Ch2A more elaborate example.docx

Ch2A more elaborate example.docx

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Ch2Amoreelaborateexample

Chapter2,Amoreelaborateexample

ToillustrateseveralofthefundamentalpointsofPikewewillnowintroduceanexampleprogram,thatwillbeextendedaswego.Wewillbuildadatabaseprogramthatkeepstrackofarecordcollectionandthesongsontherecords.Inthefirstversionwehard-codeour"database"intotheprogram.Thedatabaseisamappingwheretheindexistherecordnameandthedataisanarrayofstrings.Thestringsareofcoursethesongnames.Thedefaultregisterconsistsofonerecord.

#!

/usr/local/bin/pike

mapping（string:

array（string））records=

（[

    "StarWarsTrilogy":

（{

        "FoxFanfare",

        "MainTitle",

        "PrincessLeia'sTheme",

        "HereTheyCome",

        "TheAsteroidField",

        "Yoda'sTheme",

        "TheImperialMarch",

        "ParadeoftheEwoks",

        "LukeandLeia",

        "FightwithTieFighters",

        "JabbatheHut",

        "DarthVader'sDeath",

        "TheForestBattle",

        "Finale"

    }）

]）;

Wewanttobeabletogetasimplelistoftherecordsinourdatabase.Thefunctionlist_recordsjustgoesthroughthemappingrecordsandputstheindices,i.e.therecordnames,inanarrayofstrings,record_names.Byusingthebuiltinfunctionsortweputtherecordnamesintothearrayinalphabeticalorderwhichmightbeanicetouch.Fortheprintoutwejustprintaheader,"Records:

",followedbyanewline.Thenweusetheloopcontrolstructurefortotraversethearrayandprinteveryiteminit,includingthenumberoftherecord,bycountingupfromzerotothelastitemofthearray.Thebuiltinfunctionsizeofgivesthenumberofitemsinanarray.TheprintoutisformattedthroughtheuseofsprintfwhichworksmoreorlessliketheCfunctionofthesamename.

voidlist_records（）

{

    inti;

    array（string）record_names=sort（indices（records））;

    write（"Records:

\n"）;

    for（i=0;i

        write（sprintf（"%3d:

%s\n",i+1,record_names[i]））;

}

Ifthecommandlinecontainedanumberourprogramwillfindtherecordofthatnumberandprintitsnamealongwiththesongsofthisrecord.Firstwecreatethesamearrayofrecordnamesasinthepreviousfunction,thenwefindthenameoftherecordwhosenumber（num）wegaveasanargumenttothisfunction.Nextweputthesongsofthisrecordinthearraysongsandprinttherecordnamefollowedbythesongs,eachsongonaseparateline.

voidshow_record（intnum）

{

    inti;

    array（string）record_names=sort（indices（records））;

    stringname=record_names[num-1];

    array（string）songs=records[name];

    write（sprintf（"Record%d,%s\n",num,name））;

    for（i=0;i

        write（sprintf（"%3d:

%s\n",i+1,songs[i]））;

}

Themainfunctiondoesn'tdomuch;itcheckswhethertherewasanythingonthecommandlineaftertheinvocation.Ifthisisnotthecaseitcallsthelist_recordsfunction,otherwiseitsendsthegivenargumenttotheshow_recordfunction.Whenthecalledfunctionisdonetheprogramjustquits.

intmain（intargc,array（string）argv）

{

    if（argc<=1）

    {

        list_records（）;

    }else{

        show_record（（int）argv[1]）;

    }

}

2.1Takingcareofinput

Now,itwouldbebetterandmoregeneralifwecouldentermorerecordsintoourdatabase.Let'saddsuchafunctionandmodifythemain（）functiontoaccept"commands".

2.1.1add_record（）

UsingthemethodStdio.Readline（）->read（）wewaitforinputwhichwillbeputintothevariablerecord_name.Theargumentto->read（）isprintedasapromptinfrontoftheuser'sinput.Readlinetakeseverythinguptoanewlinecharacter.Nowweusethecontrolstructurewhiletocheckwhetherweshouldcontinueinputtingsongs.Thewhile

（1）means"loopforever",because1isalwaystrue.Thisprogramdoesnotinfactloopforever,becauseitusesreturntoexitthefunctionfromwithintheloopwhenyoutypeaperiod.Whensomethinghasbeenreadintothevariablesongitischecked.Ifitisa"."wereturnanullvaluethatwillbeusedinthewhilestatementtoindicatethatitisnotoktocontinueaskingforsongnames.Ifitisnotadot,thestringwillbeaddedtothearrayofsongsforthisrecord,unlessit'sanemptystring.Notethe+=operator.Itisthesameassayingrecords[record_name]=records[record_name]+（{song}）.

voidadd_record（）

{

    stringrecord_name=Stdio.Readline（）->read（"Recordname:

"）;

    records[record_name]=（{}）;

    write（"Inputsongnames,oneperline.Endwith'.'onitsownline.\n"）;

    while

（1）

    {

        stringsong;

        song=Stdio.Readline（）->read（sprintf（"Song%2d:

",

                                                                sizeof（records[record_name]）+1））;

        if（song=="."）

            return;

        if（strlen（song））

            records[record_name]+=（{song}）;

    }

}

2.1.2main（）

Themainfunctionnowdoesnotcareaboutanycommandlinearguments.InsteadweuseStdio.Readline（）->read（）toprompttheuserforinstructionsandarguments.Theavailableinstructionsare"add","list"and"quit".Whatyouenterintothevariablescmdandargsischeckedintheswitch（）block.Ifyouentersomethingthatisnotcoveredinanyofthecasestatementstheprogramjustsilentlyignoresitandasksforanewcommand.Inaswitch（）theargument（inthiscasecmd）ischeckedinthecasestatements.Thefirstcasewheretheexpressionequalscmdthenexecutesthestatementafterthecolon.Ifnoexpressionisequal,wejustfallthroughwithoutanyaction.Theonlycommandthattakesanargumentis"list"whichworksasinthefirstversionoftheprogram.If"list"receivesanargument,thatrecordisshownalongwithallthesongsonit.Ifthereisnoargumentitshowsalistoftherecordsinthedatabase.Whentheprogramreturnsfromeitherofthelistingfunctions,thebreakinstructiontellstheprogramtojumpoutoftheswitch（）block."add"ofcourseturnscontrolovertothefunctiondescribedabove.Ifthecommandgivenis"quit"theexit（0）statementstopstheexecutionoftheprogramandreturns0（zero）totheoperatingsystem,tellingitthateverythingwasok.

intmain（intargc,array（string）argv）

{

    stringcmd;

    while（cmd=Stdio.Readline（）->read（"Command:

"））

    {

        stringargs;

        sscanf（cmd,"%s%s",cmd,args）;

        switch（cmd）

        {

        case"list":

            if（（int）args）

            {

                show_record（（int）args）;

            }else{

                list_records（）;

            }

            break;

        case"quit":

            exit（0）;

        case"add":

            add_record（）;

            break;

        }

    }

}

2.2Communicatingwithfiles

Nowifwewanttosavethedatabaseandalsobeabletoretrievepreviouslystoreddatawehavetocommunicatewiththeenvironment,i.e.withfilesondisk.Nowwewillintroduceyoutoprogrammingwithobjects.ToopenafileforreadingorwritingwewilluseoneoftheprogramswhichisbuiltininPikecalledStdio.File.ToPike,aprogramisadatatypewhichcontainscode,functionsandvariables.AprogramcanbeclonedwhichmeansthatPikecreatesadataareainmemoryfortheprogram,placesareferencetotheprograminthedataarea,andinitializesittoactonthedatainquestion.Themethods（i.e.functionsintheobject）andvariablesintheobjectStdio.Fileenableustoperformactionsontheassociateddatafile.Themethodsweneedtouseareopen,read,writeandclose.Seechapter9"FileI/O"formoredetails.

2.2.1save（）

FirstwecloneaStdio.Fileprogramtotheobjecto.Thenweuseittoopenthefilewhosenameisgiveninthestringfile_nameforwriting.Weusethefactthatifthereisanerrorduringopening,open（）willreturnafalsevaluewhichwecandetectandactuponbyexiting.Thearrowoperator（->）iswhatyouusetoaccessmethodsandvariablesinanobject.Ifthereisnoerrorweuseyetanothercontrolstructure,foreach,togothroughthemappingrecordsonerecordatatime.Weprecederecordnameswiththestring"Record:

"andsongnameswith"Song:

".Wealsoputeveryentry,beitsongorrecord,onitsownlinebyaddinganewlinetoeverythingwewritetothefile.

Finally,remembertoclosethefile.

voidsave（stringfile_name）

{

    stringname,song;

    Stdio.Fileo=Stdio.File（）;

    if（!

o->open（file_name,"wct"））

    {

        write（"Failedtoopenfile.\n"）;

        return;

    }

    foreach（indices（records）,name）

    {

        o->write（"Record:

"+name+"\n"）;

        foreach（records[name],song）

            o->write（"Song:

"+song+"\n"）;

    }

    o->close（）;

}

2.2.2load（）

Theloadfunctionbeginsmuchthesame,exceptweopenthefilenamedfileforreadinginstead.Whenreceivingdatafromthefileweputitinthestringfile_contents.Theabsenceofargumentstothemethodo->readmeansthatthereadingshouldnotenduntiltheendofthefile.Afterhavingclosedthefileweinitializeourdatabase,i.e.themappingrecords.Thenwehavetoputfile_contentsintothemappingandwedothisbysplittingthestringonnewlines（cf.thesplitoperatorinPerl）usingthedivisionoperator.Yes,that'sright:

bydividingonestringwithanotherwecanobtainanarrayconsistingofpartsfromthefirst.Andbyusingaforeachstatementwecantakethestringfile_contentsapartpiecebypiece,puttingeachpiecebackinitsproperplaceinthemappingrecords.

voidload（stringfile_name）

{

    stringname="ERROR";

    stringfile_contents,line;

    Stdio.Fileo=Stdio.File（）;

    if（!

o->open（file_name,"r"））

    {

        write（"Failedtoopenfile.\n"）;

        return;

    }

    file_contents=o->read（）;

    o->close（）;

    record