Go Cheat Sheet

• 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

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())
}

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.

type Rectangle struct {
  Length, Width float64
}

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

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

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

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

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

See: Defer, panic and recover

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.

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

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

See: Buffered 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

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

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

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

See: Exported names

package main
// An internal package may be imported only by another package
// that is inside the tree rooted at the parent of the internal directory
package internal

See: Internal packages

import r "math/rand"

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

r.Intn()

import "fmt"
import "math/rand"

#Same as

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

See: Importing

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

func scope() func() int{
    outer_var := 2
    foo := func() int {return outer_var}
    return foo
}
// Outpus: 2
fmt.Println(scope()())

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
}

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

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

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

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

Note that using naked returns hurts readability.

r1, r2 := func() (string, string) {
    x := []string{"hello", "quickref.me"}
    return x[0], x[1]
}()
// => hello quickref.me
fmt.Println(r1, r2)

func vals() (int, int) {
    return 3, 7
}
a, b := vals()
fmt.Println(a)    // => 3
fmt.Println(b)    // => 7

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))

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

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

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]

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.

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

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

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

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

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

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

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

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

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

a := 10
if a > 20 {
    fmt.Println(">")
} else if a < 20 {
    fmt.Println("<")
} else {
    fmt.Println("=")
}

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

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]))
}

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

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

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

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

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

a := new(int)
*a = 234

See: Pointers

┌────┬────┬────┬────┬─────┬─────┐
| 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)

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

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

s1 := "Hello" + "World"
s2 := `A "raw" string literal
can include line breaks.`
// Outputs: 10
fmt.Println(len(s1))
// Outputs: Hello
fmt.Println(string(s1[0:5]))

Strings are of type string.

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

See: Flow control

// Single line comment
/\* Multi-
 line comment \*/

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

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

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