## Getting started

### Introduction

- A tour of Go (tour.golang.org)
- Go repl (repl.it)
- Golang wiki (github.com)

### Hello world

#### hello.go

    package main

    import "fmt"

    func main() {
      message := greetMe("world")
      fmt.Println(message)
    }

    func greetMe(name string) string {
      return "Hello, " + name + "!"
    }

    $ go build

Or try it out in the Go repl, or A Tour of Go.

### Variables

#### Variable declaration

    var msg string
    msg = "Hello"

#### Shortcut of above (Infers type)

    msg := "Hello"

### Constants

    const Phi = 1.618

Constants can be character, string, boolean, or numeric values.

See: Constants

## Basic types

### Strings

    str := "Hello"

    str := `Multiline
    string`

Strings are of type `string`.

### Numbers

#### Typical types

    num := 3          // int
    num := 3.         // float64
    num := 3 + 4i     // complex128
    num := byte('a')  // byte (alias for uint8)

#### Other types

    var u uint = 7        // uint (unsigned)
    var p float32 = 22.7  // 32-bit float

### Arrays

    // var numbers [5]int
    numbers := [...]int{0, 0, 0, 0, 0}

Arrays have a fixed size.

### Slices

    slice := []int{2, 3, 4}

    slice := []byte("Hello")

Slices have a dynamic size, unlike arrays.

### Pointers

    func main () {
      b := *getPointer()
      fmt.Println("Value is", b)
    }

    func getPointer () (myPointer *int) {
      a := 234
      return &a
    }

    a := new(int)
    *a = 234

Pointers point to a memory location of a variable. Go is fully garbage-collected.

See: Pointers

### Type conversions

    i := 2
    f := float64(i)
    u := uint(i)

See: Type conversions

## Flow control

### Conditional

    if day == "sunday" || day == "saturday" {
      rest()
    } else if day == "monday" && isTired() {
      groan()
    } else {
      work()
    }

See: If

### Statements in if

    if _, err := doThing(); err != nil {
      fmt.Println("Uh oh")
    }

A condition in an `if` statement can be preceded with a statement before a `;`. Variables declared by the statement are only in scope until the end of the `if`.

See: If with a short statement

### Switch

    switch day {
      case "sunday":
        // cases don't "fall through" by default!
        fallthrough

      case "saturday":
        rest()

      default:
        work()
    }

See: Switch

### For loop

    for count := 0; count <= 10; count++ {
      fmt.Println("My counter is at", count)
    }

See: For loops

### For-Range loop

    entry := []string{"Jack","John","Jones"}
    for i, val := range entry {
      fmt.Printf("At position %d, the character %s is present\n", i, val)
    }

See: For-Range loops

### While loop

    n := 0
    x := 42
    for n != x {
      n := guess()
    }

See: Go’s “while”

## Functions

### Lambdas

    myfunc := func() bool {
      return x > 10000
    }

Functions are first class objects.

### Multiple return types

    a, b := getMessage()

    func getMessage() (a string, b string) {
      return "Hello", "World"
    }

### Named return values

    func split(sum int) (x, y int) {
      x = sum * 4 / 9
      y = sum - x
      return
    }

By defining the return value names in the signature, a `return` (no args) will return variables with those names.

See: Named return values

## Packages

### Importing

    import "fmt"
    import "math/rand"

    import (
      "fmt"        // gives fmt.Println
      "math/rand"  // gives rand.Intn
    )

Both are the same.

See: Importing

### Aliases

    import r "math/rand"

    r.Intn()

### Exporting names

    func Hello () {
      ···
    }

Exported names begin with capital letters.

See: Exported names

### Packages

    package hello

Every package file has to start with `package`.

## Concurrency

### Goroutines

    func main() {
      // A "channel"
      ch := make(chan string)

      // Start concurrent routines
      go push("Moe", ch)
      go push("Larry", ch)
      go push("Curly", ch)

      // Read 3 results
      // (Since our goroutines are concurrent,
      // the order isn't guaranteed!)
      fmt.Println(<-ch, <-ch, <-ch)
    }

    func push(name string, ch chan string) {
      msg := "Hey, " + name
      ch <- msg
    }

Channels are concurrency-safe communication objects, used in goroutines.

See: Goroutines, Channels

### Buffered channels

    ch := make(chan int, 2)
    ch <- 1
    ch <- 2
    ch <- 3
    // fatal error:
    // all goroutines are asleep - deadlock!

Buffered channels limit the amount of messages it can keep.

See: Buffered channels

### Closing channels

#### Closes a channel

    ch <- 1
    ch <- 2
    ch <- 3
    close(ch)

#### Iterates across a channel until its closed

    for i := range ch {
      ···
    }

#### Closed if `ok == false`

    v, ok := <- ch

See: Range and close

### WaitGroup

    import "sync"

    func main() {
      var wg sync.WaitGroup

      for _, item := range itemList {
        // Increment WaitGroup Counter
        wg.Add(1)
        go doOperation(item)
      }
      // Wait for goroutines to finish
      wg.Wait()

    }

    func doOperation(item string) {
      defer wg.Done()
      // do operation on item
      // ...
    }

A WaitGroup waits for a collection of goroutines to finish. The main goroutine calls Add to set the number of goroutines to wait for. The goroutine calls `wg.Done()` when it finishes. See: WaitGroup

## Error control

### Defer

    func main() {
      defer fmt.Println("Done")
      fmt.Println("Working...")
    }

Defers running a function until the surrounding function returns. The arguments are evaluated immediately, but the function call is not ran until later.

See: Defer, panic and recover

### Deferring functions

    func main() {
      defer func() {
        fmt.Println("Done")
      }()
      fmt.Println("Working...")
    }

Lambdas are better suited for defer blocks.

    func main() {
      var d = int64(0)
      defer func(d *int64) {
        fmt.Printf("& %v Unix Sec\n", *d)
      }(&d)
      fmt.Print("Done ")
      d = time.Now().Unix()
    }

The defer func uses current value of d, unless we use a pointer to get final value at end of main.

## Structs

### Defining

    type Vertex struct {
      X int
      Y int
    }

    func main() {
      v := Vertex{1, 2}
      v.X = 4
      fmt.Println(v.X, v.Y)
    }

See: Structs

### Literals

    v := Vertex{X: 1, Y: 2}

    // Field names can be omitted
    v := Vertex{1, 2}

    // Y is implicit
    v := Vertex{X: 1}

You can also put field names.

### Pointers to structs

    v := &Vertex{1, 2}
    v.X = 2

Doing `v.X` is the same as doing `(*v).X`, when `v` is a pointer.

## Methods

### Receivers

    type Vertex struct {
      X, Y float64
    }

    func (v Vertex) Abs() float64 {
      return math.Sqrt(v.X * v.X + v.Y * v.Y)
    }

    v := Vertex{1, 2}
    v.Abs()

There are no classes, but you can define functions with receivers.

See: Methods

### Mutation

    func (v *Vertex) Scale(f float64) {
      v.X = v.X * f
      v.Y = v.Y * f
    }

    v := Vertex{6, 12}
    v.Scale(0.5)
    // `v` is updated

By defining your receiver as a pointer (`*Vertex`), you can do mutations.

See: Pointer receivers

## Interfaces

### A basic interface

    type Shape interface {
      Area() float64
      Perimeter() float64
    }

### Struct

    type Rectangle struct {
      Length, Width float64
    }

Struct `Rectangle` implicitly implements interface `Shape` by implementing all of its methods.

### Methods

    func (r Rectangle) Area() float64 {
      return r.Length * r.Width
    }

    func (r Rectangle) Perimeter() float64 {
      return 2 * (r.Length + r.Width)
    }

The methods defined in `Shape` are implemented in `Rectangle`.

### Interface example

    func main() {
      var r Shape = Rectangle{Length: 3, Width: 4}
      fmt.Printf("Type of r: %T, Area: %v, Perimeter: %v.", r, r.Area(), r.Perimeter())
    }

## References

### Official resources

- A tour of Go (tour.golang.org)
- Golang wiki (github.com)
- Effective Go (golang.org)

### Other links

- Go by Example (gobyexample.com)
- Awesome Go (awesome-go.com)
- JustForFunc Youtube (youtube.com)
- Style Guide (github.com)

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