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Go (Programming Language)

Q1. What do you need for two functions to be the same type?

  • They should share the same signatures, including parameter types and return types.
  • They should share the same parameter types but can return different types.
  • All functions should be the same type.
  • The functions should not be a first class type.

User defined function types in Go (Golang)

Q2. What does the len() function return if passed a UTF-8 encoded string?

  • the number of characters
  • the number of bytes
  • It does not accept string types.
  • the number of code points

Length of string in Go (Golang).

Q3. Which is not a valid loop construct in Go?

  • do { ... } while i < 5
  • for _,c := range "hello" { ... }
  • for i := 1; i < 5; i++ { ... }
  • for i < 5 { ... }

Explanation: Go has only for-loops

Q4. How will you add the number 3 to the right side?

values := []int{1, 1, 2}

  • values.append(3)
  • values.insert(3, 3)
  • append(values, 3)
  • values = append(values, 3)

Explanation: slices in GO are immutable, so calling append does not modify the slice

Q5. What is the value of Read?

const (
  Write = iota
  Read
  Execute
)
  • 0
  • 1
  • 2
  • a random value

IOTA in Go (Golang)

Q6. Which is the only valid import statement in Go?

  • import "github/gin-gonic/gin"
  • import "https://github.com/gin-gonic/gin"
  • import "../template"
  • import "github.com/gin-gonic/gin"

Import in GoLang

Q7. What would happen if you attempted to compile and run this Go program?

package main
var GlobalFlag string
func main() {
  print("["+GlobalFlag+"]")
}
  • It would not compile because GlobalFlag was never initialized.
  • It would compile and print [].
  • It would compile and print nothing because "[" +nil+"]" is also nil.
  • It would compile but then panic because GlobalFlag was never initialized.

variables in Go have initial values. For string type, it's an empty string.

Go Playground

Q8. From where is the variable myVar accessible if it is declared outside of any functions in a file in package myPackage located inside module myModule?

  • It can be accessed anywhere inside myPackage, not the rest of myModule.
  • It can be accessed by any application that imports myModule.
  • It can be accessed from anywhere in myModule.
  • It can be accessed by other packages in myModule as long as they import myPackage

Explanation: to make the variable available outside of myPackage change the name to MyVar. See also an example of Exported names in the Tour of Go.

Q9. How do you tell go test to print out the tests it is running?

  • go test
  • go test -x
  • go test --verbose
  • go test -v

test package

Q10. This code printed {0, 0}. How can you fix it?

type Point struct {
  x int
  y int
}

func main() {
  data := []byte(`{"x":1, "y": 2}`)
  var p Point
  if err := json.Unmarshal(data, &p); err != nil {
    fmt.Println("error: ", err)
  } else {
    fmt.Println(p)
  }
}
  • use json.Decoder
  • Pass a pointer to data
  • Make X and Y exported (uppercase)
  • Use field tags

How to Parse JSON in Golang?

Go Playground

Q11. What does a sync.Mutex block while it is locked?

  • all goroutines
  • any other call to lock that Mutex
  • any reads or writes of the variable it is locking
  • any writes to the variable it is locking

Mutex in GoLang

Q12. What is an idiomatic way to pause execution of the current scope until an arbitrary number of goroutines have returned?

  • Pass an int and Mutex to each and count when they return.
  • Loop over a select statement.
  • Sleep for a safe amount of time.
  • sync.WaitGroup

Explanation: this is exactly what sync.WaitGroup is designed for - Use sync.WaitGroup in Golang

Q13. What is a side effect of using time.After in a select statement?

  • It blocks the other channels.
  • It is meant to be used in select statements without side effects.
  • It blocks the select statement until the time has passed.
  • The goroutine does not end until the time passes.

Note: it doesn't block select and does not block other channels.

Q14. What is the select statement used for?

  • executing a function concurrently
  • executing a different case based on the type of a variable
  • executing a different case based on the value of a variable
  • executing a different case based on which channel returns first

Q15. According to the Go documentation standard, how should you document this function?

func Add(a, b int) {
  return a + b
}
  • A
// Calculate a + b
// - a: int
// - b: int
// - returns: int
func Add(a, b int) {
  return a + b
}
  • B
// Does a + b
func Add(a, b int) {
  return a + b
}
  • C
// Add returns the sum of a and b
func Add(a, b int) {
  return a + b
}
  • D
// returns the sum of a and b
func Add(a, b int) {
  return a + b
}

Explanation: documentation block should start with a function name

Q16. What restriction is there on the type of var to compile this i := myVal.(int)?

  • myVal must be an integer type, such as int, int64, int32, etc.
  • myVal must be able to be asserted as an int.
  • myVal must be an interface.
  • myVal must be a numeric type, such as float64 or int64.

Explanation: This kind of type casting (using .(type)) is used on interfaces only. this example

Primitive types are type-casted differently

Go Playground

Q17. What is a channel?

  • a global variable
  • a medium for sending values between goroutines
  • a dynamic array of values
  • a lightweight thread for concurrent programming

Channels

Q18. How can you make a file build only on Windows?

  • Check runtime.GOOS.
  • Add a // +build windows comment anywhere in the file.
  • Add a _ prefix to the file name.
  • Add a // +build windows comment at the top of the file.

How to use conditional compilation with the go build tool, Oct 2013

Q19. What is the correct way to pass this as a body of an HTTP POST request?

data := "A group of Owls is called a parliament"

net/http#Client.Post

Q20. What should the idiomatic name be for an interface with a single method and the signature Save() error?

  • Saveable
  • SaveInterface
  • ISave
  • Saver

Effective Go, Interface names

Q21. A switch statement _ its own lexical block. Each case statement _ an additional lexical block.

  • does not create; creates
  • does not create; does not create
  • creates; creates
  • creates; does not create

Go Language Core technology (Volume one) 1.5-scope

Relevant excerpt from the article:

The second if statement is nested inside the first, so a variable declared in the first if statement is visible to the second if statement. There are similar rules in switch: Each case has its own lexical block in addition to the conditional lexical block.

Q22. What is the default case sensitivity of the JSON Unmarshal function?

  • The default behavior is case insensitive, but it can be overridden.
  • Fields are matched case sensitive.
  • Fields are matched case insensitive.
  • The default behavior is case sensitive, but it can be overridden.

encoding/json#Unmarshal

Relevant excerpt from the article:

To unmarshal JSON into a struct, Unmarshal matches incoming object keys to the keys used by Marshal (either the struct field name or its tag), preferring an exact match but also accepting a case-insensitive match. By default, object keys which don't have a corresponding struct field are ignored (see Decoder.DisallowUnknownFields for an alternative).

Q23. What is the difference between the time package’s Time.Sub() and Time.Add() methods?

  • Time.Add() is for performing addition while Time.Sub() is for nesting timestamps.
  • Time.Add() always returns a later time while time.Sub always returns an earlier time.
  • They are opposites. Time.Add(x) is the equivalent of Time.Sub(-x).
  • Time.Add() accepts a Duration parameter and returns a Time while Time.Sub() accepts a Time parameter and returns a Duration.

time#Time.Add

time#Time.Sub

Q24. What is the risk of using multiple field tags in a single struct?

  • Every field must have all tags to compile.
  • It tightly couples different layers of your application.
  • Any tags after the first are ignored.
  • Missing tags panic at runtime.

Example Code / b29r0fUD1cp

Q25. Where is the built-in recover method useful?

  • in the main function
  • immediately after a line that might panic
  • inside a deferred function
  • at the beginning of a function that might panic

Example of Recover Function in Go (Golang)

Relevant excerpt from the article:

Recover is useful only when called inside deferred functions. Executing a call to recover inside a deferred function stops the panicking sequence by restoring normal execution and retrieves the error message passed to the panic function call. If recover is called outside the deferred function, it will not stop a panicking sequence.

Q26. Which choice does not send output to standard error?

  • println(message)
  • log.New(os.Stderr, "", 0).Println(message)
  • fmt.Errorf("%s\n", message)
  • fmt.Fprintln(os.Stderr, message)

Q27. How can you tell Go to import a package from a different location?

  • Use a proxy.
  • Change the import path.
  • Use a replace directive in go.mod.
  • Use a replace directory.

Q28. If your current working directory is the top level of your project, which command will run all its test packages?

  • go test all
  • go run --all
  • go test .
  • go test ./...

Example of testing in Go (Golang)

Example of cmd in Go (Golang)

Relevant excerpt from the article:

Relative patterns are also allowed, like "go test ./..." to test all subdirectories.

Q29. Which encodings can you put in a string?

  • any, it accepts arbitary bytes
  • any Unicode format
  • UTF-8 or ASCII
  • UTF-8

Example of encoding in Go (Golang)

Relevant excerpt from the article:

Package encoding defines an interface for character encodings, such as Shift JIS and Windows 1252, that can convert to and from UTF-8.

Q30. How is the behavior of t.Fatal different inside a t.Run?

  • There is no difference.
  • t.Fatal does not crash the test harness, preserving output messages.
  • t.Fatal stops execution of the subtest and continues with other test cases.
  • t.Fatal stops all tests and contains extra information about the failed subtest.

Reference:

Explanation:
Fatal is equivalent to Log followed by FailNow.
Log formats its arguments using default formatting, analogous to Println, and records the text in the error log.
FailNow marks the function as having failed and stops its execution by calling runtime.Goexit (which then runs all deferred calls in the current goroutine). Execution will continue at the next test or benchmark. FailNow must be called from the goroutine running the test or benchmark function, not from other goroutines created during the test. Calling FailNow does not stop those other goroutines.
Run runs f as a subtest of t called name. It runs f in a separate goroutine and blocks until f returns or calls t.Parallel to become a parallel test. Run reports whether f succeeded (or at least did not fail before calling t.Parallel).
Run may be called simultaneously from multiple goroutines, but all such calls must return before the outer test function for t returns.

Q31. What does log.Fatal do?

  • It raises a panic.
  • It prints the log and then raises a panic.
  • It prints the log and then safely exits the program.
  • It exits the program.

Example of func Fatal in Go (Golang)

Relevant excerpt from the article:

Fatal is equivalent to Print() followed by a call to os.Exit(1).

Q32. Which is a valid Go time format literal?

  • "2006-01-02"
  • "YYYY-mm-dd"
  • "y-mo-d"
  • "year-month-day"

Example of func Time in Go (Golang)

Relevant excerpt from the article:

Most programs can use one of the defined constants as the layout passed to Format or Parse. The rest of this comment can be ignored unless you are creating a custom layout string.

Q33. How should you log an error (err)

  • log.Error(err)
  • log.Printf("error: %v", err)
  • log.Printf(log.ERROR, err)
  • log.Print("error: %v", err)

Explanation: There is defined neither log.ERROR, nor log.Error() in log package; log.Print() arguments are handled in the manner of fmt.Print(); log.Printf() arguments are handled in the manner of fmt.Printf().

Q34. Which file names will the go test command recognize as test files?

  • any that starts with test
  • any files that include the word test
  • only files in the root directory that end in _test.go
  • any that ends in _test.go

Q35. What will be the output of this code?

ch := make(chan int)
ch <- 7
val := <-ch
fmt.Println(val)
  • 0
  • It will deadlock
  • It will not compile
  • 2.718

Q36. What will be the output of this program?

ch := make(chan int)
close(ch)
val := <-ch
fmt.Println(val)
  • It will deadlock
  • It will panic
  • 0
  • NaN

Q37. What will be printed in this code?

var stocks map[string]float64 // stock -> price
price := stocks["MSFT"]
fmt.Printf("%f\n", price)
  • 0
  • 0.000000
  • The code will panic
  • NaN

Q38. What is the common way to have several executables in your project?

  • Have a cmd directory and a directory per executable inside it.
  • Comment out main.
  • Use build tags.
  • Have a pkg directory and a directory per executable inside it.
  1. stackoverflow
  2. medium
  3. medium

Q39. How can you compile main.go to an executable that will run on OSX arm64 ?

  • Set GOOS to arm64 and GOARCH to darwin.
  • Set GOOS to osx and GOARCH to arm64.
  • Set GOOS to arm64 and GOARCH to osx.
  • Set GOOS to darwin and GOARCH to arm64.

documentation

Q40. What is the correct syntax to start a goroutine that will print Hello Gopher!?

  • go(fmt.Println("Hello Gopher!"))
  • go func() { fmt.Println("Hello Gopher!") }
  • go fmt.Println("Hello Gopher!")
  • Go fmt.Println("Hello Gopher!")

Example of start a goroutine

Q41. If you iterate over a map in a for range loop, in which order will the key:value pairs be accessed?

  • in pseudo-random order that cannot be predicted
  • in reverse order of how they were added, last in first out
  • sorted by key in ascending order
  • in the order they were added, first in first out

Reference

Q42. What is an idiomatic way to customize the representation of a custom struct in a formatted string?

  • There is no customizing the string representation of a type.
  • Build it in pieces each time by calling individual fields.
  • Implement a method String() string
  • Create a wrapper function that accepts your type and outputs a string.

Reference

Q43. How can you avoid a goroutine leak in this code?

func findUser(ctx context.Context, login string) (*User, error) {
	ch := make(chan *User)
	go func() {
		ch <- findUserInDB(login)
	}()

	select {
	case user := <-ch:
		return user, nil
	case <-ctx.Done():
		return nil, fmt.Errorf("timeout")
	}
}

  • Use a sync.WaitGroup.

  • Make ch a buffered channel.

  • Add a default case to the select.

  • Use runtime.SetFinalizer.

Explanation: I am not sure, but I believe we can avoid the goroutine leak by adding a default case to the select statement. The leak happens because if the context timeout is not set correctly and if findUserInDB stalls or is slow, then the findUser function will just stay at the select statement, waiting for something to happen. We can get rid of this waiting by adding a default case, but since the function would just execute the default case every time without waiting for the goroutine, I don't believe this function would do what is intended.

44. What will this code print?

var i int8 = 120 i += 10 fmt.Println(i)

  • -126
  • 0
  • NaN
  • 130

45. Given the definition of worker below, what is the right syntax to start a start a goroutine that will call worker and send the result to a channel named ch?

func worker(m Message) Result

  • go func() { r := worker(m) ch <- r }
  • go func() { r := worker(m) r -> ch } ()
  • go func() { r := worker(m) ch <- r } ()
  • go ch <- worker(m)