book

Head First Go

by Jay McGavren

April 2019

Beginner

556 pages

14h 21m

English

O'Reilly Media, Inc.

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Who is this book for?Who should probably back away from this book?We know what you’re thinkingWe know what your brain is thinkingMetacognition: thinking about thinkingHere’s what WE didHere’s what YOU can do to bend your brain into submissionRead meIt helps if you’ve done a little programming in some other language.We don’t cover every type, function, and package ever created.The activities are NOT optional.The redundancy is intentional and important.The code examples are as lean as possible.AcknowledgmentsO’Reilly Online Learning
Ready, set, Go!The Go PlaygroundWhat does it all mean?The typical Go file layoutthere are no Dumb QuestionsWhat if something goes wrong?Breaking Stuff is Educational!Calling functionsThe Println functionUsing functions from other packagesFunction return valuesPool PuzzleA Go program templateStringsRunesBooleansNumbersMath operations and comparisonsTypesDeclaring variablesZero valuesCode MagnetsShort variable declarationsBreaking Stuff is Educational!Naming rulesConversionsInstalling Go on your computerCompiling Go codeGo toolsTry out code quickly with “go run”Your Go ToolboxPool Puzzle SolutionCode Magnets Solution
Calling methodsMaking the gradeCommentsGetting a grade from the userMultiple return values from a function or methodOption 1: Ignore the error return value with the blank identifierOption 2: Handle the errorConditionalsthere are no Dumb QuestionsLogging a fatal error, conditionallyCode MagnetsAvoid shadowing namesConverting strings to numbersBlocksBlocks and variable scopeWe’ve finished the grading program!Only one variable in a short variable declaration has to be newLet’s build a gamePackage names vs. import pathsGenerating a random numberGetting an integer from the keyboardComparing the guess to the targetLoopsInit and post statements are optionalLoops and scopeBreaking Stuff is Educational!Using a loop in our guessing gameSkipping parts of a loop with “continue” and “break”Breaking out of our guessing loopRevealing the targetThe finishing touchesCongratulations, your game is complete!Your Go ToolboxCode Magnets Solution
Some repetitive codeFormatting output with Printf and SprintfFormatting verbsFormatting value widthsFormatting fractional number widthsUsing Printf in our paint calculatorDeclaring functionsDeclaring function parametersUsing functions in our paint calculatorFunctions and variable scopeFunction return valuesUsing a return value in our paint calculatorBreaking Stuff is Educational!The paintNeeded function needs error handlingError valuesDeclaring multiple return valuesUsing multiple return values with our paintNeeded functionAlways handle errors!Breaking Stuff is Educational!Pool PuzzleFunction parameters receive copies of the argumentsPointersPointer typesGetting or changing the value at a pointerCode MagnetsUsing pointers with functionsFixing our “double” function using pointersYour Go ToolboxPool Puzzle SolutionCode Magnets Solution
Different programs, same functionSharing code between programs using packagesThe Go workspace directory holds package codethere are no Dumb QuestionsCreating a new packageImporting our package into a programPackages use the same file layoutBreaking Stuff is Educational!Pool PuzzlePackage naming conventionsPackage qualifiersMoving our shared code to a packageConstantsNested package directories and import pathsInstalling program executables with “go install”Changing workspaces with the GOPATH environment variableSetting GOPATHOn Mac or Linux systems:On Windows systems:Publishing packagesDownloading and installing packages with “go get”Reading package documentation with “go doc”Documenting your packages with doc commentsViewing documentation in a web browserServing HTML documentation to yourself with “godoc”The “godoc” server includes YOUR packages!Your Go ToolboxPool Puzzle Solution
Arrays hold collections of valuesZero values in arraysArray literalsFunctions in the “fmt” package know how to handle arraysAccessing array elements within a loopChecking array length with the “len” functionLooping over arrays safely with “for...range”Using the blank identifier with “for...range” loopsGetting the sum of the numbers in an arrayGetting the average of the numbers in an arrayPool PuzzleReading a text fileReading a text file into an arrayUpdating our “average” program to read a text fileOur program can only process three values!Your Go ToolboxPool Puzzle Solution
SlicesSlice literalsPool PuzzleThe slice operatorUnderlying arraysChange the underlying array, change the sliceAdd onto a slice with the “append” functionSlices and zero valuesReading additional file lines using slices and “append”Trying our improved programReturning a nil slice in the event of an errorCommand-line argumentsGetting command-line arguments from the os.Args sliceThe slice operator can be used on other slicesUpdating our program to use command-line argumentsVariadic functionsUsing variadic functionsCode MagnetsUsing a variadic function to calculate averagesPassing slices to variadic functionsSlices have saved the day!Your Go ToolboxPool Puzzle SolutionCode Magnets Solution
Counting votesReading names from a fileCounting names the hard way, with slicesMapsMap literalsZero values within mapsThe zero value for a map variable is nilHow to tell zero values apart from assigned valuesRemoving key/value pairs with the “delete” functionUpdating our vote counting program to use mapsUsing for...range loops with mapsThe for...range loop handles maps in random order!Updating our vote counting program with a for...range loopThe vote counting program is complete!Code MagnetsYour Go ToolboxCode Magnets Solution
Slices and maps hold values of ONE typeStructs are built out of values of MANY typesAccess struct fields using the dot operatorStoring subscriber data in a structDefined types and structsUsing a defined type for magazine subscribersUsing defined types with functionsCode MagnetsModifying a struct using a functionAccessing struct fields through a pointerthere are no Dumb QuestionsPass large structs using pointersMoving our struct type to a different packageA defined type’s name must be capitalized to be exportedStruct field names must be capitalized to be exportedStruct literalsPool PuzzleCreating an Employee struct typeCreating an Address struct typeAdding a struct as a field on another typeSetting up a struct within another structAnonymous struct fieldsEmbedding structsOur defined types are complete!Your Go ToolboxCode Magnets SolutionPool Puzzle Solution

Type errors in real lifeDefined types with underlying basic typesDefined types and operatorsPool PuzzleConverting between types using functionsthere are no Dumb QuestionsFixing our function name conflict using methodsDefining methodsThe receiver parameter is (pretty much) just another parameterthere are no Dumb QuestionsA method is (pretty much) just like a functionPointer receiver parametersBreaking Stuff is Educational!Converting Liters and Milliliters to Gallons using methodsConverting Gallons to Liters and Milliliters using methodsYour Go ToolboxPool Puzzle Solution
Creating a Date struct typePeople are setting the Date struct field to invalid values!Setter methodsSetter methods need pointer receiversAdding the remaining setter methodsAdding validation to the setter methodsThe fields can still be set to invalid values!Moving the Date type to another packageMaking Date fields unexportedAccessing unexported fields through exported methodsGetter methodsEncapsulationthere are no Dumb QuestionsEmbedding the Date type in an Event typeUnexported fields don’t get promotedExported methods get promoted just like fieldsEncapsulating the Event Title fieldPromoted methods live alongside the outer type’s methodsOur calendar package is complete!Your Go Toolbox
Two different types that have the same methodsA method parameter that can only accept one typeInterfacesDefining a type that satisfies an interfaceConcrete types, interface typesAssign any type that satisfies the interfaceYou can only call methods defined as part of the interfaceBreaking Stuff is Educational!Fixing our playList function using an interfacethere are no Dumb QuestionsType assertionsType assertion failuresAvoiding panics when type assertions failTesting TapePlayers and TapeRecorders using type assertionsPool PuzzleThe “error” interfacethere are no Dumb QuestionsThe Stringer interfaceThe empty interfaceYour Go ToolboxPool Puzzle Solution
Reading numbers from a file, revisitedAny errors will prevent the file from being closed!Deferring function callsRecovering from errors using deferred function callsEnsuring files get closed using deferred function callsCode Magnetsthere are no Dumb QuestionsListing the files in a directoryListing the files in subdirectories (will be trickier)Recursive function callsRecursively listing directory contentsError handling in a recursive functionStarting a panicStack tracesDeferred calls completed before crashUsing “panic” with scanDirectoryWhen to panicThe “recover” functionThe panic value is returned from recoverRecovering from panics in scanDirectoryReinstating a panicthere are no Dumb QuestionsYour Go ToolboxCode Magnets Solution
Retrieving web pagesMultitaskingConcurrency using goroutinesUsing goroutinesUsing goroutines with our responseSize functionWe don’t directly control when goroutines runCode MagnetsGo statements can’t be used with return valuesSending and receiving values with channelsSynchronizing goroutines with channelsObserving goroutine synchronizationBreaking Stuff is Educational!Fixing our web page size program with channelsUpdating our channel to carry a structYour Go ToolboxCode Magnets Solution
Automated tests find your bugs before someone else doesA function we should have had automated tests forWe’ve introduced a bug!Writing testsRunning tests with the “go test” commandTesting our actual return valuesMore detailed test failure messages with the “Errorf” methodTest “helper” functionsGetting the tests to passTest-driven developmentAnother bug to fixthere are no Dumb QuestionsCode MagnetsRunning specific sets of testsTable-driven testsFixing panicking code using a testYour Go ToolboxCode Magnets Solution
Writing web apps in GoBrowsers, requests, servers, and responsesA simple web appYour computer is talking to itselfthere are no Dumb QuestionsOur simple web app, explainedResource pathsResponding differently for different resource pathsFirst-class functionsPassing functions to other functionsFunctions as typesPool PuzzleWhat’s nextYour Go ToolboxPool Puzzle Solution
A guestbook appFunctions to handle a request and check errorsSetting up a project directory and trying the appMaking a signature list in HTMLMaking our app respond with HTMLThe “text/template” packageUsing the io.Writer interface with a template’s Execute methodResponseWriters and os.Stdout both satisfy io.WriterInserting data into templates using actionsMaking parts of a template optional with “if” actionsRepeating parts of a template with “range” actionsInserting struct fields into a template with actionsReading a slice of signatures in from a filethere are no Dumb QuestionsA struct to hold the signatures and signature countUpdating our template to include our signaturesthere are no Dumb QuestionsLetting users add data with HTML formsResponding with the HTML formForm submission requestsPath and HTTP method for form submissionsGetting values of form fields from the requestSaving the form dataHTTP redirectsLet’s try it all out!Our complete app codeYour Go Toolbox
Understanding os.OpenFilePassing flag constants to os.OpenFileBinary notationBitwise operatorsThe bitwise AND operatorThe bitwise OR operatorUsing bitwise OR on the “os” package constantsUsing bitwise OR to fix our os.OpenFile optionsUnix-style file permissionsRepresenting permissions with the os.FileMode typeOctal notationConverting octal values to FileMode valuesCalls to os.OpenFile, explainedthere are no Dumb Questions
#1 Initialization statements for “if”#2 The switch statementthere are no Dumb Questions#3 More basic types#4 More about runes#5 Buffered channels#6 Further reading

Content preview from Head First Go

Chapter 7. labeling data: Maps

Throwing things in piles is fine, until you need to find something again. You’ve already seen how to create lists of values using arrays and slices. You’ve seen how to apply the same operation to every value in an array or slice. But what if you need to work with a particular value? To find it, you’ll have to start at the beginning of the array or slice, and look through Every. Single. Value.

What if there were a kind of collection where every value had a label on it? You could quickly find just the value you needed! In this chapter, we’ll look at maps, which do just that.

Counting votes

A seat on the Sleepy Creek County School Board is up for grabs this year, and polls have been showing that the election is really close. Now that it’s election night, the candidates are excitedly watching the votes roll in.

Note

This is another example that debuted in Head First Ruby, in the hashes chapter. Ruby hashes are a lot like Go maps, so this example works great here, too!