Golang

Golang is a programming language developed by Google in 2007 for the Google App Engine platform.

Getting started 

package main

import "fmt"

func main() {
    fmt.Println("Hello, world!")
}

Run directly

$ go run hello.go
Hello, world!

Or try it out in the Go repl

Variables 

var s1 string
s1 = "Learn Go!"

// declare multiple variables at once
var b, c int = 1, 2
var d = true

Short declaration

s1 := "Learn Go!"        // string
b, c := 1, 2             // int
d := true                // bool

See: Basic types

Functions 

package main

import "fmt"

// The entry point of the programs
func main() {
    fmt.Println("Hello world!")
    say("Hello Go!")
}

func say(message string) {
    fmt.Println("You said: ", message)
}

See: Functions

Comments 

// Single line comment

/* Multi-
 line comment */

If statement 

if true {
    fmt.Println("Yes!")
}

See: Flow control

Basic types

Strings 

s1 := "Hello" + "World"

s2 := `A "raw" string literal
can include line breaks.`

// Outputs: 11
fmt.Println(len(s1))

// Outputs: Hello
fmt.Println(string(s1[0:5]))

Strings are of type string.

Numbers 

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

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

Operators 

x := 5
x++
fmt.Println("x + 4 =", x + 4)
fmt.Println("x * 4 =", x * 4)

See: More Operators

Booleans 

isTrue   := true
isFalse  := false

Operators 

fmt.Println(true && true)   // true 
fmt.Println(true && false)  // false
fmt.Println(true || true)   // true
fmt.Println(true || false)  // true
fmt.Println(!true)          // false

See: More Operators

Arrays 

┌────┬────┬────┬────┬─────┬─────┐
| 2  | 3  | 5  | 7  | 11  | 13  |
└────┴────┴────┴────┴─────┴─────┘
  0    1    2    3     4     5

primes := [...]int{2, 3, 5, 7, 11, 13}
fmt.Println(len(primes)) // => 6

// Outputs: [2 3 5 7 11 13]
fmt.Println(primes)

// Same as [:3], Outputs: [2 3 5]
fmt.Println(primes[0:3])

var a [2]string
a[0] = "Hello"
a[1] = "World"

fmt.Println(a[0], a[1]) //=> Hello World
fmt.Println(a)   // => [Hello World]

2d array 

var twoDimension [2][3]int
for i := 0; i < 2; i++ {
    for j := 0; j < 3; j++ {
        twoDimension[i][j] = i + j
    }
}
// => 2d:  [[0 1 2] [1 2 3]]
fmt.Println("2d: ", twoDimension)

Pointers 

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

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

a := new(int)
*a = 234

See: Pointers

Slices 

s := make([]string, 3)
s[0] = "a"
s[1] = "b"
s = append(s, "d")
s = append(s, "e", "f")

fmt.Println(s)
fmt.Println(s[1])
fmt.Println(len(s))
fmt.Println(s[1:3])

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

See also: Slices example

Constants 

const s string = "constant"
const Phi = 1.618
const n = 500000000
const d = 3e20 / n
fmt.Println(d)

Type conversions 

i := 90
f := float64(i)
u := uint(i)

// Will be equal to the character Z
s := string(i)

How to get int string? 

i := 90

// need import "strconv"
s := strconv.Itoa(i)
fmt.Println(s) // Outputs: 90

Strings

Strings function 

package main

import (
    "fmt"
    s "strings"
)

func main() {
    /* Need to import strings as s */
    fmt.Println(s.Contains("test", "e"))

    /* Build in */
    fmt.Println(len("hello"))  // => 5
    // Outputs: 101
    fmt.Println("hello"[1])
    // Outputs: e
    fmt.Println(string("hello"[1]))

}

fmt.Printf 

package main

import (
    "fmt"
    "os"
)

type point struct {
    x, y int
}

func main() {
    p := point{1, 2}
    fmt.Printf("%v\n", p)                        // => {1 2}
    fmt.Printf("%+v\n", p)                       // => {x:1 y:2}
    fmt.Printf("%#v\n", p)                       // => main.point{x:1, y:2}
    fmt.Printf("%T\n", p)                        // => main.point
    fmt.Printf("%t\n", true)                     // => TRUE
    fmt.Printf("%d\n", 123)                      // => 123
    fmt.Printf("%b\n", 14)                       // => 1110
    fmt.Printf("%c\n", 33)                       // => !
    fmt.Printf("%x\n", 456)                      // => 1c8
    fmt.Printf("%f\n", 78.9)                     // => 78.9
    fmt.Printf("%e\n", 123400000.0)              // => 1.23E+08
    fmt.Printf("%E\n", 123400000.0)              // => 1.23E+08
    fmt.Printf("%s\n", "\"string\"")             // => "string"
    fmt.Printf("%q\n", "\"string\"")             // => "\"string\""
    fmt.Printf("%x\n", "hex this")               // => 6.86578E+15
    fmt.Printf("%p\n", &p)                       // => 0xc00002c040
    fmt.Printf("|%6d|%6d|\n", 12, 345)           // => |    12|   345|
    fmt.Printf("|%6.2f|%6.2f|\n", 1.2, 3.45)     // => |  1.20|  3.45|
    fmt.Printf("|%-6.2f|%-6.2f|\n", 1.2, 3.45)   // => |1.20  |3.45  |
    fmt.Printf("|%6s|%6s|\n", "foo", "b")        // => |   foo|     b|
    fmt.Printf("|%-6s|%-6s|\n", "foo", "b")      // => |foo   |b     |

    s := fmt.Sprintf("a %s", "string")
    fmt.Println(s)

    fmt.Fprintf(os.Stderr, "an %s\n", "error")
}

See also: fmt

Function examples

Example Result
Contains(“test”, “es”) true
Count(“test”, “t”) 2
HasPrefix(“test”, “te”) true
HasSuffix(“test”, “st”) true
Index(“test”, “e”) 1
Join([]string{“a”, “b”}, “-“) a-b
Repeat(“a”, 5) aaaaa
Replace(“foo”, “o”, “0”, -1) f00
Replace(“foo”, “o”, “0”, 1) f0o
Split(“a-b-c-d-e”, “-“) [a b c d e]
ToLower(“TEST”) test
ToUpper(“test”) TEST

Flow control

Conditional 

a := 10

if a > 20 {
    fmt.Println(">")
} else if a < 20 {
    fmt.Println("<")
} else {
    fmt.Println("=")
}

Statements in if 

x := "hello go!"

if count := len(x); count > 0 {
    fmt.Println("Yes")
}

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

Switch 

x := 42.0 switch x { 
    case 0: 
    case 1, 2: 
        fmt.Println("Multiple matches") 
    case 42: // Don't "fall through". 
        fmt.Println("reached") 
    case 43: 
        fmt.Println("Unreached") 
    default: 
        fmt.Println("Optional") }

See: Switch

For loop 

for i := 0; i <= 10; i++ {
  fmt.Println("i: ", i)
}

For-Range loop 

nums := []int{2, 3, 4} 
sum := 0
for _, num := range nums { 
    sum += num 
}
fmt.Println("sum:", sum)

While loop 

i := 1
for i <= 3 {
    fmt.Println(i)
    i++
}

Continue keyword 

for i := 0; i <= 5; i++ {
    if i % 2 == 0 {
        continue
    }
    fmt.Println(i)
}

Break keyword 

for {
    fmt.Println("loop")
    break
}

Structs & Maps

Defining 

package main

import (
    "fmt"
)

type Vertex struct {
    X int
    Y int
}

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

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.

Maps 

m := make(map[string]int)
m["k1"] = 7
m["k2"] = 13
fmt.Println(m) // => map[k1:7 k2:13]

v1 := m["k1"]
fmt.Println(v1)     // => 7
fmt.Println(len(m)) // => 2

delete(m, "k2")
fmt.Println(m) // => map[k1:7]

_, prs := m["k2"]
fmt.Println(prs) // => false

n := map[string]int{"foo": 1, "bar": 2}
fmt.Println(n) // => map[bar:2 foo:1]

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.

Functions

Multiple arguments 

func plus(a int, b int) int {
    return a + b
}
func plusPlus(a, b, c int) int {
    return a + b + c
}
fmt.Println(plus(1, 2))
fmt.Println(plusPlus(1, 2, 3))

Multiple return 

func vals() (int, int) {
    return 3, 7
}

a, b := vals()
fmt.Println(a)    // => 3
fmt.Println(b)    // => 7

Anonymous function 

r1, r2 := func() (string, string) {
    x := []string{"hello", "world"}
    return x[0], x[1]
}()

// => hello world
fmt.Println(r1, r2)

Named return 

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

x, y := split(17)
fmt.Println(x)   // => 7
fmt.Println(y)   // => 10

Variadic functions 

func sum(nums ...int) {
    fmt.Print(nums, " ")
    total := 0
    for _, num := range nums {
        total += num
    }
    fmt.Println(total)
}
sum(1, 2)     //=> [1 2] 3
sum(1, 2, 3)  // => [1 2 3] 6

nums := []int{1, 2, 3, 4}
sum(nums...)  // => [1 2 3 4] 10

init function 

import --> const --> var --> init()

var num = setNumber()

func setNumber() int {
    return 42
}
func init() {
    num = 0
}
func main() {
    fmt.Println(num) // => 0
}

Functions as values 

func main() {
    // assign a function to a name
    add := func(a, b int) int {
        return a + b
    }
    // use the name to call the function
    fmt.Println(add(3, 4)) // => 7
}

Closures 1 

func scope() func() int{
    outer_var := 2
    foo := func() int {return outer_var}
    return foo
}

// Outpus: 2
fmt.Println(scope()())

Closures 2 

func outer() (func() int, int) {
    outer_var := 2
    inner := func() int {
        outer_var += 99
        return outer_var
    }
    inner()
    return inner, outer_var
}
inner, val := outer()
fmt.Println(inner()) // => 200
fmt.Println(val)     // => 101

Packages

Importing 

import "fmt"
import "math/rand"

Same as 

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

See: Importing

Aliases 

import r "math/rand"

import (
    "fmt"
    r "math/rand"
)

r.Intn()

Packages 

package main

Exporting names 

// Begin with a capital letter
func Hello () {
  // ···
}

See: Exported names

Concurrency

Goroutines 

package main

import (
    "fmt"
    "time"
)

func f(from string) {
    for i := 0; i < 3; i++ {
        fmt.Println(from, ":", i)
    }
}

func main() {
    f("direct")
    go f("goroutine")

    go func(msg string) {
        fmt.Println(msg)
    }("going")

    time.Sleep(time.Second)
    fmt.Println("done")
}

See: Goroutines, Channels

WaitGroup 

package main

import (
    "fmt"
    "sync"
    "time"
)

func w(id int, wg *sync.WaitGroup) {
    defer wg.Done()
    fmt.Printf("%d starting\n", id)

    time.Sleep(time.Second)
    fmt.Printf("%d done\n", id)
}

func main() {
    var wg sync.WaitGroup
    for i := 1; i <= 5; i++ {
        wg.Add(1)
        go w(i, &wg)
    }
    wg.Wait()
}

See: WaitGroup

Closing channels 

ch <- 1
ch <- 2
ch <- 3
close(ch) // Closes a channel

// Iterate the channel until closed
for i := range ch {
  // ···
}

// Closed if `ok == false`
v, ok := <- ch

See: Range and close

Buffered channels 

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

See: Buffered channels

Error control

Deferring functions 

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

Lambda defer 

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.

Defer 

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

See: Defer, panic and recover

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()

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

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()) 
}

Miscellaneous

Keywords

  • break
  • default
  • func
  • interface
  • select
  • case
  • defer
  • go
  • map
  • struct
  • chan
  • else
  • goto
  • package
  • switch
  • const
  • fallthrough
  • if
  • range
  • type
  • continue
  • for
  • import
  • return
  • var

Operators and punctuation

                 
+ & += &= && == != ( )
- | -= |= || < <= [ ]
* ^ *= ^= <- > >= { }
/ << /= <<= ++ = := , ;
% >> %= >>= – ! … . :
  &^ &^=            

Snippets 

type H map[string]interface{}

func (s *Server) Render(w http.ResponseWriter, name string, data H) {
  t, err := template.ParseFiles("./templates/" + name + ".html")
  if err != nil {
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
  }
  w.Header().Add("Content-Type", "text/html; charset=utf-8")
  err = t.Execute(w, data)
  if err != nil {
    http.Error(w, err.Error(), http.StatusInternalServerError)
  }
}

func (s *Server) IndexView(w http.ResponseWriter, r *http.Request) {
  s.Render(w, "index", H{})
}

// Render renders an HTML template with the provided data.
func (s *Server) Render(w http.ResponseWriter, name string, data H) {
  if data == nil {
    data = H{}
  }
  // Parse templates from embedded file system
  // tmpl, err := template.New("").ParseFS(templates.Files, "layout.html", templateName+".html")
  tmpl, err := template.ParseFiles("templates/layout.html", "templates/"+name+".html")
  if err != nil {
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
  }
  err = tmpl.ExecuteTemplate(w, "layout", data)
  if err != nil {
    log.Println(err)
    http.Error(w, err.Error(), http.StatusInternalServerError)
    return
  }
}

layout.html

\{\{define "layout"\}\}
<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>

<body>
  <div class="container">
    <h1><a href="/">Server</a></h1>
    <nav>
      <a href="/">Home</a>
    </nav>
    <main>
      \{\{template "page" .\}\}
    </main>
  </div>
</body>

</html>

\{\{end\}\}

index.html

\{\{define "page"\}\}
<h2>Index</h2>
\{\{end\}\}

References