edu.anarcho-copy.org Languages/Go...1.1 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.3 1.3.1 1.3.2 1.3.3...

1.1

1.2

1.2.1

1.2.2

1.2.3

1.2.4

1.2.5

1.3

1.3.1

1.3.2

1.3.3

1.3.4

1.3.5

1.3.6

1.3.7

1.3.8

1.4

1.4.1

1.4.2

1.4.3

1.4.4

1.4.5

1.5

1.5.1

1.5.2

1.5.3

1.5.4

1.5.5

1.5.6

1.6

1.6.1

1.6.2

1.6.3

1.6.4

1.6.5

1.6.6

1.6.7

1.7

1.7.1

TableofContentsIntroduction

GoEnvironmentConfiguration

Installation

$GOPATHandworkspace

Gocommands

Godevelopmenttools

Summary

Gobasicknowledge

Hello,Go

Gofoundation

Controlstatementsandfunctions

struct

Object-oriented

interface

Concurrency

Summary

Webfoundation

Webworkingprinciples

Buildasimplewebserver

HowGoworkswithweb

Getintohttppackage

Summary

HTTPForm

Processforminputs

Validationofinputs

Crosssitescripting

Duplicatesubmissions

Fileupload

Summary

Database

database/sqlinterface

HowtouseMySQL

HowtouseSQLite

HowtousePostgreSQL

HowtousebeedbORM

NOSQL

Summary

Datastorageandsession

Sessionandcookies

2

1.7.2

1.7.3

1.7.4

1.7.5

1.8

1.8.1

1.8.2

1.8.3

1.8.4

1.8.5

1.8.6

1.8.7

1.9

1.9.1

1.9.2

1.9.3

1.9.4

1.9.5

1.10

1.10.1

1.10.2

1.10.3

1.10.4

1.10.5

1.10.6

1.10.7

1.11

1.11.1

1.11.2

1.11.3

1.11.4

1.12

1.12.1

1.12.2

1.12.3

1.12.4

1.13

1.13.1

1.13.2

1.13.3

1.13.4

1.13.5

HowtousesessioninGo

Sessionstorage

Preventhijackofsession

Summary

Textfiles

XML

JSON

Regexp

Templates

Files

Strings

Summary

Webservices

Sockets

WebSocket

REST

RPC

Summary

Securityandencryption

CSRFattacks

Filterinputs

XSSattacks

SQLinjection

Passwordstorage

Encryptanddecryptdata

Summary

Internationalizationandlocalization

Timezone

Localizedresources

Internationalsites

Summary

Errorhandling,debuggingandtesting

Errorhandling

DebuggingbyusingGDB

Writetestcases

Summary

Deploymentandmaintenance

Logs

Errorsandcrashes

Deployment

Backupandrecovery

Summary

3

1.14

1.14.1

1.14.2

1.14.3

1.14.4

1.14.5

1.14.6

1.15

1.15.1

1.15.2

1.15.3

1.15.4

1.15.5

1.15.6

1.15.7

1.16

1.17

Buildawebframework

Projectprogram

Customizedrouters

Designcontrollers

Logsandconfigurations

Add,deleteandupdateblogs

Summary

Developwebframework

Staticfiles

Session

Form

Uservalidation

Multi-languagesupport

pprof

Summary

References

preface

4

BuildWebApplicationwithGolangPurpose

BecauseI'minterestedinwebapplicationdevelopment,Iusedmyfreetimetowritethisbookasanopensourceversion.Itdoesn'tmeanthatIhaveaverygoodabilitytobuildwebapplications;IwouldliketosharewhatI'vedonewithGoinbuildingwebapplications.

ForthoseofyouwhoareworkingwithPHP/Python/Ruby,youwilllearnhowtobuildawebapplicationwithGo.ForthoseofyouwhoareworkingwithC/C++,youwillknowhowthewebworks.

Ibelievethepurposeofstudyingissharingwithothers.ThehappiestthinginmylifeissharingeverythingI'veknownwithmorepeople.

Donate

AliPay:

alipay

EnglishDonate:donate

CommunityQQ群：386056972

BBS：http://gocn.io/

Acknowledgments四月份平民AprilCitizen(reviewcode)洪瑞琦HongRuiqi(reviewcode)边疆BianJiang(writetheconfigurationsaboutVimandEmacsforGodevelopment)欧林猫OlingCat(reviewcode)吴文磊WenleiWu(providesomepictures)北极星Polaris(reviewwholebook)雨痕RainTrail(reviewchapter2and3)

LicenseThisbookislicensedundertheCCBY-SA3.0License,thecodeislicensedunderaBSD3-ClauseLicense,unlessotherwisespecified.

GetStarted

Index

Introduction

5

http://beego.me/donatehttp://gocn.io/https://plus.google.com/110445767383269817959https://github.com/hongruiqihttps://github.com/borderhttps://github.com/OlingCatmailto:[email protected]://github.com/polaris1119https://github.com/qyuhenhttp://creativecommons.org/licenses/by-sa/3.0/https://github.com/astaxie/build-web-application-with-golang/blob/master/LICENSE.md

Introduction

6

1GoEnvironmentConfigurationWelcometotheworldofGo,let'sstartexploring!

Goisafast-compiled,garbage-collected,concurrentsystemsprogramminglanguage.Ithasthefollowingadvantages:

Compilesalargeprojectwithinafewseconds.Providesasoftwaredevelopmentmodelthatiseasytoreasonabout,avoidingmostoftheproblemsassociatedwithC-styleheaderfiles.Isastaticlanguagethatdoesnothavelevelsinitstypesystem,sousersdonotneedtospendmuchtimedealingwithrelationsbetweentypes.Itismorelikealightweightobject-orientedlanguage.Performsgarbagecollection.Itprovidesbasicsupportforconcurrencyandcommunication.Designedformulti-corecomputers.

Goisacompiledlanguage.Itcombinesthedevelopmentefficiencyofinterpretedordynamiclanguageswiththesecurityofstaticlanguages.Itisgoingtobethelanguageofchoiceformodern,multi-corecomputerswithnetworking.Forthesepurposes,therearesomeproblemsthatneedtoinherentlyberesolvedatthelevelofthelanguageofchoice,suchasarichlyexpressivelightweighttypesystem,anativeconcurrencymodel,andstrictlyregulatedgarbagecollection.Forquitesometime,nopackagesortoolshaveemergedthathaveaimedtosolvealloftheseproblemsinapragmaticfashion;thuswasbornthemotivationfortheGolanguage.

Inthischapter,IwillshowyouhowtoinstallandconfigureyourownGodevelopmentenvironment.

LinksDirectoryNextsection:Installation

GoEnvironmentConfiguration

7

1.1Installation

ThreewaystoinstallGoTherearemanywaystoconfiguretheGodevelopmentenvironmentonyourcomputer,andyoucanchoosewhicheveroneyoulike.Thethreemostcommonwaysareasfollows.

Officialinstallationpackages.TheGoteamprovidesconvenientinstallationpackagesinWindows,Linux,Macandotheroperatingsystems.Thisisprobablytheeasiestwaytogetstarted.YoucangettheinstallersfromtheGolangDownloadPage.

Installityourselffromsourcecode.PopularwithdeveloperswhoarefamiliarwithUnix-likesystems.

Usingthird-partytools.Therearemanythird-partytoolsandpackagemanagersforinstallingGo,likeapt-getinUbuntuandhomebrewforMac.

IncaseyouwanttoinstallmorethanoneversionofGoonacomputer,youshouldtakealookatatoolcalledGVM.ItisthebesttoolI'veseensofarforaccomplishingthistask,otherwiseyou'dhavetodealwithityourself.

InstallfromsourcecodeTocompileGo1.5andupwards,youonlyneedthepreviousversionofGo,asGohasachievedbootstrapping.YouonlyneedGotocompileGo.

TocompileGo1.4downwards,youwillneedaCcompilerassomepartsofGoarestillwritteninPlan9CandAT&Tassembler.

OnaMac,ifyouhaveinstalledXcode,youalreadyhavethecompiler.

OnUnix-likesystems,youneedtoinstallgccorasimilarcompiler.Forexample,usingthepackagemanagerapt-get(includedwithUbuntu),onecaninstalltherequiredcompilersasfollows:

sudoapt-getinstallgcclibc6-dev

OnWindows,youneedtoinstallMinGWinordertoinstallgcc.Don'tforgettoconfigureyourenvironmentvariablesaftertheinstallationhascompleted.(Everythingthatlookslikethismeansit'scommentedbyatranslator:Ifyouareusing64-bitWindows,youshouldinstallthe64-bitversionofMinGW)

Atthispoint,executethefollowingcommandstoclonetheGosourcecodeandcompileit.(Itwillclonethesourcecodetoyourcurrentdirectory.Switchyourworkpathbeforeyoucontinue.Thismaytakesometime.)

gitclonehttps://go.googlesource.com/gocdgo/src./all.bash

Asuccessfulinstallationwillendwiththemessage"ALLTESTSPASSED."

OnWindows,youcanachievethesamebyrunningall.bat.

IfyouareusingWindows,theinstallationpackagewillsetyourenvironmentvariablesautomatically.InUnix-likesystems,youneedtosetthesevariablesmanuallyasfollows.(IfyourGoversionisgreaterthan1.0,youdon'thavetoset$GOBIN,anditwillautomaticallyberelatedtoyour$GOROOT/bin,whichwewilltalkaboutinthenextsection)

Installation

8

https://golang.org/dl/https://github.com/moovweb/gvm

exportGOROOT=$HOME/goexportGOBIN=$GOROOT/binexportPATH=$PATH:$GOROOT/bin

Ifyouseethefollowinginformationonyourscreen,you'reallset.

Figure1.1Informationafterinstallingfromsourcecode

OnceyouseetheusageinformationofGo,itmeansyouhavesuccessfullyinstalledGoonyourcomputer.Ifitsays"nosuchcommand",checkthatyour$PATHenvironmentvariablecontainstheinstallationpathofGo.

UsingthestandardinstallationpackagesGohasone-clickinstallationpackagesforeverysupportedoperatingsystem.ThesepackageswillinstallGoin/usr/local/go(c:\GoinWindows)bydefault.Ofcoursethiscanbemodified,butyoualsoneedtochangealltheenvironmentvariablesmanuallyasI'veshownabove.

Howtocheckifyouroperatingsystemis32-bitor64-bit?

Ournextstepdependsonyouroperatingsystemtype,sowehavetocheckitbeforewedownloadthestandardinstallationpackages.

IfyouareusingWindows,pressWin+Randthenrunthecommandtool.Typethesysteminfocommandanditwillshowyousomeusefulsysteminformation.Findthelinethatsays"systemtype"-ifyousee"x64-basedPC"thatmeansyouroperatingsystemis64-bit,32-bitotherwise.

Istronglyrecommenddownloadingthe64-bitpackageifyouareaMacuser,asGonolongersupportspure32-bitprocessorsonMacOSX.

Linuxuserscantypeuname-aintheterminaltoseesysteminformation.A64-bitoperatingsystemwillshowthefollowing:

x86_64x86_64x86_64GNU/Linux//somemachinessuchasUbuntu10.04willshowasfollowingx86_64GNU/Linux

32-bitoperatingsystemsinsteadshow:

i686i686i386GNU/Linux

Mac

Gotothedownloadpage,choosego1.4.2.darwin-386.pkg(Thelaterversionhasno32-bitdownload.)for32-bitsystemsandgo1.8.3.darwin-amd64.pkgfor64-bitsystems.Goingallthewaytotheendbyclicking"next",~/go/binwillbeaddedtoyoursystem's$PATHafteryoufinishtheinstallation.Nowopentheterminalandtypego.Youshouldseethesameoutputshowninfigure1.1.

Linux

Gotothedownloadpage,choosego1.8.3.linux-386.tar.gzfor32-bitsystemsandgo1.8.3.linux-amd64.tar.gzfor64-bitsystems.SupposeyouwanttoinstallGointhe$GO_INSTALL_DIRpath.Uncompressthetar.gztoyourchosenpathusingthecommandtarzxvfgo1.8.3.linux-amd64.tar.gz-C$GO_INSTALL_DIR.Thensetyour$PATHwiththefollowing:exportPATH=$PATH:$GO_INSTALL_DIR/go/bin.Nowjustopentheterminalandtypego.Youshouldnowseethesameoutputdisplayedinfigure1.1.

Installation

9

https://golang.org/dl/https://golang.org/dl/

Windows

Gotothedownloadpage,choosego1.8.3.windows-386.msifor32-bitsystemsandgo1.8.3.windows-amd64.msifor64-bitsystems.Goingallthewaytotheendbyclicking"next",c:/go/binwillbeaddedtopath.Nowjustopenacommandlinewindowandtypego.Youshouldnowseethesameoutputdisplayedinfigure1.1.

Usethird-partytools

GVM

GVMisaGomulti-versioncontroltooldevelopedbyathird-party,likervmforruby.It'squiteeasytouse.Installgvmbytypingthefollowingcommandsinyourterminal:

bash<

1. InstallGo

brewupdate&&brewupgradebrewinstallgo

LinksDirectoryPrevioussection:GoenvironmentconfigurationNextsection:$GOPATHandworkspace

Installation

11

1.2$GOPATHandworkspace

$GOPATHGotakesauniqueapproachtomanagethecodefileswiththeintroductionofa$GOPATHdirectorywhichcontainsallthegocodeinthemachine.Notethatthisisdifferentfromthe$GOROOTenvironmentvariablewhichstateswheregoisinstalledonthemachine.Wehavetodefinethe$GOPATHvariablebeforeusingthelanguage,in*nixsystemsthereisafilecalled.profileweneedtoappendthebelowexportstatementtothefile.Theconceptbehindgopathisanovelone,wherewecanlinktoanygocodeatanyinstantoftimewithoutambiguity.

Startingfromgo1.8,theGOPATHenvironmentvariablenowhasadefaultvalueifitisunset.Itdefaultsto$HOME/goonUnixand%USERPROFILE%/goonWindows.

InUnix-likesystems,thevariableshouldbeusedlikethis:

exportGOPATH=${HOME}/mygo

InWindows,youneedtocreateanewenvironmentvariablecalledGOPATH,thensetitsvaluetoc:\mygo(Thisvaluedependsonwhereyourworkspaceislocated)

It'sOKtohavemorethanonepath(workspace)in$GOPATH,butrememberthatyouhavetouse:(;inWindows)tobreakthemup.Atthispoint,gogetwillsavethecontenttoyourfirstpathin$GOPATH.Itishighlyrecommendedtonothavemultiplesversions,theworstcaseistocreateafolderbythenameofyourprojectrightinside$GOPATH,itbreakseverythingthatthecreatorswerewishingtochangeinprogrammingwiththecreationofgolanguagebecausewhenyoucreateafolderinside$GOPATHyouwillreferenceyourpackagesasdirectlyas,andthisbreaksalltheapplicationswhichwillimportyourpackagebecausethegogetwon'tfindyourpackage.Pleasefollowconventions,thereisareasonconventionsarecreated.

In$GOPATH,youmusthavethreefoldersasfollows:

srcforsourcefileswhosesuffixis.go,.c,.g,.s.pkgforcompiledfileswhosesuffixis.a.binforexecutablefiles

Inthisbook,Iusemygoasmyonlypathin$GOPATH.

PackagedirectoryCreatepackagesourcefilesandfolderslike$GOPATH/src/mymath/sqrt.go(mymathisthepackagename)(Authorusesmymathashispackagename,andthesamenameforthefolderthatcontainsthepackagesourcefiles)

Everytimeyoucreateapackage,youshouldcreateanewfolderinthesrcdirectory,withthenotableexceptionofmain,forwhichmainfoldercreationisoptional.Foldernamesareusuallythesameasthepackagethatyouaregoingtouse.Youcanhavemulti-leveldirectoriesifyouwantto.Forexample,ifyoucreatethedirectory$GOPATH/src/github.com/astaxie/beedb,thenthepackagepathwouldbegithub.com/astaxie/beedb.Thepackagenamewillbethelastdirectoryinyourpath,whichisbeedbinthiscase.

Executefollowingcommands.(Nowauthorgoesbacktotalkexamples)

cd$GOPATH/srcmkdirmymath

Createanewfilecalledsqrt.go,typethefollowingcontenttoyourfile.

$GOPATHandworkspace

12

//Sourcecodeof$GOPATH/src/mymath/sqrt.gopackagemymath

funcSqrt(xfloat64)float64{z:=0.0fori:=0;i<1000;i++{z-=(z*z-x)/(2*x)}returnz}

Nowmypackagedirectoryhasbeencreatedandit'scodehasbeenwritten.Irecommendthatyouusethesamenameforyourpackagesastheircorrespondingdirectories,andthatthedirectoriescontainallofthepackagesourcefiles.

CompilepackagesWe'vealreadycreatedourpackageabove,buthowdowecompileitforpracticalpurposes?Therearetwowaystodothis.

1. Switchyourworkpathtothedirectoryofyourpackage,thenexecutethegoinstallcommand.2. Executetheabovecommandexceptwithafilename,likegoinstallmymath.

Aftercompiling,wecanopenthefollowingfolder.

cd$GOPATH/pkg/${GOOS}_${GOARCH}//youcanseethefilewasgeneratedmymath.a

Thefilewhosesuffixis.aisthebinaryfileofourpackage.Howdoweuseit?

Obviously,weneedtocreateanewapplicationtouseit.

Createanewapplicationpackagecalledmathapp.

cd$GOPATH/srcmkdirmathappcdmathappvimmain.go

Writethefollowingcontenttomain.go.

//$GOPATH/src/mathapp/main.gosourcecode.packagemain

import("mymath""fmt")

funcmain(){fmt.Printf("Hello,world.Sqrt(2)=%v\n",mymath.Sqrt(2))}

Tocompilethisapplication,youneedtoswitchtotheapplicationdirectory,whichinthiscaseis$GOPATH/src/mathapp,thenexecutethegoinstallcommand.Nowyoushouldseeanexecutablefilecalledmathappwasgeneratedinthedirectory$GOPATH/bin/.Torunthisprogram,usethe./mathappcommand.Youshouldseethefollowingcontentinyourterminal.

Helloworld.Sqrt(2)=1.414213562373095

$GOPATHandworkspace

13

InstallremotepackagesGohasatoolforinstallingremotepackages,whichisacommandcalledgoget.Itsupportsmostopensourcecommunities,includingGithub,GoogleCode,BitBucket,andLaunchpad.

gogetgithub.com/astaxie/beedb

Youcanusegoget-u…toupdateyourremotepackagesanditwillautomaticallyinstallallthedependentpackagesaswell.

Thistoolwillusedifferentversioncontroltoolsfordifferentopensourceplatforms.Forexample,gitforGithubandhgforGoogleCode.Therefore,youhavetoinstalltheseversioncontroltoolsbeforeyouusegoget.

Afterexecutingtheabovecommands,thedirectorystructureshouldlooklikefollowing.

$GOPATHsrc|-github.com|-astaxie|-beedbpkg|--${GOOS}_${GOARCH}|-github.com|-astaxie|-beedb.a

Actually,gogetclonessourcecodetothe$GOPATH/srcofthelocalfilesystem,thenexecutesgoinstall.

Youcanuseremotepackagesinthesamewaythatweuselocalpackages.

import"github.com/astaxie/beedb"

DirectorycompletestructureIfyou'vefollowedalloftheabovesteps,yourdirectorystructureshouldnowlooklikethefollowing.

bin/mathapppkg/${GOOS}_${GOARCH},suchasdarwin_amd64,linux_amd64mymath.agithub.com/astaxie/beedb.asrc/mathappmain.gomymath/sqrt.gogithub.com/astaxie/beedb/beedb.goutil.go

Nowyouareabletoseethedirectorystructureclearly;bincontainsexecutablefiles,pkgcontainscompiledfilesandsrccontainspackagesourcefiles.

(TheformatofenvironmentvariablesinWindowsis%GOPATH%,howeverthisbookmainlyfollowstheUnix-style,soWindowsusersneedtoreplacetheseyourself.)

$GOPATHandworkspace

14

LinksDirectoryPrevioussection:InstallationNextsection:Gocommands

$GOPATHandworkspace

15

1.3Gocommands

GocommandsTheGolanguagecomeswithacompletesetofcommandoperationtools.Youcanexecutethegocommandontheterminaltoseethem:

Figure1.3Gocommanddisplaysdetailedinformation

Theseareallusefulforus.Let'sseehowtousesomeofthem.

gobuildThiscommandisforcompilingtests.Itwillcompilepackagesanddependenciesifit'snecessary.

Ifthepackageisnotthemainpackagesuchasmymathinsection1.2,nothingwillbegeneratedafteryouexecutegobuild.Ifyouneedthepackagefile.ain$GOPATH/pkg,usegoinstallinstead.Ifthepackageisthemainpackage,itwillgenerateanexecutablefileinthesamefolder.Ifyouwantthefiletobegeneratedin$GOPATH/bin,usegoinstallorgobuild-o${PATH_HERE}/a.exe.Iftherearemanyfilesinthefolder,butyoujustwanttocompileoneofthem,youshouldappendthefilenameaftergobuild.Forexample,gobuilda.go.gobuildwillcompileallthefilesinthefolder.Youcanalsoassignthenameofthefilethatwillbegenerated.Forinstance,inthemathappproject(insection1.2),usinggobuild-oastaxie.exewillgenerateastaxie.exeinsteadofmathapp.exe.Thedefaultnameisyourfoldername(non-mainpackage)orthefirstsourcefilename(mainpackage).

(AccordingtoTheGoProgrammingLanguageSpecification,packagenamesshouldbethenameafterthewordpackageinthefirstlineofyoursourcefiles.Itdoesn'thavetobethesameasthefoldername,andtheexecutablefilenamewillbeyourfoldernamebydefault.)

gobuildignoresfileswhosenamesstartwith_or..Ifyouwanttohavedifferentsourcefilesforeveryoperatingsystem,youcannamefileswiththesystemnameasasuffix.Supposetherearesomesourcefilesforloadingarrays.Theycouldbenamedasfollows:

array_linux.go|array_darwin.go|array_windows.go|array_freebsd.go

gobuildchoosestheonethat'sassociatedwithyouroperatingsystem.Forexample,itonlycompilesarray_linux.goinLinuxsystems,andignoresalltheothers.

gocleanThiscommandisforcleaningfilesthataregeneratedbycompilers,includingthefollowingfiles:

_obj///olddirectoryofobject,leftbyMakefiles_test///olddirectoryoftest,leftbyMakefiles_testmain.go//olddirectoryofgotest,leftbyMakefilestest.out//olddirectoryoftest,leftbyMakefilesbuild.out//olddirectoryoftest,leftbyMakefiles*.[568ao]//objectfiles,leftbyMakefiles

DIR(.exe)//generatedbygobuildDIR.test(.exe)//generatedbygotest-cMAINFILE(.exe)//generatedbygobuildMAINFILE.go

Gocommands

16

https://golang.org/ref/spec

IusuallyusethiscommandtocleanupmyfilesbeforeIuploadmyprojecttoGithub.Theseareusefulforlocaltests,butuselessforversioncontrol.

gofmtandgofmtThepeoplewhoareworkingwithC/C++shouldknowthatpeoplearealwaysarguingaboutwhichcodestyleisbetter:K&R-styleorANSI-style.HoweverinGo,thereisonlyonecodestylewhichisenforced.Forexample,leftbracesmustonlybeinsertedattheendoflines,andtheycannotbeontheirownlines,otherwiseyouwillgetcompileerrors!Fortunately,youdon'thavetoremembertheserules.gofmtdoesthisjobforyou.Justexecutethecommandgofmt.gointerminal.Idon'tusethiscommandverymuchbecauseIDEsusuallyexecutethiscommandautomaticallywhenyousavesourcefiles.IwilltalkmoreaboutIDEsinthenextsection.

gofmtisjustanalias,whichrunsthecommand'gofmt-l-w'onthepackagesnamedbytheimportpaths.

Weusuallyusegofmt-winsteadofgofmt.Thelatterwillnotrewriteyoursourcefilesafterformattingcode.gofmt-wsrcformatsthewholeproject.

gogetThiscommandisforgettingremotepackages.Sofar,itsupportsBitBucket,Github,GoogleCodeandLaunchpad.Thereareactuallytwothingsthathappenafterweexecutethiscommand.ThefirstthingisthatGodownloadsthesourcecode,thenexecutesgoinstall.Beforeyouusethiscommand,makesureyouhaveinstalledalloftherelatedtools.

BitBucket(MercurialGit)Github(git)GoogleCode(Git,Mercurial,Subversion)Launchpad(Bazaar)

Inordertousethiscommand,youhavetoinstallthesetoolscorrectly.Don'tforgettoupdatethe$PATHvariable.Bytheway,italsosupportscustomizeddomainnames.Usegohelpimportpathformoredetailsaboutthis.

goinstallThiscommandcompilesallpackagesandgeneratesfiles,thenmovesthemto$GOPATH/pkgor$GOPATH/bin.

gotestThiscommandloadsallfileswhosenameinclude*_test.goandgeneratestestfiles,thenprintsinformationthatlookslikethefollowing.

okarchive/tar0.011sFAILarchive/zip0.022sokcompress/gzip0.033s...

Ittestsallyourtestfilesbydefault.Usecommandgohelptestflagformoredetails.

godocManypeoplesaythatwedon'tneedanythird-partydocumentationforprogramminginGo(actuallyI'vemadeaCHMalready).Gohasapowerfultooltomanagedocumentationnatively.

Gocommands

17

https://github.com/astaxie/godoc

Sohowdowelookuppackageinformationindocumentation?Forinstance,ifyouwanttogetmoredetailsaboutthebuiltinpackage,usethegodocbuiltincommand.Similarly,usethegodocnet/httpcommandtolookupthehttppackagedocumentation.Ifyouwanttoseemoredetailsaboutspecificfunctions,usethegodocfmtPrintfandgodoc-srcfmtPrintfcommandstoviewthesourcecode.

Executethegodoc-http=:8080command,thenopen127.0.0.1:8080inyourbrowser.Youshouldseealocalizedgolang.org.Itcannotonlyshowthestandardpackages'information,butalsopackagesinyour$GOPATH/pkg.It'sgreatforpeoplewhoaresufferingfromtheGreatFirewallofChina.

OthercommandsGoprovidesmorecommandsthanthosewe'vejusttalkedabout.

gofix//upgradecodefromanoldversionbeforego1toanewversionaftergo1goversion//getinformationaboutyourversionofGogoenv//viewenvironmentvariablesaboutGogolist//listallinstalledpackagesgorun//compiletemporaryfilesandruntheapplication

TherearealsomoredetailsaboutthecommandsthatI'vetalkedabout.Youcanusegohelptolookthemup.

LinksDirectoryPrevioussection:$GOPATHandworkspaceNextsection:Godevelopmenttools

Gocommands

18

GodevelopmenttoolsInthissection,I'mgoingtoshowyouafewIDEsthatcanhelpyoubecomeamoreefficientprogrammer,withcapabilitiessuchasintelligentcodecompletionandauto-formatting.Theyareallcross-platform,sothestepsIwillbeshowingyoushouldnotbeverydifferent,evenifyouarenotusingthesameoperatingsystem.

LiteIDELiteIDEisanopensource,lightweightIDEfordevelopingGoprojectsonly,developedbyvisualfc.

Figure1.4MainpanelofLiteIDE

LiteIDEfeatures.

Cross-platformWindowsLinuxMacOS

Cross-compileManagemultiplecompileenvironmentsSupportscross-compilationofGo

ProjectmanagementstandardDocumentationviewbasedon$GOPATHCompilationsystembasedon$GOPATHAPIdocumentationindexbasedon$GOPATH

GosourcecodeeditorCodeoutliningFullsupportofgocodeGodocumentationviewandAPIindexViewcodeexpressionusingF1FunctiondeclarationjumpusingF2GdbsupportAuto-formatwithgofmt

OthersMulti-languagePluginsystemTexteditorthemesSyntaxsupportbasedonKateintelligentcompletionbasedonfull-textCustomizedshortcutsMarkdownsupport

Real-timepreviewCustomizedCSSExportHTMLandPDFConvertandmergetoHTMLandPDF

LiteIDEinstallationInstallLiteIDE

Downloadpage

Godevelopmenttools

19

https://sourceforge.net/projects/liteide/files/

Sourcecode

YouneedtoinstallGofirst,thendownloadtheversionappropriateforyouroperatingsystem.Decompressthepackagetodirectlyuseit.

Installgocode

Youhavetoinstallgocodeinordertouseintelligentcompletion

goget-ugithub.com/nsf/gocode

Compilationenvironment

SwitchconfigurationinLiteIDEtosuityouroperatingsystem.InWindowsandusingthe64-bitversionofGo,youshouldchoosewin64astheconfigurationenvironmentinthetoolbar.Then,chooseOptions,findLiteEnvintheleftlistandopenfilewin64.envintherightlist.

GOROOT=c:\goGOBIN=GOARCH=amd64GOOS=windowsCGO_ENABLED=1

PATH=%GOBIN%;%GOROOT%\bin;%PATH%。。。

ReplaceGOROOT=c:\gotoyourGoinstallationpath,saveit.IfyouhaveMinGW64,addc:\MinGW64\bintoyourpathenvironmentvariableforcgosupport.

InLinuxandusingthe64-bitversionofGo,youshouldchooselinux64astheconfigurationenvironmentinthetoolbar.Then,chooseOptions,findLiteEnvintheleftlistandopenthelinux64.envfileintherightlist.

GOROOT=$HOME/goGOBIN=GOARCH=amd64GOOS=linuxCGO_ENABLED=1

PATH=$GOBIN:$GOROOT/bin:$PATH。。。

ReplaceGOROOT=$HOME/gotoyourGoinstallationpath,saveit.

$GOPATH$GOPATHisthepaththatcontainsalistofprojects.Openthecommandtool(orpressCtrl+`inLiteIDE),thentypegohelpgopathformoredetails.It'sveryeasytoviewandchange$GOPATHinLiteIDE.FollowView-SetupGOPATHtoviewandchangethesevalues.

SublimeTextHereI'mgoingtointroduceyoutheSublimeText3(Sublimeforshort)+GoSublime+gocode.Letmeexplainwhy.

Intelligentcompletion

Figure1.5Sublimeintelligentcompletion

Auto-formatsourcefilesProjectmanagement

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https://github.com/visualfc/liteide

Figure1.6Sublimeprojectmanagement

Syntaxhighlight

Freetrialforeverwithnofunctionallimitations.Youmaybepromptedonceinawhiletoremindyoutopurchasealicense,butyoucansimplyignoreitifyouwish.Ofcourse,ifyoudofindthatitenhancesyourproductivityandyoureallyenjoyusingit,pleasepurchaseacopyofitandsupportitscontinueddevelopment!

First,downloadtheversionofSublimesuitableforyouroperatingsystem.

1. PressCtrl+`,openthecommandtoolandinputthefollowingcommands.

ApplicabletoSublimeText3：

importurllib.request,os;pf='PackageControl.sublime-package';ipp=sublime.installed_packages_path();urllib.request.install_opener(urllib.request.build_opener(urllib.request.ProxyHandler()));open(os.path.join(ipp,pf),'wb').write(urllib.request.urlopen('http://sublime.wbond.net/'+pf.replace('','%20')).read())

ApplicabletoSublimeText2：

importurllib2,os;pf='PackageControl.sublime-package';ipp=sublime.installed_packages_path();os.makedirs(ipp)ifnotos.path.exists(ipp)elseNone;urllib2.install_opener(urllib2.build_opener(urllib2.ProxyHandler()));open(os.path.join(ipp,pf),'wb').write(urllib2.urlopen('http://sublime.wbond.net/'+pf.replace('','%20')).read());print('PleaserestartSublimeTexttofinishinstallation')

RestartSublimeTextwhentheinstallationhasfinished.Youshouldthenfinda`PackageControl`optioninthe"Preferences"menu.

![](images/1.4.sublime3.png?raw=true)

Figure1.7SublimePackageControl

1. ToinstallGoSublime,SidebarEnhancementsandGoBuild,pressCtrl+Shift+ptoopenPackageControl,thentypepcip(shortfor"PackageControl:InstallPackage").

Figure1.8SublimeInstallPackages

Nowtypein"GoSublime",pressOKtoinstallthepackage,andrepeatthesamestepsforinstallingSidebarEnhancementsandGoBuild.Onceagain,restarttheeditorwhenitcompletestheinstallation.

2. Toverifythattheinstallationissuccessful,openSublime,thenopenthemain.gofiletoseeifithasthepropersyntaxhighlighting.Typeimporttoseeifcodecompletionpromptsappear.Aftertypingimport"fmt",typefmt.anywhereaftertheimportdeclarationtoseewhetherornotintelligentcodecompletionforfunctionswassuccessfullyenabled.

Ifeverythingisfine,you'reallset.

Ifnot,checkyour$PATHagain.Openaterminal,typegocode.Ifitdoesnotrun,your$PATHwasnotconfiguredcorrectly.

VimVimisapopulartexteditorforprogrammers,whichevolvedfromitsslimmerpredecessor,Vi.Ithasfunctionsforintelligentcompletion,compilationandjumpingtoerrors.

vim-goisvimaboveanopen-sourcegolanguageusingthemostextensivedevelopmentenvironmentplug-ins

Thepluginaddress：github.com/fatih/vim-go

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http://www.sublimetext.com/https://github.com/fatih/vim-go

VimpluginmanagementarethemainstreamPathogenandVundle，Buttheaspectsthereofaredifferent.Pathogenistosolveeachplug-inaftertheinstallationoffilesscatteredtomultipledirectoriesandpoormanagementoftheexistence.Vundleistosolvetheautomaticsearchanddownloadplug-insexist.Thesetwoplug-inscanbeusedsimultaneously.

1.InstallVundle

mkdir~/.vim/bundlegitclonehttps://github.com/gmarik/Vundle.vim.git~/.vim/bundle/Vundle.vim

Edit.vimrc，Vundletherelevantconfigurationwillbeplacedinthebeginning(RefertotheVundledocumentationfordetails)

setnocompatible"beiMproved,requiredfiletypeoff"required

"settheruntimepathtoincludeVundleandinitializesetrtp+=~/.vim/bundle/Vundle.vimcallvundle#begin()

"letVundlemanageVundle,requiredPlugin'gmarik/Vundle.vim'

"AllofyourPluginsmustbeaddedbeforethefollowinglinecallvundle#end()"requiredfiletypepluginindenton"required

2.InstallVim-go

Edit~/.vimrc，Addalinebetweenvundle#beginandvundle#end：

Plugin'fatih/vim-go'

ExecutedwithinVim:PluginInstall

3.InstallYCM(YourCompleteMe)toAutoCompleteAddalineto~/.vimrc:

Plugin'Valloric/YouCompleteMe'

ExecutedwithinVim:PluginInstall

Figure1.8VimintelligentcompletionforGo

1. SyntaxhighlightingforGo

cp-r$GOROOT/misc/vim/*~/.vim/

2. Enablingsyntaxhighlighting

filetypepluginindentonsyntaxon

3. Installgocode


gocodewillbeinstalledin$GOBINasdefault

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https://github.com/tpope/vim-pathogenhttps://github.com/VundleVim/Vundle.vimhttps://github.com/VundleVim/Vundle.vimhttps://github.com/nsf/gocode/

4. Configuregocode

~cd$GOPATH/src/github.com/nsf/gocode/vim~./update.sh~gocodesetpropose-builtinstruepropose-builtinstrue~gocodesetlib-path"/home/border/gocode/pkg/linux_amd64"lib-path"/home/border/gocode/pkg/linux_amd64"~gocodesetpropose-builtinstruelib-path"/home/border/gocode/pkg/linux_amd64"

Explanationofgocodeconfiguration:

propose-builtins:specifieswhetherornottoopenintelligentcompletion;falsebydefault.lib-path:gocodeonlysearchesforpackagesin$GOPATH/pkg/$GOOS_$GOARCHand$GOROOT/pkg/$GOOS_$GOARCH.Thissettingcanbeusedtoaddadditionalpaths.

5. Congratulations!Try:emain.gotoexperiencetheworldofGo!

EmacsEmacsistheso-calledWeaponofGod.Sheisnotonlyaneditor,butalsoapowerfulIDE.

Figure1.10EmacsmainpanelofGoeditor

1. Syntaxhighlighting

cp$GOROOT/misc/emacs/*~/.emacs.d/

2. Installgocode


gocodewillbeinstalledin$GOBINasdefault

3. Configuregocode

~cd$GOPATH/src/github.com/nsf/gocode/vim~./update.bash~gocodesetpropose-builtinstruepropose-builtinstrue~gocodesetlib-path"/home/border/gocode/pkg/linux_amd64"lib-path"/home/border/gocode/pkg/linux_amd64"~gocodesetpropose-builtinstruelib-path"/home/border/gocode/pkg/linux_amd64"

4. InstallAutoCompletionDownloadanduncompress

~makeinstallDIR=$HOME/.emacs.d/auto-complete

Configure~/.emacsfile

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https://github.com/nsf/gocode/https://github.com/nsf/gocode/https://github.com/nsf/gocode/http://www.emacswiki.org/emacs/AutoComplete

;;auto-complete(require'auto-complete-config)(add-to-list'ac-dictionary-directories"~/.emacs.d/auto-complete/ac-dict")(ac-config-default)(local-set-key(kbd"M-/")'semantic-complete-analyze-inline)(local-set-key"."'semantic-complete-self-insert)(local-set-key">"'semantic-complete-self-insert)

Followthislinkformoredetails.

5. Configure.emacs

;;golangmode(require'go-mode-load)(require'go-autocomplete);;speedbar;;(speedbar1)(speedbar-add-supported-extension".go")(add-hook'go-mode-hook'(lambda();;gocode(auto-complete-mode1)(setqac-sources'(ac-source-go));;Imenu&Speedbar(setqimenu-generic-expression'(("type""^type*\$[^\t\n\r\f]*\$"1)("func""^func*\$.*\${"1)))(imenu-add-to-menubar"Index");;Outlinemode(make-local-variable'outline-regexp)(setqoutline-regexp"//\\.\\|//[^\r\n\f][^\r\n\f]\\|pack\\|func\\|impo\\|cons\\|var.\\|type\\|\t\t*....")(outline-minor-mode1)(local-set-key"\M-a"'outline-previous-visible-heading)(local-set-key"\M-e"'outline-next-visible-heading);;Menubar(require'easymenu)(defconstgo-hooked-menu'("Gotools"["Gorunbuffer"got]["Goreformatbuffer"go-fmt-buffert]["Gocheckbuffer"go-fix-buffert]))(easy-menu-definego-added-menu(current-local-map)"Gotools"go-hooked-menu)

;;Other(setqshow-trailing-whitespacet)));;helperfunction(defungo()"runcurrentbuffer"(interactive)(compile(concat"gorun"(buffer-file-name))))

;;helperfunction(defungo-fmt-buffer()"rungofmtoncurrentbuffer"(interactive)(ifbuffer-read-only(progn(ding)(message"Bufferisreadonly"))(let((p(line-number-at-pos))(filename(buffer-file-name))(old-max-mini-window-heightmax-mini-window-height))(show-all)(if(get-buffer"*GoReformatErrors*")

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http://www.emacswiki.org/emacs/AutoComplete

(progn(delete-windows-on"*GoReformatErrors*")(kill-buffer"*GoReformatErrors*")))(setqmax-mini-window-height1)(if(=0(shell-command-on-region(point-min)(point-max)"gofmt""*GoReformatOutput*"nil"*GoReformatErrors*"t))(progn(erase-buffer)(insert-buffer-substring"*GoReformatOutput*")(goto-char(point-min))(forward-line(1-p)))(with-current-buffer"*GoReformatErrors*"(progn(goto-char(point-min))(while(re-search-forward""nilt)(replace-matchfilename))(goto-char(point-min))(compilation-mode))))(setqmax-mini-window-heightold-max-mini-window-height)(delete-windows-on"*GoReformatOutput*")(kill-buffer"*GoReformatOutput*"))));;helperfunction(defungo-fix-buffer()"rungofixoncurrentbuffer"(interactive)(show-all)(shell-command-on-region(point-min)(point-max)"gotoolfix-diff"))

6. Congratulations,you'redone!Speedbarisclosedbydefault-removethecommentsymbolsintheline;;(speedbar1)toenablethisfeature,oryoucanuseitthroughM-xspeedbar.

EclipseEclipseisalsoagreatdevelopmenttool.I'llshowyouhowtouseittowriteGoprograms.

Figure1.1EclipsemainpanelforeditingGo

1. DownloadandinstallEclipse2. Downloadgoclipsehttp://code.google.com/p/goclipse/wiki/InstallationInstructions3. Downloadgocode

gocodeinGithub.

https://github.com/nsf/gocode

YouneedtoinstallgitinWindows,usuallyweusemsysgit

Installgocodeinthecommandtool


Youcaninstallfromsourcecodeifyoulike.

4. DownloadandinstallMinGW5. Configureplugins.

Windows->Preferences->Go

(1).ConfigureGocompiler

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http://www.eclipse.org/https://code.google.com/p/goclipse/http://code.google.com/p/goclipse/wiki/InstallationInstructionshttps://code.google.com/p/msysgit/http://sourceforge.net/projects/mingw/files/MinGW/

Figure1.12GoSettinginEclipse

(2).Configuregocode(optional),setgocodepathtowherethegocode.exeis.

Figure1.13gocodeSetting

(3).Configuregdb(optional),setgdbpathtowherethegdb.exeis.

Figure1.14gdbSetting

6. Checktheinstallation

CreateanewGoprojectandhello.gofileasfollowing.

Figure1.15Createanewprojectandfile

Testinstallationasfollows.(youneedtotypecommandinconsoleinEclipse)

Figure1.16TestGoprograminEclipse

IntelliJIDEAPeoplewhohaveworkedwithJavashouldbefamiliarwiththisIDE.ItsupportsGosyntaxhighlightingandintelligentcodecompletion,implementedbyaplugin.

1. DownloadIDEA,thereisnodifferencebetweentheUltimateandCommunityeditions

2. InstalltheGoplugin.ChooseFile-Setting-Plugins,thenclickBrowserrepo.

3. Searchgolang,doubleclickdownloadandinstallandwaitforthedownloadtocomplete.

ClickApply,thenrestart.

4. NowyoucancreateaGoproject.

InputthepositionofyourGosdkinthenextstep-basicallyit'syour$GOROOT.

(SeeablogpostforsetupanduseIntelliJIDEAwithGostepbystep)

VisualStudioVSCodeThisisanawesometexteditorreleasedasopensourcecrossplatformmyMicrosoftwhichtakesthedevelopmentexperiencetoawholenewlevel,https://code.visualstudio.com/.Ithaseverythingamoderntexteditorisexpectedtohaveanddespitebeingbasedonthesamebackendthatatom.ioisbased,itisveryfast.

ItworkswithWindows,Mac,Linux.Ithasgopackagebuilt,itprovidescodelinting.

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http://wuwen.org/tips-about-using-intellij-idea-and-go/https://code.visualstudio.com/

AtomAtomisanawesometexteditorreleasedasopensourcecrossplatform,builtonElectron,andbasedoneverythingweloveaboutourfavoriteeditors.Wedesignedittobedeeplycustomizable,butstillapproachableusingthedefaultconfiguration.

Download:https://atom.io/

GoglandGoglandisthecodenameforanewcommercialIDEbyJetBrainsaimedatprovidinganergonomicenvironmentforGodevelopment.

Theofficialversionisnotyetreleased。

Download:https://www.jetbrains.com/go/

LinksDirectoryPrevioussection:GocommandsNextsection:Summary

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https://atom.io/https://www.jetbrains.com/go/

1.5SummaryInthischapter,wetalkedabouthowtoinstallGousingthreedifferentmethodsincludingfromsourcecode,thestandardpackageandviathird-partytools.ThenweshowedyouhowtoconfiguretheGodevelopmentenvironment,mainlycoveringhowtosetupyour$GOPATH.Afterthat,weintroducedsomestepsforcompilinganddeployingGoprograms.WethencoveredGocommands,includingthecompile,install,formatandtestcommands.Finally,therearemanypowerfultoolstodevelopGoprogramssuchasLiteIDE,SublimeText,VSCode,Atom,Goglang,Vim,Emacs,Eclipse,IntelliJIDEA,etc.YoucanchooseanyoneyoulikeexploringtheworldofGo.

LinksDirectoryPrevioussection:GodevelopmenttoolsNextchapter:Gobasicknowledge

Summary

28

2GobasicknowledgeGoisacompiledsystemprogramminglanguage,anditbelongstotheC-family.However,itscompilationspeedismuchfasterthanotherC-familylanguages.Ithasonly25keywords...evenlessthanthe26lettersoftheEnglishalphabet!Let'stakealookatthesekeywordsbeforewegetstarted.

breakdefaultfuncinterfaceselectcasedefergomapstructchanelsegotopackageswitchconstfallthroughifrangetypecontinueforimportreturnvar

Inthischapter,I'mgoingtoteachyousomebasicGoknowledge.YouwillfindouthowconcisetheGoprogramminglanguageis,andthebeautifuldesignofthelanguage.ProgrammingcanbeveryfuninGo.Afterwecompletethischapter,you'llbefamiliarwiththeabovekeywords.

LinksDirectoryPreviouschapter:Chapter1SummaryNextsection:"Hello,Go"

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29

WhatmakesGodifferentfromotherlanguages?TheGoprogramminglanguagewascreatedwithonegoalinmind,tobeabletobuildscalableweb-applicationsforlargescaleaudiencesinalargeteam.Sothatisthereasontheymadethelanguageasstandardizedaspossible,hencethegofmttoolandthestrictusageguidelinestothelanguagewasforthesakeofnothavingtwofactionsinthedeveloperbase,inotherlanguagestherearereligiouswarsonwheretokeeptheopeningbrace?

publicstaticvoidmain(){

}

orpublicstaticvoidmain(){

}

orforpythonshouldweuse4spacesor6spacesoratabortwotabsandotheruserpreferences.IfyouknowpythonthenyoumightbeawareofPEP8,whichisasetofguidelinesabouthowtowriteelegantcode.

Whilethismightseemtobeashallowproblem,whenthecodebasegrowsandmoreandmorepeopleworkonthesamecodebaseitisbecomesincreasinglydifficulttomaintainthecode's"beauty."Weliveinaworldwhererobotscandriveacar,soweshouldn'tjustwritecode,weshouldwriteelegantcode.

Forotherlanguagestherearemanyvariableswhenitcomestowritingcode.Everylanguageisgoodforitsusecase,butGoisalittlespecialbecauseitwasdesignedatacompanywhichistheverysynonymoftheInternet(anddistributedcomputing).Typicallyinordertooptimizeprograms,developerschoosetowriteJavaoverPythonandC++overJava,butalmostallavailablelanguageswidelyinusewerewrittendecadesagowhen1GBstoragewasmuchpricier.Nowstorageandcomputingisrelativelycheapandcomputersaregettingmultiplescores,butthe"oldlanguages"arenotharnessingconcurrencyinawaythatgodoes.It'snotbecausethoselanguagesarebad;utilizingconcurrencywasn'tarelevantusecasewhiletheolderlanguagesevolved.

TomitigatealltheproblemsthatGooglefacedwithcurrenttools,theywroteasystemslanguagecalledGowhichyouareabouttolearn!Therearemanyadvantagestousinggolangandtherearedisadvantagestoo,foreverycoinhasbothsides.Oneofthesignificantimprovementsinincodeformatting.Googlehasdesignedthelanguagetoavoiddebatesoncodeformatting.Gocodewrittenbyanyoneintheworld(assumingtheyknowandusegofmt)willlookexactlythesame.Thiswon'tseemtomatteruntilyouworkinateam!Alsowhenthecompanyusesautomatedcoderevieworsomeotherfancytechnique,theformattedcodemaybreakinotherlanguageswhichdon'thavestrictandstandardformattingrules,butnotingo!

Gowasdesignedwithconcurrencyinmind,pleasenotethatparallelism!=concurrency,thereisanamazingpostbyRobPikeonthegolangblog,youwillfinditthere,itiswortharead.

AnotherveryimportantchangethatistheconceptofGOPATH.Gonearethedayswhenyouhadtocreateafoldercalledcodeandthencreateworkspacesforeclipseandwhatnot.Nowyouhavetokeeponefoldertreeforgocodewhichwillbeupdatedbythepackagemanagerautomatically.Itisalsorecommendedtocreatefolderswitheitheracustomdomainorthegithubdomain,forexampleIcreatedataskmanagerusinggolangsoIcreatedasetoffolders~/go/src/github.com/thewhitetulip/Tasks

Note:In*nixsystems~standsforhomedirectory,whichisthewindowsequivalentofC:\\Users\\username.Nowthe~/go/istheuniverseforthegocodeinyourmachine.Thisisasignificantimprovementoverotherlanguages;wecanstorethecodeefficientlywithouthassles.Whileitmightseemstrangeatfirst,thisapproachmakealotofsensethantheridiculouspackagenames,i.e.packagenamesgeneratedforotherlanguagesusingreversedomains.

Note:Alongwithsrctherearetwofolderspkgwhichisforpackagesandbinwhichisforbinary.

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30

https://blog.golang.org/concurrency-is-not-parallelismhttps://blog.golang.org/

ThisGOPATHadvantageisn'tjustrestrictedtostoringcodeinparticularfolder.Whenyouhavecreatedfivepackagesforyourproject,youdon'thavetoimportthemlike"import./db".Insteadyoucanuseimport"github.com/thewhitetulip/Tasks/db"sothatwhenexecutinggogetonmyrepo,thegotoolwillfindthepackagefromgithub.com/...pathifitwasn'tdownloadedinitially.Thisstandardizesalotofscrewedupthingsintheprogrammingdiscipline.(

Onthefifthline,weusethekeywordfunctodefinethemainfunction.Thebodyofthefunctionisinsideof{},justlikeC,C++andJava.

Asyoucansee,therearenoarguments.Wewilllearnhowtowritefunctionswithargumentsinjustasecond,andyoucanalsohavefunctionsthathavenoreturnvalueorhaveseveralreturnvalues.

Onthesixthline,wecalledthefunctionPrintfwhichisfromthepackagefmt.Thiswascalledbythesyntax.,whichisverylikePython-style.

Aswementionedinchapter1,thepackage'snameandthenameofthefolderthatcontainsthatpackagecanbedifferent.Herethecomesfromthenameinpackage,notthefolder'sname.

Youmaynoticethattheexampleabovecontainsmanynon-ASCIIcharacters.ThepurposeofshowingthisistotellyouthatGosupportsUTF-8bydefault.YoucanuseanyUTF-8characterinyourprograms.

Eachgofileisinsomepackage,andthatpackageshouldbeadistinctfolderintheGOPATH,butmainisaspecialpackagewhichdoesn'trequireamainfolder.Thisisoneaspectwhichtheyleftoutforstandardization!Butshouldyouchoosetomakeamainfolderthenyouhavetoensurethatyourunthebinaryproperly.Alsoonegocodecan'thavemorethanonemaingofile.

~/go/src/github.com/thewhitetulip/Tasks/main$gobuild~/go/src/github.com/thewhitetulip/Tasks$./main/main

thethinghereisthatwhenyourcodeisusingsomestaticfilesorsomethingelse,thenyououghttorunthebinaryfromtherootoftheapplicationasweseeinthesecondlineabove,Iamrunningthemainbinaryoutsidethemainpackage,sometimesyoumightwonderwhyyourapplicationisn'tworkingthenthismightbeoneofthepossibleproblems,pleasekeepthisinmind.

Onethingyouwillnoticehereisthatgodoesn'tseetousesemicolonstoendastatement,well,itdoes,justthereisaminorcatch,theprogrammerisn'texpectedtoputsemicolons,thecompileraddssemicolonstothegocodewhenitcompileswhichisthereasonthatthis(thankfully!)isasyntaxerror

funcmain(){}

becausethecompileraddsasemicolonattheendofmain()whichisasyntaxerrorandasstatedabove,ithelpsavoidreligiouswars,iwishtheycombinevimandemacsandcreateauniversaleditorwhich'llhelpsavesomemorewars!Butfornowwe'lllearnGo.

ConclusionGousespackage(likemodulesinPython)toorganizeprograms.Thefunctionmain.main()(thisfunctionmustbeinthemainpackage)istheentrypointofanyprogram.Gostandardizeslanguageandmostoftheprogrammingmethodology,savingtimeofdeveloperswhichthey'dhavewastedinreligiouswars.Therecanbeonlyonemainpackageandonlyonemainfunctioninsideagomainpackage.GosupportsUTF-8charactersbecauseoneofthecreatorsofGoisacreatorofUTF-8,soGohassupportedmultiplelanguagesfromthetimeitwasborn.

LinksDirectoryPrevioussection:GobasicknowledgeNextsection:Gofoundation

Hello,Go

32

Hello,Go

33

2.2GofoundationInthissection,wearegoingtoteachyouhowtodefineconstants,variableswithelementarytypesandsomeskillsinGoprogramming.

DefinevariablesTherearemanyformsofsyntaxthatcanbeusedtodefinevariablesinGo.

Thekeywordvaristhebasicformtodefinevariables,noticethatGoputsthevariabletypeafterthevariablename.

//defineavariablewithname“variableName”andtype"type"varvariableNametype

Definemultiplevariables.

//definethreevariableswhichtypesare"type"varvname1,vname2,vname3type

Defineavariablewithinitialvalue.

//defineavariablewithname“variableName”,type"type"andvalue"value"varvariableNametype=value

Definemultiplevariableswithinitialvalues.

/*Definethreevariableswithtype"type",andinitializetheirvalues.vname1isv1,vname2isv2,vname3isv3*/varvname1,vname2,vname3type=v1,v2,v3

Doyouthinkthatit'stootedioustodefinevariablesusethewayabove?Don'tworry,becausetheGoteamhasalsofoundthistobeaproblem.Thereforeifyouwanttodefinevariableswithinitialvalues,wecanjustomitthevariabletype,sothecodewilllooklikethisinstead:

/*Definethreevariableswithouttype"type",andinitializetheirvalues.vname1isv1，vname2isv2，vname3isv3*/varvname1,vname2,vname3=v1,v2,v3

Well,Iknowthisisstillnotsimpleenoughforyou.Let'sseehowwefixit.

/*Definethreevariableswithouttype"type"andwithoutkeyword"var",andinitializetheirvalues.vname1isv1，vname2isv2，vname3isv3*/vname1,vname2,vname3:=v1,v2,v3

Nowitlooksmuchbetter.Use:=toreplacevarandtype,thisiscalledashortassignment.Ithasonelimitation:thisformcanonlybeusedinsideofafunctions.Youwillgetcompileerrorsifyoutrytouseitoutsideoffunctionbodies.Therefore,weusuallyusevartodefineglobalvariables.

Gofoundation

34

_(blank)isaspecialvariablename.Anyvaluethatisgiventoitwillbeignored.Forexample,wegive35tob,anddiscard34.(Thisexamplejustshowyouhowitworks.Itlooksuselessherebecauseweoftenusethissymbolwhenwegetfunctionreturnvalues.)

_,b:=34,35

Ifyoudon'tusevariablesthatyou'vedefinedinyourprogram,thecompilerwillgiveyoucompilationerrors.Trytocompilethefollowingcodeandseewhathappens.

packagemain

funcmain(){variint}

ConstantsSo-calledconstantsarethevaluesthataredeterminedduringcompiletimeandyoucannotchangethemduringruntime.InGo,youcanusenumber,booleanorstringastypesofconstants.

Defineconstantsasfollows.

constconstantName=value//youcanassigntypeofconstantsifit'snecessaryconstPifloat32=3.1415926

Moreexamples.

constPi=3.1415926consti=10000constMaxThread=10constprefix="astaxie_"

Elementarytypes

Boolean

InGo,weusebooltodefineavariableasbooleantype,thevaluecanonlybetrueorfalse,andfalsewillbethedefaultvalue.(Youcannotconvertvariables'typebetweennumberandboolean!)

//samplecodevarisActivebool//globalvariablevarenabled,disabled=true,false//omittypeofvariables

functest(){varavailablebool//localvariablevalid:=false//briefstatementofvariableavailable=true//assignvaluetovariable}

Numericaltypes

Integertypesincludebothsignedandunsignedintegertypes.Gohasintanduintatthesametime,theyhavesamelength,butspecificlengthdependsonyouroperatingsystem.Theyuse32-bitin32-bitoperatingsystems,and64-bitin64-bitoperatingsystems.Goalsohastypesthathavespecificlengthincludingrune,int8,int16,int32,int64,byte,uint8,uint16,uint32,uint64.Notethatruneisaliasofint32andbyteisaliasofuint8.

Gofoundation

35

Oneimportantthingyoushouldknowthatyoucannotassignvaluesbetweenthesetypes,thisoperationwillcausecompileerrors.

varaint8

varbint32

c:=a+b

Althoughint32hasalongerlengththanint8,andhasthesametypeasint,youcannotassignvaluesbetweenthem.(cwillbeassertedastypeinthere)

Floattypeshavethefloat32andfloat64typesandnotypecalledfloat.Thelatteroneisthedefaulttypeifusingbriefstatement.

That'sall?No!Gosupportscomplexnumbersaswell.complex128(witha64-bitrealand64-bitimaginarypart)isthedefaulttype,ifyouneedasmallertype,thereisonecalledcomplex64(witha32-bitrealand32-bitimaginarypart).ItsformisRE+IMi,whereREisrealpartandIMisimaginarypart,thelastiistheimaginarynumber.Thereisaexampleofcomplexnumber.

varccomplex64=5+5i//output:(5+5i)fmt.Printf("Valueis:%v",c)

String

WejusttalkedabouthowGousestheUTF-8characterset.Stringsarerepresentedbydoublequotes""orbackticks ̀ `.

//samplecodevarfrenchHellostring//basicformtodefinestringvaremptyStringstring=""//defineastringwithemptystringfunctest(){no,yes,maybe:="no","yes","maybe"//briefstatementjapaneseHello:="Ohaiou"frenchHello="Bonjour"//basicformofassignvalues}

It'simpossibletochangestringvaluesbyindex.Youwillgeterrorswhenyoucompilethefollowingcode.

varsstring="hello"s[0]='c'

WhatifIreallywanttochangejustonecharacterinastring?Trythefollowingcode.

s:="hello"c:=[]byte(s)//convertstringto[]bytetypec[0]='c's2:=string(c)//convertbacktostringtypefmt.Printf("%s\n",s2)

Youusethe+operatortocombinetwostrings.

s:="hello,"m:="world"a:=s+mfmt.Printf("%s\n",a)

andalso.

Gofoundation

36

s:="hello"s="c"+s[1:]//youcannotchangestringvaluesbyindex,butyoucangetvaluesinstead.fmt.Printf("%s\n",s)

WhatifIwanttohaveamultiple-linestring?

m:=`helloworld`

̀ willnotescapeanycharactersinastring.

ErrortypesGohasoneerrortypeforpurposeofdealingwitherrormessages.Thereisalsoapackagecallederrorstohandleerrors.

err:=errors.New("emitmachodwarf:elfheadercorrupted")iferr!=nil{fmt.Print(err)}

Underlyingdatastructure

ThefollowingpicturecomesfromanarticleaboutGodatastructureinRussCox'sBlog.Asyoucansee,Goutilizesblocksofmemorytostoredata.

Figure2.1Gounderlyingdatastructure

Someskills

Definebygroup

Ifyouwanttodefinemultipleconstants,variablesorimportpackages,youcanusethegroupform.

Basicform.

import"fmt"import"os"

consti=100constpi=3.1415constprefix="Go_"

variintvarpifloat32varprefixstring

Groupform.

Gofoundation

37

http://research.swtch.com/godatahttp://research.swtch.com/

import("fmt""os")

const(i=100pi=3.1415prefix="Go_")

var(iintpifloat32prefixstring)

Unlessyouassignthevalueofconstantisiota,thefirstvalueofconstantinthegroupconst()willbe0.Iffollowingconstantsdon'tassignvaluesexplicitly,theirvalueswillbethesameasthelastone.Ifthevalueoflastconstantisiota,thevaluesoffollowingconstantswhicharenotassignedareiotaalso.

iotaenumerate

Gohasonekeywordcallediota,thiskeywordistomakeenum,itbeginswith0,increasedby1.

const(x=iota//x==0y=iota//y==1z=iota//z==2w//Ifthereisnoexpressionaftertheconstantsname,itusesthelastexpression,//soit'ssayingw=iotaimplicitly.Thereforew==3,andyandzbothcanomit"=iota"aswell.)

constv=iota//onceiotameetskeyword`const`,itresetsto`0`,sov=0.

const(e,f,g=iota,iota,iota//e=0,f=0,g=0valuesofiotaaresameinoneline.)

Somerules

ThereasonthatGoisconcisebecauseithassomedefaultbehaviors.

Anyvariablethatbeginswithacapitallettermeansitwillbeexported,privateotherwise.Thesameruleappliesforfunctionsandconstants,nopublicorprivatekeywordexistsinGo.

array,slice,map

array

arrayisanarrayobviously,wedefineoneasfollows.

vararr[n]type

in[n]type,nisthelengthofthearray,typeisthetypeofitselements.Likeotherlanguages,weuse[]togetorsetelementvalueswithinarrays.

Gofoundation

38

vararr[10]int//anarrayoftype[10]intarr[0]=42//arrayis0-basedarr[1]=13//assignvaluetoelementfmt.Printf("Thefirstelementis%d\n",arr[0])//getelementvalue,itreturns42fmt.Printf("Thelastelementis%d\n",arr[9])//itreturnsdefaultvalueof10thelementinthisarray,whichis0inthiscase.

Becauselengthisapartofthearraytype,[3]intand[4]intaredifferenttypes,sowecannotchangethelengthofarrays.Whenyouusearraysasarguments,functionsgettheircopiesinsteadofreferences!Ifyouwanttousereferences,youmaywanttouseslice.We'lltalkaboutlater.

It'spossibletouse:=whenyoudefinearrays.

a:=[3]int{1,2,3}//defineanintarraywith3elements

b:=[10]int{1,2,3}//defineaintarraywith10elements,ofwhichthefirstthreeareassigned.//Therestofthemusethedefaultvalue0.

c:=[...]int{4,5,6}//use`…`toreplacethelengthparameterandGowillcalculateitforyou.

Youmaywanttousearraysasarrays'elements.Let'sseehowtodothis.

//defineatwo-dimensionalarraywith2elements,andeachelementhas4elements.doubleArray:=[2][4]int{[4]int{1,2,3,4},[4]int{5,6,7,8}}

//Thedeclarationcanbewrittenmoreconciselyasfollows.easyArray:=[2][4]int{{1,2,3,4},{5,6,7,8}}

Arrayunderlyingdatastructure.

Figure2.2Multidimensionalarraymappingrelationship

sliceInmanysituations,thearraytypeisnotagoodchoice-forinstancewhenwedon'tknowhowlongthearraywillbewhenwedefineit.Thus,weneeda"dynamicarray".ThisiscalledsliceinGo.

sliceisnotreallyadynamicarray.It'sareferencetype.slicepointstoanunderlyingarraywhosedeclarationissimilartoarray,butdoesn'tneedlength.

//justlikedefininganarray,butthistime,weexcludethelength.varfslice[]int

Thenwedefineaslice,andinitializeitsdata.

slice:=[]byte{'a','b','c','d'}

slicecanredefineexistingslicesorarrays.sliceusesarray[i:j]toslice,whereiisthestartindexandjisendindex,butnoticethatarray[j]willnotbeslicedsincethelengthofthesliceisj-i.

Gofoundation

39

//defineanarraywith10elementswhosetypesarebytesvarar=[10]byte{'a','b','c','d','e','f','g','h','i','j'}

//definetwosliceswithtype[]bytevara,b[]byte

//'a'pointstoelementsfrom3rdto5thinarrayar.a=ar[2:5]//now'a'haselementsar[2],ar[3]andar[4]

//'b'isanothersliceofarrayarb=ar[3:5]//now'b'haselementsar[3]andar[4]

Noticethedifferencesbetweensliceandarraywhenyoudefinethem.Weuse[…]toletGocalculatelengthbutuse[]todefinesliceonly.

Theirunderlyingdatastructure.

Figure2.3Correspondencebetweensliceandarray

slicehassomeconvenientoperations.

sliceis0-based,ar[:n]equalstoar[0:n]Thesecondindexwillbethelengthofsliceifomitted,ar[n:]equalstoar[n:len(ar)].Youcanusear[:]toslicewholearray,reasonsareexplainedinfirsttwostatements.

Moreexamplespertainingtoslice

//defineanarrayvararray=[10]byte{'a','b','c','d','e','f','g','h','i','j'}//definetwoslicesvaraSlice,bSlice[]byte

//someconvenientoperationsaSlice=array[:3]//equalstoaSlice=array[0:3]aSlicehaselementsa,b,caSlice=array[5:]//equalstoaSlice=array[5:10]aSlicehaselementsf,g,h,i,jaSlice=array[:]//equalstoaSlice=array[0:10]aSlicehasallelements

//slicefromsliceaSlice=array[3:7]//aSlicehaselementsd,e,f,g，len=4，cap=7bSlice=aSlice[1:3]//bSlicecontainsaSlice[1],aSlice[2],soithaselementse,fbSlice=aSlice[:3]//bSlicecontainsaSlice[0],aSlice[1],aSlice[2],soithasd,e,fbSlice=aSlice[0:5]//slicecouldbeexpandedinrangeofcap,nowbSlicecontainsd,e,f,g,hbSlice=aSlice[:]//bSlicehassameelementsasaSlicedoes,whichared,e,f,g

sliceisareferencetype,soanychangeswillaffectothervariablespointingtothesamesliceorarray.Forinstance,inthecaseofaSliceandbSliceabove,ifyouchangethevalueofanelementinaSlice,bSlicewillbechangedaswell.

sliceislikeastructbydefinitionanditcontains3parts.

Apointerthatpointstowhereslicestarts.Thelengthofslice.Capacity,thelengthfromstartindextoendindexofslice.

Array_a:=[10]byte{'a','b','c','d','e','f','g','h','i','j'}Slice_a:=Array_a[2:5]

Theunderlyingdatastructureofthecodeaboveasfollows.

Gofoundation

40

Figure2.4Arrayinformationofslice

Therearesomebuilt-infunctionsforslice.

lengetsthelengthofslice.capgetsthemaximumlengthofsliceappendappendsoneormoreelementstoslice,andreturnsslice.copycopieselementsfromoneslicetotheother,andreturnsthenumberofelementsthatwerecopied.

Attention:appendwillchangethearraythatslicepointsto,andaffectotherslicesthatpointtothesamearray.Also,ifthereisnotenoughlengthfortheslice((cap-len)==0),appendreturnsanewarrayforthisslice.Whenthishappens,otherslicespointingtotheoldarraywillnotbeaffected.

map

mapbehaveslikeadictionaryinPython.Usetheformmap[keyType]valueTypetodefineit.

Let'sseesomecode.The'set'and'get'valuesinmaparesimilartoslice,howevertheindexinslicecanonlybeoftype'int'whilemapcanusemuchmorethanthat:forexampleint,string,orwhateveryouwant.Also,theyareallabletouse==and!=tocomparevalues.

//usestringasthekeytype,intasthevaluetype,and`make`initializeit.varnumbersmap[string]int//anotherwaytodefinemapnumbers:=make(map[string]int)numbers["one"]=1//assignvaluebykeynumbers["ten"]=10numbers["three"]=3

fmt.Println("Thethirdnumberis:",numbers["three"])//getvalues//Itprints:Thethirdnumberis:3

Somenoteswhenyouusemap.

mapisdisorderly.Everytimeyouprintmapyouwillgetdifferentresults.It'simpossibletogetvaluesbyindex-youhavetousekey.mapdoesn'thaveafixedlength.It'sareferencetypejustlikeslice.lenworksformapalso.Itreturnshowmanykeysthatmaphas.It'squiteeasytochangethevaluethroughmap.Simplyusenumbers["one"]=11tochangethevalueofkeyoneto11.

Youcanuseformkey:valtoinitializemap'svalues,andmaphasbuilt-inmethodstocheckifthekeyexists.

Usedeletetodeleteanelementinmap.

//Initializeamaprating:=map[string]float32{"C":5,"Go":4.5,"Python":4.5,"C++":2}//maphastworeturnvalues.Forthesecondreturnvalue,ifthekeydoesn't//exist，'ok'returnsfalse.Itreturnstrueotherwise.csharpRating,ok:=rating["C#"]ifok{fmt.Println("C#isinthemapanditsratingis",csharpRating)}else{fmt.Println("WehavenoratingassociatedwithC#inthemap")}

delete(rating,"C")//deleteelementwithkey"c"

AsIsaidabove,mapisareferencetype.Iftwomapspointtosameunderlyingdata,anychangewillaffectbothofthem.

Gofoundation

41

m:=make(map[string]string)m["Hello"]="Bonjour"m1:=mm1["Hello"]="Salut"//nowthevalueofm["hello"]isSalut

make,newmakedoesmemoryallocationforbuilt-inmodels,suchasmap,slice,andchannel,whilenewisfortypes'memoryallocation.

new(T)allocateszero-valuetotypeT'smemory,returnsitsmemoryaddress,whichisthevalueoftype*T.ByGo'sdefinition,itreturnsapointerwhichpointstotypeT'szero-value.

newreturnspointers.

Thebuilt-infunctionmake(T,args)hasdifferentpurposesthannew(T).makecanbeusedforslice,map,andchannel,andreturnsatypeTwithaninitialvalue.Thereasonfordoingthisisbecausetheunderlyingdataofthesethreetypesmustbeinitializedbeforetheypointtothem.Forexample,aslicecontainsapointerthatpointstotheunderlyingarray,lengthandcapacity.Beforethesedataareinitialized,sliceisnil,soforslice,mapandchannel,makeinitializestheirunderlyingdataandassignssomesuitablevalues.

makereturnsnon-zerovalues.

Thefollowingpictureshowshownewandmakearedifferent.

Figure2.5Underlyingmemoryallocationofmakeandnew

Zero-valuedoesnotmeanemptyvalue.It'sthevaluethatvariablesdefaulttoinmostcases.Hereisalistofsomezero-values.

int0int80int320int640uint0x0rune0//theactualtypeofruneisint32byte0x0//theactualtypeofbyteisuint8float320//lengthis4bytefloat640//lengthis8byteboolfalsestring""

LinksDirectoryPrevioussection:"Hello,Go"Nextsection:Controlstatementsandfunctions

Gofoundation

42

2.3ControlstatementsandfunctionsInthissection,wearegoingtotalkaboutcontrolstatementsandfunctionoperationsinGo.

ControlstatementThegreatestinventioninprogrammingisflowcontrol.Becauseofthem,youareabletousesimplecontrolstatementsthatcanbeusedtorepresentcomplexlogic.Therearethreecategoriesofflowcontrol:conditional,cyclecontrolandunconditionaljump.

if

ifwillmostlikelybethemostcommonkeywordinyourprograms.Ifitmeetstheconditions,thenitdoessomethinganditdoessomethingelseifnot.

ifdoesn'tneedparenthesesinGo.

ifx>10{fmt.Println("xisgreaterthan10")}else{fmt.Println("xislessthanorequalto10")}

ThemostusefulthingconcerningifinGoisthatitcanhaveoneinitializationstatementbeforetheconditionalstatement.Thescopeofthevariablesdefinedinthisinitializationstatementareonlyavailableinsidetheblockofthedefiningif.

//initializex,thencheckifxgreaterthanifx:=computedValue();x>10{fmt.Println("xisgreaterthan10")}else{fmt.Println("xislessthan10")}

//thefollowingcodewillnotcompilefmt.Println(x)

Useif-elseformultipleconditions.

ifinteger==3{fmt.Println("Theintegerisequalto3")}elseifinteger<3{fmt.Println("Theintegerislessthan3")}else{fmt.Println("Theintegerisgreaterthan3")}

gotoGohasagotokeyword,butbecarefulwhenyouuseit.gotoreroutesthecontrolflowtoapreviouslydefinedlabelwithinthebodyofsamecodeblock.


43

funcmyFunc(){i:=0Here://labelendswith":"fmt.Println(i)i++gotoHere//jumptolabel"Here"}

Thelabelnameiscasesensitive.

for

foristhemostpowerfulcontrollogicinGo.Itcanreaddatainloopsanditerativeoperations,justlikewhile.

forexpression1;expression2;expression3{//...}

expression1,expression2andexpression3areallexpressions,whereexpression1andexpression3arevariabledefinitionsorreturnvaluesfromfunctions,andexpression2isaconditionalstatement.expression1willbeexecutedoncebeforelooping,andexpression3willbeexecutedaftereachloop.

Examplesaremoreusefulthanwords.

packagemain

import"fmt"

funcmain(){sum:=0;forindex:=0;index<10;index++{sum+=index}fmt.Println("sumisequalto",sum)}//Print：sumisequalto45

Sometimesweneedmultipleassignments,butGodoesn'thavethe,operator,soweuseparallelassignmentlikei,j=i+1,j-1.

Wecanomitexpression1andexpression3iftheyarenotnecessary.

sum:=1for;sum<1000;{sum+=sum}

Omit;aswell.Feelfamiliar?Yes,it'sidenticaltowhile.

sum:=1forsum<1000{sum+=sum}

Therearetwoimportantoperationsinloopswhicharebreakandcontinue.breakjumpsoutoftheloop,andcontinueskipsthecurrentloopandstartsthenextone.Ifyouhavenestedloops,usebreakalongwithlabels.


44

forindex:=10;index>0;index--{ifindex==5{break//orcontinue}fmt.Println(index)}//breakprints10、9、8、7、6//continueprints10、9、8、7、6、4、3、2、1

forcanreaddatafromarray,slice,mapandstringwhenitisusedtogetherwithrange.

fork,v:=rangemap{fmt.Println("map'skey:",k)fmt.Println("map'sval:",v)}

BecauseGosupportsmulti-valuereturnsandgivescompileerrorswhenyoudon'tusevaluesthatweredefined,youmaywanttouse_todiscardcertainreturnvalues.

for_,v:=rangemap{fmt.Println("map'sval:",v)}

Withgoyoucanaswellcreateaninfiniteloop,whichisequivalenttowhiletrue{...}inotherlanguges.

for{//yourlogic}

switchSometimesyoumayfindthatyouareusingtoomanyif-elsestatementstoimplementsomelogic,whichmaymakeitdifficulttoreadandmaintaininthefuture.Thisistheperfecttimetousetheswitchstatementtosolvethisproblem.

switchsExpr{caseexpr1:someinstructionscaseexpr2:someotherinstructionscaseexpr3:someotherinstructionsdefault:othercode}

ThetypeofsExpr,expr1,expr2,andexpr3mustbethesame.switchisveryflexible.Conditionsdon'thavetobeconstantsanditexecutesfromtoptobottomuntilitmatchesconditions.Ifthereisnostatementafterthekeywordswitch,thenitmatchestrue.

i:=10switchi{case1:fmt.Println("iisequalto1")case2,3,4:fmt.Println("iisequalto2,3or4")case10:fmt.Println("iisequalto10")default:fmt.Println("AllIknowisthatiisaninteger")}


45

Inthefifthline,weputmanyvaluesinonecase,andwedon'tneedtoaddthebreakkeywordattheendofcase'sbody.Itwilljumpoutoftheswitchbodyonceitmatchedanycase.Ifyouwanttocontinuetomatchingmorecases,youneedtousethefallthroughstatement.

integer:=6switchinteger{case4:fmt.Println("integer

packagemain

import"fmt"

//returngreatervaluebetweenaandbfuncmax(a,bint)int{ifa>b{returna}returnb}

funcmain(){x:=3y:=4z:=5

max_xy:=max(x,y)//callfunctionmax(x,y)max_xz:=max(x,z)//callfunctionmax(x,z)

fmt.Printf("max(%d,%d)=%d\n",x,y,max_xy)fmt.Printf("max(%d,%d)=%d\n",x,z,max_xz)fmt.Printf("max(%d,%d)=%d\n",y,z,max(y,z))//callfunctionhere}

Intheaboveexample,therearetwoargumentsinthefunctionmax,theirtypesarebothintsothefirsttypecanbeomitted.Forinstance,a,bintinsteadofaint,bint.Thesamerulesapplyforadditionalarguments.Noticeherethatmaxonlyhasonereturnvalue,soweonlyneedtowritethetypeofitsreturnvalue-thisistheshortformofwritingit.

Multi-valuereturn

OnethingthatGoisbetteratthanCisthatitsupportsmulti-valuereturns.

We'llusethefollowingexamplehere.

packagemain

import"fmt"

//returnresultsofA+BandA*BfuncSumAndProduct(A,Bint)(int,int){returnA+B,A*B}

funcmain(){x:=3y:=4

xPLUSy,xTIMESy:=SumAndProduct(x,y)

fmt.Printf("%d+%d=%d\n",x,y,xPLUSy)fmt.Printf("%d*%d=%d\n",x,y,xTIMESy)}

Theaboveexamplereturnstwovalueswithoutnames-youhavetheoptionofnamingthemalso.Ifwenamedthereturnvalues,wewouldjustneedtousereturntoreturnthevaluessincetheyareinitializedinthefunctionautomatically.Noticethatifyourfunctionsaregoingtobeusedoutsideofthepackage,whichmeansyourfunctionnamesstartwithacapitalletter,you'dbetterwritecompletestatementsforreturn;itmakesyourcodemorereadable.

funcSumAndProduct(A,Bint)(addint,multipliedint){add=A+Bmultiplied=A*Breturn}


47

Variadicfunctions

Gosupportsfunctionswithavariablenumberofarguments.Thesefunctionsarecalled"variadic",whichmeansthefunctionallowsanuncertainnumbersofarguments.

funcmyfunc(arg...int){}

arg…inttellsGothatthisisafunctionthathasvariablearguments.Noticethattheseargumentsaretypeint.Inthebodyoffunction,theargbecomesasliceofint.

for_,n:=rangearg{fmt.Printf("Andthenumberis:%d\n",n)}

Passbyvalueandpointers

Whenwepassanargumenttothefunctionthatwascalled,thatfunctionactuallygetsthecopyofourvariablessoanychangewillnotaffecttotheoriginalvariable.

Let'sseeoneexampleinordertoprovewhati'msaying.

packagemain

import"fmt"

//simplefunctiontoadd1toafuncadd1(aint)int{a=a+1//wechangevalueofareturna//returnnewvalueofa}

funcmain(){x:=3

fmt.Println("x=",x)//shouldprint"x=3"

x1:=add1(x)//calladd1(x)

fmt.Println("x+1=",x1)//shouldprint"x+1=4"fmt.Println("x=",x)//shouldprint"x=3"}

Canyouseethat?Eventhoughwecalledadd1withx,theoriginvalueofxdoesn'tchange.

Thereasonisverysimple:whenwecalledadd1,wegaveacopyofxtoit,notthexitself.

NowyoumayaskhowIcanpasstherealxtothefunction.

Weneedusepointershere.Weknowvariablesarestoredinmemoryandtheyhavesomememoryaddresses.So,ifwewanttochangethevalueofavariable,wemustchangeitsmemoryaddress.Thereforethefunctionadd1hastoknowthememoryaddressofxinordertochangeitsvalue.Herewepass&xtothefunction,andchangetheargument'stypetothepointertype*int.Beawarethatwepassacopyofthepointer,notcopyofvalue.


48

packagemain

import"fmt"

//simplefunctiontoadd1toafuncadd1(a*int)int{*a=*a+1//wechangedvalueofareturn*a//returnnewvalueofa}

funcmain(){x:=3

fmt.Println("x=",x)//shouldprint"x=3"

x1:=add1(&x)//calladd1(&x)passmemoryaddressofx

fmt.Println("x+1=",x1)//shouldprint"x+1=4"fmt.Println("x=",x)//shouldprint"x=4"}

Nowwecanchangethevalueofxinthefunctions.Whydoweusepointers?Whataretheadvantages?

Allowsustousemorefunctionstooperateononevariable.Lowcostbypassingmemoryaddresses(8bytes),copyisnotanefficientway,bothintermsoftimeandspace,topassvariables.channel,sliceandmaparereferencetypes,sotheyusepointerswhenpassingtofunctionsbydefault.(Attention:Ifyouneedtochangethelengthofslice,youhavetopasspointersexplicitly)

deferGohasawelldesignedkeywordcalleddefer.Youcanhavemanydeferstatementsinonefunction;theywillexecuteinreverseorderwhentheprogramexecutestotheendoffunctions.Inthecasewheretheprogramopenssomeresourcefiles,thesefileswouldhavetobeclosedbeforethefunctioncanreturnwitherrors.Let'sseesomeexamples.

funcReadWrite()bool{file.Open("file")//DosomeworkiffailureX{file.Close()returnfalse}

iffailureY{file.Close()returnfalse}

file.Close()returntrue}

Wesawsomecodebeingrepeatedseveraltimes.defersolvesthisproblemverywell.Itdoesn'tonlyhelpyoutowritecleancodebutalsomakesyourcodemorereadable.


49

funcReadWrite()bool{file.Open("file")deferfile.Close()iffailureX{returnfalse}iffailureY{returnfalse}returntrue}

Iftherearemorethanonedefers,theywillexecutebyreverseorder.Thefollowingexamplewillprint43210.

fori:=0;i<5;i++{deferfmt.Printf("%d",i)}

Functionsasvaluesandtypes

FunctionsarealsovariablesinGo,wecanusetypetodefinethem.Functionsthathavethesamesignaturecanbeseenasthesametype.

typetypeNamefunc(input1inputType1,input2inputType2[,...])(result1resultType1[,...])

What'stheadvantageofthisfeature?Theansweristhatitallowsustopassfunctionsasvalues.

packagemain

import"fmt"

typetestIntfunc(int)bool//defineafunctiontypeofvariable

funcisOdd(integerint)bool{returninteger%2!=0}

funcisEven(integerint)bool{returninteger%2==0}

//passthefunction`f`asanargumenttoanotherfunctionfuncfilter(slice[]int,ftestInt)[]int{varresult[]intfor_,value:=rangeslice{iff(value){result=append(result,value)}}returnresult}

varslice=[]int{1,2,3,4,5,7}

funcmain(){odd:=filter(slice,isOdd)even:=filter(slice,isEven)

fmt.Println("slice=",slice)fmt.Println("Oddelementsofsliceare:",odd)fmt.Println("Evenelementsofsliceare:",even)}


50

It'sveryusefulwhenweuseinterfaces.AsyoucanseetestIntisavariablethathasafunctionastypeandthereturnedvaluesandargumentsoffilterarethesameasthoseoftestInt.Therefore,wecanhavecomplexlogicinourprograms,whilemaintainingflexibilityinourcode.

PanicandRecover

Godoesn'thavetry-catchstructurelikeJavadoes.Insteadofthrowingexceptions,Gousespanicandrecovertodealwitherrors.However,youshouldn'tusepanicverymuch,althoughit'spowerful.

Panicisabuilt-infunctiontobreakthenormalflowofprogramsandgetintopanicstatus.WhenafunctionFcallspanic,Fwillnotcontinueexecutingbutitsdeferfunctionswillcontinuetoexecute.ThenFgoesbacktothebreakpointwhichcausedthepanicstatus.Theprogramwillnotterminateuntilallofthesefunctionsreturnwithpanictothefirstlevelofthatgoroutine.paniccanbeproducedbycallingpanicintheprogram,andsomeerrorsalsocausepaniclikearrayaccessoutofboundserrors.

Recoverisabuilt-infunctiontorecovergoroutinesfrompanicstatus.Callingrecoverindeferfunctionsisusefulbecausenormalfunctionswillnotbeexecutedwhentheprogramisinthepanicstatus.Itcatchespanicvaluesiftheprogramisinthepanicstatus,anditgetsniliftheprogramisnotinpanicstatus.

Thefollowingexampleshowshowtousepanic.

varuser=os.Getenv("USER")

funcinit(){ifuser==""{panic("novaluefor$USER")}}

Thefollowingexampleshowshowtocheckpanic.

functhrowsPanic(ffunc())(bbool){deferfunc(){ifx:=recover();x!=nil{b=true}}()f()//iffcausespanic,itwillrecoverreturn}

mainfunctionandinitfunction

Gohastworetentionswhicharecalledmainandinit,whereinitcanbeusedinallpackagesandmaincanonlybeusedinthemainpackage.Thesetwofunctionsarenotabletohaveargumentsorreturnvalues.Eventhoughwecanwritemanyinitfunctionsinonepackage,Istronglyrecommendwritingonlyoneinitfunctionforeachpackage.

Goprogramswillcallinit()andmain()automatically,soyoudon'tneedtocallthembyyourself.Foreverypackage,theinitfunctionisoptional,butpackagemainhasoneandonlyonemainfunction.

Programsinitializeandbeginexecutionfromthemainpackage.Ifthemainpackageimportsotherpackages,theywillbeimportedinthecompiletime.Ifonepackageisimportedmanytimes,itwillbeonlycompiledonce.Afterimportingpackages,programswillinitializetheconstantsandvariableswithintheimportedpackages,thenexecutetheinitfunctionifitexists,andsoon.Afteralltheotherpackagesareinitialized,programswillinitializeconstantsandvariablesinthemainpackage,thenexecutetheinitfunctioninsidethepackageifitexists.Thefollowingfigureshowstheprocess.

Figure2.6FlowofprogramsinitializationinGo


51

import

WeuseimportveryofteninGoprogramsasfollows.

import("fmt")

Thenweusefunctionsinthatpackageasfollows.

fmt.Println("helloworld")

fmtisfromGostandardlibrary,itislocatedwithin$GOROOT/pkg.Gosupportsthird-partypackagesintwoways.

1. Relativepathimport"./model"//loadpackageinthesamedirectory,Idon'trecommendthisway.2. Absolutepathimport"shorturl/model"//loadpackageinpath"$GOPATH/pkg/shorturl/model"

Therearesomespecialoperatorswhenweimportpackages,andbeginnersarealwaysconfusedbytheseoperators.

1. Dotoperator.Sometimeweseepeopleusefollowingwaytoimportpackages.

import(."fmt")

Thedotoperatormeansyoucanomitthepackagenamewhenyoucallfunctionsinsideofthatpackage.Nowfmt.Printf("Helloworld")becomestoPrintf("Helloworld").

2. Aliasoperation.Itchangesthenameofthepackagethatweimportedwhenwecallfunctionsthatbelongtothatpackage.

import(f"fmt")

Nowfmt.Printf("Helloworld")becomestof.Printf("Helloworld").3. _operator.Thisistheoperatorthatisdifficulttounderstandwithoutsomeoneexplainingittoyou.

import("database/sql"_"github.com/ziutek/mymysql/godrv")

The_operatoractuallymeanswejustwanttoimportthatpackageandexecuteitsinitfunction,andwearenotsureifwewanttousethefunctionsbelongingtothatpackage.

LinksDirectoryPrevioussection:GofoundationNextsection:struct


52

2.4struct

structWecandefinenewtypesofcontainersofotherpropertiesorfieldsinGojustlikeinotherprogramminglanguages.Forexample,wecancreateatypecalledpersontorepresentaperson,withfieldsnameandage.Wecallthiskindoftypeastruct.

typepersonstruct{namestringageint}

Lookhoweasyitistodefineastruct!

Therearetwofields.

nameisastringusedtostoreaperson'sname.ageisaintusedtostoreaperson'sage.

Let'sseehowtouseit.

typepersonstruct{namestringageint}

varPperson//pispersontype

P.name="Astaxie"//assign"Astaxie"tothefield'name'ofpP.age=25//assign25tofield'age'ofpfmt.Printf("Theperson'snameis%s\n",P.name)//accessfield'name'ofp

Therearethreemorewaystoinitializeastruct.

Assigninitialvaluesbyorder

P:=person{"Tom",25}

Usetheformatfield:valuetoinitializethestructwithoutorder

P:=person{age:24,name:"Bob"}

Defineananonymousstruct,theninitializeit

P:=struct{namestring;ageint}{"Amy",18}

Let'sseeacompleteexample.

struct

53

packagemain

import"fmt"

//defineanewtypetypepersonstruct{namestringageint}

//structispassedbyvalue//comparetheageoftwopeople,thenreturntheolderpersonanddifferencesofagefuncOlder(p1,p2person)(person,int){ifp1.age>p2.age{returnp1,p1.age-p2.age}returnp2,p2.age-p1.age}

funcmain(){vartomperson

tom.name,tom.age="Tom",18bob:=person{age:25,name:"Bob"}paul:=person{"Paul",43}

tb_Older,tb_diff:=Older(tom,bob)tp_Older,tp_diff:=Older(tom,paul)bp_Older,bp_diff:=Older(bob,paul)

fmt.Printf("Of%sand%s,%sisolderby%dyears\n",tom.name,bob.name,tb_Older.name,tb_diff)fmt.Printf("Of%sand%s,%sisolderby%dyears\n",tom.name,paul.name,tp_Older.name,tp_diff)fmt.Printf("Of%sand%s,%sisolderby%dyears\n",bob.name,paul.name,bp_Older.name,bp_diff)}

embeddedfieldsinstructI'vejustintroducedtoyouhowtodefineastructwithfieldnamesandtype.Infact,Gosupportsfieldswithoutnames,butwithtypes.Wecalltheseembeddedfields.

Whentheembeddedfieldisastruct,allthefieldsinthatstructwillimplicitlybethefieldsinthestructinwhichithasbeenembedded.

Let'sseeoneexample.

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packagemain

import"fmt"

typeHumanstruct{namestringageintweightint}

typeStudentstruct{Human//embeddedfield,itmeansStudentstructincludesallfieldsthatHumanhas.specialtystring}

funcmain(){//instantiateandinitializeastudentmark:=Student{Human{"Mark",25,120},"ComputerScience"}

//accessfieldsfmt.Println("Hisnameis",mark.name)fmt.Println("Hisageis",mark.age)fmt.Println("Hisweightis",mark.weight)fmt.Println("Hisspecialtyis",mark.specialty)

//modifymark'sspecialtymark.specialty="AI"fmt.Println("Markchangedhisspecialty")fmt.Println("Hisspecialtyis",mark.specialty)

fmt.Println("Markbecomeold.Heisnotanathleteanymore")mark.age=46mark.weight+=60fmt.Println("Hisageis",mark.age)fmt.Println("Hisweightis",mark.weight)}

Figure2.7EmbeddinginStudentandHuman

WeseethatwecanaccesstheageandnamefieldsinStudentjustlikewecaninHuman.Thisishowembeddedfieldswork.It'sverycool,isn'tit?Holdon,there'ssomethingcooler!YoucanevenuseStudenttoaccessHumaninthisembeddedfield!

mark.Human=Human{"Marcus",55,220}mark.Human.age-=1

AllthetypesinGocanbeusedasembeddedfields.

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packagemain

import"fmt"

typeSkills[]string

typeHumanstruct{namestringageintweightint}

typeStudentstruct{Human//structasembeddedfieldSkills//stringsliceasembeddedfieldint//built-intypeasembeddedfieldspecialtystring}

funcmain(){//initializeStudentJanejane:=Student{Human:Human{"Jane",35,100},specialty:"Biology"}//accessfieldsfmt.Println("Hernameis",jane.name)fmt.Println("Herageis",jane.age)fmt.Println("Herweightis",jane.weight)fmt.Println("Herspecialtyis",jane.specialty)//modifyvalueofskillfieldjane.Skills=[]string{"anatomy"}fmt.Println("Herskillsare",jane.Skills)fmt.Println("Sheacquiredtwonewones")jane.Skills=append(jane.Skills,"physics","golang")fmt.Println("Herskillsnoware",jane.Skills)//modifyembeddedfieldjane.int=3fmt.Println("Herpreferrednumberis",jane.int)}

Intheaboveexample,wecanseethatalltypescanbeembeddedfieldsandwecanusefunctionstooperateonthem.

Thereisonemoreproblemhowever.IfHumanhasafieldcalledphoneandStudenthasafieldwithsamename,whatshouldwedo?

Gouseaverysimplewaytosolveit.Theouterfieldsgetupperaccesslevels,whichmeanswhenyouaccessstudent.phone,wewillgetthefieldcalledphoneinstudent,nottheoneintheHumanstruct.Thisfeaturecanbesimplyseenasfieldoverloading.

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packagemain

import"fmt"

typeHumanstruct{namestringageintphonestring//Humanhasphonefield}

typeEmployeestruct{Humanspecialtystringphonestring//phoneinemployee}

funcmain(){Bob:=Employee{Human{"Bob",34,"777-444-XXXX"},"Designer","333-222"}

fmt.Println("Bob'sworkphoneis:",Bob.phone)fmt.Println("Bob'spersonalphoneis:",Bob.Human.phone)}

LinksDirectoryPrevioussection:ControlstatementsandfunctionsNextsection:Object-oriented

struct

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Object-orientedWetalkedaboutfunctionsandstructsinthelasttwosections,butdidyoueverconsiderusingfunctionsasfieldsofastruct?Inthissection,Iwillintroduceyoutoanotherformoffunctionthathasareceiver,whichiscalledamethod.

methodSupposeyoudefinea"rectangle"structandyouwanttocalculateitsarea.We'dtypicallyusethefollowingcodetoachievethisgoal.

packagemain

import"fmt"

typeRectanglestruct{width,heightfloat64}

funcarea(rRectangle)float64{returnr.width*r.height}

funcmain(){r1:=Rectangle{12,2}r2:=Rectangle{9,4}fmt.Println("Areaofr1is:",area(r1))fmt.Println("Areaofr2is:",area(r2))}

Theaboveexamplecancalculatearectangle'sarea.Weusethefunctioncalledarea,butit'snotamethodoftherectanglestruct(likeclassmethodsinclassicobject-orientedlanguages).Thefunctionandstructaretwoindependentthingsasyoumaynotice.

It'snotaproblemsofar.However,ifyoualsohavetocalculatetheareaofacircle,square,pentagon,oranyotherkindofshape,youaregoingtoneedtoaddadditionalfunctionswithverysimilarnames.

Figure2.8Relationshipbetweenfunctionandstruct

Obviouslythat'snotcool.Also,theareashouldreallybethepropertyofacircleorrectangle.

Thisiswhereamethodcomestoplay.Themethodisafunctionaffiliatedwithatype.Ithassimilarsyntaxasfunctionexcept,afterthefunckeywordhasaparametercalledthereceiver,whichisthemainbodyofthatmethod.

Usingthesameexample,Rectangle.Area()belongsdirectlytorectangle,insteadofasaperipheralfunction.Morespecifically,length,widthandArea()allbelongtorectangle.

AsRobPikesaid.

"Amethodisafunctionwithanimplicitfirstargument,calledareceiver."

Syntaxofmethod.

func(rReceiverType)funcName(parameters)(results)

Let'schangeourexampleusingmethodinstead.

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packagemain

import("fmt""math")

typeCirclestruct{radiusfloat64}

typeRectanglestruct{width,heightfloat64}

//methodfunc(cCircle)Area()float64{returnc.radius*c.radius*math.Pi}

//methodfunc(rRectangle)Area()float64{returnr.width*r.height}

funcmain(){c1:=Circle{10}c2:=Circle{25}r1:=Rectangle{9,4}r2:=Rectangle{12,2}

fmt.Println("Areaofc1is:",c1.Area())fmt.Println("Areaofc2is:",c2.Area())fmt.Println("Areaofr1is:",r1.Area())fmt.Println("Areaofr2is:",r2.Area())}

Notesforusingmethods.

Ifthenameofmethodsarethesamebuttheydon'tsharethesamereceivers,theyarenotthesame.Methodsareabletoaccessfieldswithinreceivers.Use.tocallamethodinthestruct,thesamewayfieldsarecalled.

Figure2.9Methodsaredifferentindifferentstructs

Intheexampleabove,theArea()methodsbelongtobothRectangleandCirclerespectively,sothereceiversareRectangleandCircle.

Onethingthat'sworthnotingisthatthemethodwithadottedlinemeansthereceiverispassedbyvalue,notbyreference.Thedifferencebetweenthemisthatamethodcanchangeitsreceiver'svalueswhenthereceiverispassedbyreference,anditgetsacopyofthereceiverwhenthereceiverispassedbyvalue.

Canthereceiveronlybeastruct?Ofcoursenot.Anytypecanbethereceiverofamethod.Youmaybeconfusedaboutcustomizedtypes.Structisaspecialkindofcustomizedtype-therearemorecustomizedtypes.

Usethefollowingformattodefineacustomizedtype.

typetypeNametypeLiteral

Examplesofcustomizedtypes:

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59

typeageinttypemoneyfloat32typemonthsmap[string]int

m:=months{"January":31,"February":28,..."December":31,}

Ihopethatyouknowhowtousecustomizedtypesnow.SimilartotypedefinC,weuseagestosubstituteintintheaboveexample.

Let'sgetbacktotalkingaboutmethod.

Youcanuseasmanymethodsincustomtypesasyouwant.

packagemain

import"fmt"

const(WHITE=iotaBLACKBLUEREDYELLOW)

typeBoxstruct{width,height,depthfloat64colorColor}typeColorbytetypeBoxList[]Box//asliceofboxes

//methodfunc(bBox)Volume()float64{returnb.width*b.height*b.depth}

//methodwithapointerreceiverfunc(b*Box)SetColor(cColor){b.color=c}

//methodfunc(blBoxList)BiggestsColor()Color{v:=0.00k:=Color(WHITE)for_,b:=rangebl{ifb.Volume()>v{v=b.Volume()k=b.color}}returnk}

//methodfunc(blBoxList)PaintItBlack(){fori,_:=rangebl{bl[i].SetColor(BLACK)}}

//methodfunc(cColor)String()string{

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strings:=[]string{"WHITE","BLACK","BLUE","RED","YELLOW"}returnstrings[c]}

funcmain(){boxes:=BoxList{Box{4,4,4,RED},Box{10,10,1,YELLOW},Box{1,1,20,BLACK},Box{10,10,1,BLUE},Box{10,30,1,WHITE},Box{20,20,20,YELLOW},}

fmt.Printf("Wehave%dboxesinourset\n",len(boxes))fmt.Println("Thevolumeofthefirstoneis",boxes[0].Volume(),"cm³")fmt.Println("Thecolorofthelastoneis",boxes[len(boxes)-1].color.String())fmt.Println("Thebiggestoneis",boxes.BiggestsColor().String())

//Let'spaintthemallblackboxes.PaintItBlack()

fmt.Println("Thecolorofthesecondoneis",boxes[1].color.String())fmt.Println("Obviously,now,thebiggestoneis",boxes.BiggestsColor().String())}

Wedefinesomeconstantsandcustomizedtypes.

UseColorasaliasofbyte.DefineastructBoxwhichhasfieldsheight,width,lengthandcolor.DefineastructBoxListwhichhasBoxasitsfield.

Thenwedefinedsomemethodsforourcustomizedtypes.

Volume()usesBoxasitsreceiverandreturnsthevolumeofBox.SetColor(cColor)changesBox'scolor.BiggestsColor()returnsthecolorwhichhasthebiggestvolume.PaintItBlack()setscolorforallBoxinBoxListtoblack.String()useColorasitsreceiver,returnsthestringformatofcolorname.

Isitmuchclearerwhenweusewordstodescribeourrequirements?Weoftenwriteourrequirementsbeforewestartcoding.

Usepointerasreceiver

Let'stakealookatSetColormethod.ItsreceiverisapointerofBox.Yes,youcanuse*Boxasareceiver.Whydoweuseapointerhere?BecausewewanttochangeBox'scolorinthismethod.Thus,ifwedon'tuseapointer,itwillonlychangethevalueinsideacopyofBox.

Ifweseethatareceiveristhefirstargumentofamethod,it'snothardtounderstandhowitworks.

Youmightbeaskingwhywearen'tusing(*b).Color=cinsteadofb.Color=cintheSetColor()method.EitheroneisOKherebecauseGoknowshowtointerprettheassignment.DoyouthinkGoismorefascinatingnow?

Youmayalsobeaskingwhetherweshoulduse(&bl[i]).SetColor(BLACK)inPaintItBlackbecausewepassapointertoSetColor.Again,eitheroneisOKbecauseGoknowshowtointerpretit!

Inheritanceofmethod

Welearnedaboutinheritanceoffieldsinthelastsection.Similarly,wealsohavemethodinheritanceinGo.Ifananonymousfieldhasmethods,thenthestructthatcontainsthefieldwillhaveallthemethodsfromitaswell.

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packagemain

import"fmt"

typeHumanstruct{namestringageintphonestring}

typeStudentstruct{Human//anonymousfieldschoolstring}

typeEmployeestruct{Humancompanystring}

//defineamethodinHumanfunc(h*Human)SayHi(){fmt.Printf("Hi,Iam%syoucancallmeon%s\n",h.name,h.phone)}

funcmain(){sam:=Employee{Human{"Sam",45,"111-888-XXXX"},"GolangInc"}mark:=Student{Human{"Mark",25,"222-222-YYYY"},"MIT"}

sam.SayHi()mark.SayHi()}

MethodOverridingIfwewantEmployeetohaveitsownmethodSayHi,wecandefineamethodthathasthesamenameinEmployee,anditwillhideSayHiinHumanwhenwecallit.

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packagemain

import"fmt"


typeStudentstruct{Humanschoolstring}

typeEmployeestruct{Humancompanystring}

func(h*Human)SayHi(){fmt.Printf("Hi,Iam%syoucancallmeon%s\n",h.name,h.phone)}

func(e*Employee)SayHi(){fmt.Printf("Hi,Iam%s,Iworkat%s.Callmeon%s\n",e.name,e.company,e.phone)//Yesyoucansplitinto2lineshere.}

funcmain(){sam:=Employee{Human{"Sam",45,"111-888-XXXX"},"GolangInc"}mark:=Student{Human{"Mark",25,"222-222-YYYY"},"MIT"}

sam.SayHi()mark.SayHi()}

YouareabletowriteanObject-orientedprogramnow,andmethodsuseruleofcapitallettertodecidewhetherpublicorprivateaswell.

LinksDirectoryPrevioussection:structNextsection:interface

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2.6Interface

InterfaceOneofthesubtlestdesignfeaturesinGoareinterfaces.Afterreadingthissection,youwilllikelybeimpressedbytheirimplementation.

Whatisaninterface

Inshort,aninterfaceisasetofmethodsthatweusetodefineasetofactions.

Liketheexamplesinprevioussections,bothStudentandEmployeecanSayHi(),buttheydon'tdothesamething.

Let'sdosomemorework.We'lladdonemoremethodSing()tothem,alongwiththeBorrowMoney()methodtoStudentandtheSpendSalary()methodtoEmployee.

Now,StudenthasthreemethodscalledSayHi(),Sing()andBorrowMoney(),andEmployeehasSayHi(),Sing()andSpendSalary().

ThiscombinationofmethodsiscalledaninterfaceandisimplementedbybothStudentandEmployee.So,StudentandEmployeeimplementtheinterface:SayHi()andSing().Atthesametime,Employeedoesn'timplementtheinterface:BorrowMoney(),andStudentdoesn'timplementtheinterface:SpendSalary().ThisisbecauseEmployeedoesn'thavethemethodBorrowMoney()andStudentdoesn'thavethemethodSpendSalary().

TypeofInterfaceAninterfacedefinesasetofmethods,soifatypeimplementsallthemethodswesaythatitimplementstheinterface.

interface

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typeStudentstruct{Humanschoolstringloanfloat32}

typeEmployeestruct{Humancompanystringmoneyfloat32}

//defineinterfacestypeMeninterface{SayHi()Sing(lyricsstring)Guzzle(beerSteinstring)}

typeYoungChapinterface{SayHi()Sing(songstring)BorrowMoney(amountfloat32)}

typeElderlyGentinterface{SayHi()Sing(songstring)SpendSalary(amountfloat32)}

func(h*Human)SayHi(){fmt.Printf("Hi,Iam%syoucancallmeon%s\n",h.name,h.phone)}

func(h*Human)Sing(lyricsstring){fmt.Println("Lala,lalala,lalalalala...",lyrics)}

func(h*Human)Guzzle(beerSteinstring){fmt.Println("GuzzleGuzzleGuzzle...",beerStein)}

//EmployeeoverloadsSayHifunc(e*Employee)SayHi(){fmt.Printf("Hi,Iam%s,Iworkat%s.Callmeon%s\n",e.name,e.company,e.phone)//Yesyoucansplitinto2lineshere.}

func(s*Student)BorrowMoney(amountfloat32){s.loan+=amount//(againandagainand...)}

func(e*Employee)SpendSalary(amountfloat32){e.money-=amount//Morevodkaplease!!!Getmethroughtheday!}

Weknowthataninterfacecanbeimplementedbyanytype,andonetypecanimplementma

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