogramsusuallydetectaneventintheexecutingprogramandpointtheprogrammerbacktotheoriginoftheeventintheprogramcode.Recentprogrammingsystems,suchasJava,useacombinationofapproachestocreateandexecuteprograms.AcompilertakesaJavasourceprogramandtranslatesitintoanintermediateform.SuchintermediateprogramsarethentransferredovertheInternetintocomputerswhereaninterpreterprogramthenexecutestheintermediateformasanapplicationprogram.3ProgramElementsMostprogramsarebuiltfromjustafewkindsofstepsthatarerepeatedmanytimesindifferentcontextsandindif