# Go Cheat Sheet

This cheat sheet provided basic syntax and methods to help you using Go.

## Miscellaneous

#### Operators and punctuation

+ & += &= && == != ( )
- -= =
* ^ *= ^= <- > >= { }
/ << /= <<= ++ = := , ;
% >> %= >>= -- ! . :
&^ &^=

• 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

## Go Interfaces

#### 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())
}


#### 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.

#### Struct

type Rectangle struct {
Length, Width float64
}


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

#### A basic interface

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


## Go 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


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

## Go Error control

#### Defer

func main() {
defer 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.

#### Deferring functions

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


## Go Concurrency

#### Buffered channels

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


#### Closing channels

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


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


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


See: Range and close

#### 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++ {
go w(i, &wg)
}
wg.Wait()
}


See: WaitGroup

#### 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

## Go Packages

#### Exporting names

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


See: Exported names

#### Packages

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


#### Aliases

import r "math/rand"


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


r.Intn()


#### Importing

import "fmt"
import "math/rand"


#### #Same as

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


See: Importing

## Go Functions

#### 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


#### Closures 1

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


#### 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
}


#### init function

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


#### Naked returns

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.

#### Function literals

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


#### Multiple return

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


#### 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))


## Go Structs & Maps

#### 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.

#### 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]


#### 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.

#### 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

## Go Flow control

#### Break keyword

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


#### Continue keyword

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


#### While loop

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


#### For-Range loop

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


#### For loop

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


#### 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

#### Statements in if

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


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


#### Conditional

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


## Go Strings

#### 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

#### 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")
}


#### 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]))
}


## Go Basic types

#### 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


#### Constants

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


#### 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}


#### 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

#### Arrays

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| 2  | 3  | 5  | 7  | 11  | 13  |
ââââââ´âââââ´âââââ´âââââ´ââââââ´ââââââ
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)


#### 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

#### 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

#### Strings

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.

## Getting Started

#### If statement

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


See: Flow control

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


#### 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

#### 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

#### hello.go

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