Fortran TutorialDepartment of Physics.docx

Fortran TutorialDepartment of Physics.docx

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FortranTutorialDepartmentofPhysics

RichardKass

Winter2004

APhysics416FortranTutorial

（basedontheinformationat:

http:

//macams1.bo.infn.it/tutorial/）

1.WhatisFortran?

2.Fortranbasics

3.Variables,declarations,andtypes

4.Expressionsandassignment

5.Logicalexpressions

6.Theifstatements

7.Loops

8.Arrays

9.Subprograms

10.RandomnumbersandMonteCarlosimulations

11.Simpleinputandoutput

12.Formatstatements

13.FileI/O

14.Commonblocks

15.dataandblockdata

16.Debugging

17.RunningFortranonthePhysicsDepartment’sVAX（OHSTPY）computer

18.AsampleFortranprogramforLab1

1.WhatisFortran?

Fortranisageneralpurposeprogramminglanguage,mainlyintendedformathematicalcomputationsinscienceapplications（e.g.physics）.FortranisanacronymforFORmulaTRANslation,andwasoriginallycapitalizedasFORTRAN.However,followingthecurrenttrendtoonlycapitalizethefirstletterinacronyms,wewillcallitFortran.Fortranwasthefirsthigh-levelprogramminglanguage.TheworkonFortranstartedinthe1950'satIBMandtherehavebeenmanyversionssince.Byconvention,aFortranversionisdenotedbythelasttwodigitsoftheyearthestandardwasproposed.ThuswehaveFortran66,Fortran77andFortran90（95）.

ThemostcommonFortranversiontodayisstillFortran77,althoughFortran90isgrowinginpopularity.Fortran95isarevisedversionofFortran90whichisexpectedtobeapprovedbyANSIsoon（1996）.TherearealsoseveralversionsofFortranaimedatparallelcomputers.ThemostimportantoneisHighPerformanceFortran（HPF）,whichisade-factostandard.

UsersshouldbeawarethatmostFortran77compilersallowasupersetofFortran77,i.e.theyallownon-standardextensions.InthistutorialwewillemphasizestandardANSIFortran77.

WhylearnFortran?

Fortranisthedominantprogramminglanguageusedinscientificapplications.Itisthereforeimportantforphysics（orengineering）studentstobeabletoreadandmodifyFortrancode.Fromtimetotime,so-calledexpertspredictthatFortranwillrapidlyfadeinpopularityandsoonbecomeextinct.ThismayactuallyhappenasC（orC++）israpidlygrowinginpopularity.However,previouspredictionsofthedownfallofFortranhavealwaysbeenwrong.Fortranisthemostenduringcomputerprogramminglanguageinhistory.OneofthemainreasonsFortranhassurvivedandwillsurviveissoftwareinertia.Onceacompanyhasspentmanypeople-yearsandperhapsmillionsofdollarsonasoftwareproduct,itisunlikelytotrytotranslatethesoftwaretoadifferentlanguage.Reliablesoftwaretranslationisaverydifficulttaskandthere’s40yearsofFortrancodetoreplace!

Portability

AmajoradvantageFortranhasisthatitisstandardizedbyANSI（AmericanNationalStandardsInstitute）andISO（InternationalStandardsOrganization）.Consequently,ifyourprogramiswritteninANSIFortran77thenitwillrunonanycomputerthathasaFortran77compiler.Thus,Fortranprogramsareportableacrosscomputerplatforms

2.Fortran77Basics

AFortranprogramisjustasequenceoflinesoftext.ThetexthastofollowacertainsyntaxtobeavalidFortranprogram.Westartbylookingatasimpleexamplewherewecalculatetheareaofacircle:

programcircle

realr,area

cThisprogramreadsarealnumberrandprints

ctheareaofacirclewithradiusr.

write（*,*）'Giveradiusr:

'

read（*,*）r

area=3.14159*r*r

write（*,*）'Area=',area

stop

end

Thelinesthatbeginwitha"c"arecommentsandhavenopurposeotherthantomaketheprogrammorereadableforhumans.Originally,allFortranprogramshadtobewritteninallupper-caseletters.Mostpeoplenowwritelower-casesincethisismorelegible.

Programorganization

AFortranprogramgenerallyconsistsofamainprogram（ordriver）andpossiblyseveralsubprograms（orproceduresorsubroutines）.Fornowwewillassumeallthestatementsareinthemainprogram;subprogramswillbetreatedlater.Thestructureofamainprogramis:

programname

declarations

statements

stop

end

Inthistutorial,wordsthatareinitalicsshouldnotbetakenasliteraltext,butratherasagenericdescription.Thestopstatementisoptionalandmayseemsuperfluoussincetheprogramwillstopwhenitreachestheendanywaybutitisrecommendedtoalwaysterminateaprogramwiththestopstatementtoemphasizethattheexecutionflowstopsthere.

Columnpositionrules

Fortran77isnotafree-formatlanguage,buthasaverystrictsetofrulesforhowthesourcecodeshouldbeformatted.Themostimportantrulesarethecolumnpositionrules:

Col.1:

Blank,ora"c"or"*"forcomments

Col.2-5:

Statementlabel（optional）

Col.6:

Continuationofpreviousline（optional）

Col.7-72:

Statements

Col.73-80:

Sequencenumber（optional,rarelyusedtoday）

MostlinesinaFortran77programstartswith6blanksandendsbeforecolumn72,i.e.onlythestatementfieldisused.NotethatFortran90allowsfreeformat.

Comments

Alinethatbeginswiththeletter"c"oranasteriskinthefirstcolumnisacomment.Commentsmayappearanywhereintheprogram.Well-writtencommentsarecrucialtoprogramreadability.CommercialFortrancodesoftencontainabout50%comments.YoumayalsoencounterFortranprogramsthatusetheexclamationmark（!

）forcomments.Thisishighlynon-standardinFortran77,butisallowedinFortran90.Theexclamationmarkmayappearanywhereonaline（exceptinpositions2-6）.

Continuation

Occasionally,astatementdoesnotfitintoonesingleline.Onecanthenbreakthestatementintotwoormorelines,andusethecontinuationmarkinposition6.Example:

c23456789（Thisdemonstratescolumnposition!

）

cThenextstatementgoesovertwophysicallines

area=3.14159265358979

+*r*r

Anycharactercanbeusedinsteadoftheplussignasacontinuationcharacter.Itisconsideredgoodprogrammingstyletouseeithertheplussign,anampersand,ornumbers（2forthesecondline,3forthethird,andsoon）.

Blankspaces

BlankspacesareignoredinFortran77.SoifyouremoveallblanksinaFortran77program,theprogramisstillsyntacticallycorrectbutalmostunreadableforhumans.

3.Variables,types,anddeclarations

Variablenames

VariablenamesinFortranconsistof1-6characterschosenfromthelettersa-zandthedigits0-9.Thefirstcharactermustbealetter.（Note:

Fortran90allowsvariablenamesofarbitrarylength）.Fortran77doesnotdistinguishbetweenupperandlowercase,infact,itassumesallinputisuppercase.However,nearlyallFortran77compilerswillacceptlowercase.IfyoushouldeverencounteraFortran77compilerthatinsistsonuppercaseitisusuallyeasytoconvertthesourcecodetoalluppercase.

Typesanddeclarations

Everyvariableshouldbedefinedinadeclaration.Thisestablishesthetypeofthevariable.Themostcommondeclarationsare:

integerlistofvariables

reallistofvariables

doubleprecisionlistofvariables

complexlistofvariables

logicallistofvariables

characterlistofvariables

Thelistofvariablesshouldconsistofvariablenamesseparatedbycommas.Eachvariableshouldbedeclaredexactlyonce.Ifavariableisundeclared,Fortran77usesasetofimplicitrulestoestablishthetype.Thismeansallvariablesstartingwiththelettersi-nareintegersandallothersarereal.ManyoldFortran77programsusestheseimplicitrules,butyoushouldnot!

Theprobabilityoferrorsinyourprogramgrowsdramaticallyifyoudonotconsistentlydeclareyourvariables.

Integersandfloatingpointvariables

Fortran77hasonlyonetypeforintegervariables.Integersareusuallystoredas32bits（4bytes）variables.Therefore,allintegervariablesshouldtakeonvaluesintherange[-m,m]wheremisapproximately2*10^9.

Fortran77hastwodifferenttypesforfloatingpointvariables,calledrealanddoubleprecision.Whilerealisoftenadequate,somenumericalcalculationsneedveryhighprecisionanddoubleprecisionshouldbeused.Usuallyarealisa4bytevariableandthedoubleprecisionis8bytes,butthisismachinedependent.Somenon-standardFortranversionsusethesyntaxreal*8todenote8bytefloatingpointvariables.

Theparameterstatement

Someconstantsappearmanytimesinaprogram.Itisthenoftendesirabletodefinethemonlyonce,inthebeginningoftheprogram.Thisiswhattheparameterstatementisfor.Italsomakesprogramsmorereadable.Forexample,thecircleareaprogramshouldhavebeenwrittenlikethis:

programcircle

realr,area,pi

parameter（pi=3.14159）

cThisprogramreadsarealnumberrandprints

ctheareaofacirclewithradiusr.

write（*,*）'Giveradiusr:

'

read（*,*）r

area=pi*r*r

write（*,*）'Area=',area

stop

end

Thesyntaxoftheparameterstatementis

parameter（name=constant,...,name=constant）

Therulesfortheparameterstatementare:

⎽The"variable"definedintheparameterstatementisnotavariablebutratheraconstantwhosevaluecanneverchange

⎽A"variable"canappearinatmostoneparameterstatement

⎽Theparameterstatement（s）mustcomebeforethefirstexecutablestatement

Somegoodreasonstousetheparameterstatementare:

⎽ithelpsreducethenumberoftypos

⎽itiseasytochangeaconstantthatappearsmanytimesinaprogram

4.Expressionsandassignment

Constants

Thesimplestformofanexpressionisaconstant.Thereare6typesofconstants,correspondingtothe6datatypes.Herearesomeintegerconstants:

1

0

-100

32767

+15

Thenwehaverealconstants:

1.0

-0.25

2.0E6

3.333E-1

TheE-notationmeansthatyoushouldmultiplytheconstantby10raisedto