a8m / golang-cheat-sheet

An overview of Go syntax and features.

Go Cheat Sheet

Index

1. Basic Syntax
2. Operators
   • Arithmetic
   • Comparison
   • Logical
   • Other
3. Declarations
4. Functions
   • Functions as values and closures
   • Variadic Functions
5. Built-in Types
6. Type Conversions
7. Packages
8. Control structures
   • If
   • Loops
   • Switch
9. Arrays, Slices, Ranges
   • Arrays
   • Slices
   • Operations on Arrays and Slices
10. Maps
11. Structs
12. Pointers
13. Interfaces
14. Embedding
15. Errors
16. Concurrency
    • Goroutines
    • Channels
    • Channel Axioms
17. Printing
18. Reflection
    • Type Switch
    • Examples
19. Snippets
    • Files Embedding
    • HTTP Server

Credits

Most example code taken from A Tour of Go, which is an excellent introduction to Go. If you're new to Go, do that tour. Seriously.

Go in a Nutshell

• Imperative language
• Statically typed
• Syntax tokens similar to C (but less parentheses and no semicolons) and the structure to Oberon-2
• Compiles to native code (no JVM)
• No classes, but structs with methods
• Interfaces
• No implementation inheritance. There's type embedding, though.
• Functions are first class citizens
• Functions can return multiple values
• Has closures
• Pointers, but not pointer arithmetic
• Built-in concurrency primitives: Goroutines and Channels

Basic Syntax

Hello World

File hello.go:

package main

import "fmt"

func main() {
    fmt.Println("Hello Go")
}

$ go run hello.go

Operators

Arithmetic

Operator	Description
+	addition
-	subtraction
*	multiplication
/	quotient
%	remainder
&	bitwise and
|	bitwise or
^	bitwise xor
&^	bit clear (and not)
<<	left shift
>>	right shift

Comparison

Operator	Description
==	equal
!=	not equal
<	less than
<=	less than or equal
>	greater than
>=	greater than or equal

Logical

Operator	Description
&&	logical and
||	logical or
!	logical not

Other

Operator	Description
&	address of / create pointer
*	dereference pointer
<-	send / receive operator (see 'Channels' below)

Declarations

Type goes after identifier!

var foo int // declaration without initialization
var foo int = 42 // declaration with initialization
var foo, bar int = 42, 1302 // declare and init multiple vars at once
var foo = 42 // type omitted, will be inferred
foo := 42 // shorthand, only in func bodies, omit var keyword, type is always implicit
const constant = "This is a constant"

// iota can be used for incrementing numbers, starting from 0
const (
    _ = iota
    a
    b
    c = 1 << iota
    d
)
    fmt.Println(a, b) // 1 2 (0 is skipped)
    fmt.Println(c, d) // 8 16 (2^3, 2^4)

Functions

// a simple function
func functionName() {}

// function with parameters (again, types go after identifiers)
func functionName(param1 string, param2 int) {}

// multiple parameters of the same type
func functionName(param1, param2 int) {}

// return type declaration
func functionName() int {
    return 42
}

// Can return multiple values at once
func returnMulti() (int, string) {
    return 42, "foobar"
}
var x, str = returnMulti()

// Return multiple named results simply by return
func returnMulti2() (n int, s string) {
    n = 42
    s = "foobar"
    // n and s will be returned
    return
}
var x, str = returnMulti2()

Functions As Values And Closures

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))
}

// Closures, lexically scoped: Functions can access values that were
// in scope when defining the function
func scope() func() int{
    outer_var := 2
    foo := func() int { return outer_var}
    return foo
}

func another_scope() func() int{
    // won't compile because outer_var and foo not defined in this scope
    outer_var = 444
    return foo
}


// Closures
func outer() (func() int, int) {
    outer_var := 2
    inner := func() int {
        outer_var += 99 // outer_var from outer scope is mutated.
        return outer_var
    }
    inner()
    return inner, outer_var // return inner func and mutated outer_var 101
}

Variadic Functions

func main() {
	fmt.Println(adder(1, 2, 3)) 	// 6
	fmt.Println(adder(9, 9))	// 18

	nums := []int{10, 20, 30}
	fmt.Println(adder(nums...))	// 60
}

// By using ... before the type name of the last parameter you can indicate that it takes zero or more of those parameters.
// The function is invoked like any other function except we can pass as many arguments as we want.
func adder(args ...int) int {
	total := 0
	for _, v := range args { // Iterates over the arguments whatever the number.
		total += v
	}
	return total
}

Built-in Types

bool

string

int  int8  int16  int32  int64
uint uint8 uint16 uint32 uint64 uintptr

byte // alias for uint8

rune // alias for int32 ~= a character (Unicode code point) - very Viking

float32 float64

complex64 complex128

All Go's predeclared identifiers are defined in the builtin package.

Type Conversions

var i int = 42
var f float64 = float64(i)
var u uint = uint(f)

// alternative syntax
i := 42
f := float64(i)
u := uint(f)

Packages

• Package declaration at top of every source file
• Executables are in package main
• Convention: package name == last name of import path (import path math/rand => package rand)
• Upper case identifier: exported (visible from other packages)
• Lower case identifier: private (not visible from other packages)

Control structures

If

func main() {
	// Basic one
	if x > 10 {
		return x
	} else if x == 10 {
		return 10
	} else {
		return -x
	}

	// You can put one statement before the condition
	if a := b + c; a < 42 {
		return a
	} else {
		return a - 42
	}

	// Type assertion inside if
	var val interface{} = "foo"
	if str, ok := val.(string); ok {
		fmt.Println(str)
	}
}

Loops

    // There's only `for`, no `while`, no `until`
    for i := 1; i < 10; i++ {
    }
    for ; i < 10;  { // while - loop
    }
    for i < 10  { // you can omit semicolons if there is only a condition
    }
    for { // you can omit the condition ~ while (true)
    }
    
    // use break/continue on current loop
    // use break/continue with label on outer loop
here:
    for i := 0; i < 2; i++ {
        for j := i + 1; j < 3; j++ {
            if i == 0 {
                continue here
            }
            fmt.Println(j)
            if j == 2 {
                break
            }
        }
    }

there:
    for i := 0; i < 2; i++ {
        for j := i + 1; j < 3; j++ {
            if j == 1 {
                continue
            }
            fmt.Println(j)
            if j == 2 {
                break there
            }
        }
    }

Switch

    // switch statement
    switch operatingSystem {
    case "darwin":
        fmt.Println("Mac OS Hipster")
        // cases break automatically, no fallthrough by default
    case "linux":
        fmt.Println("Linux Geek")
    default:
        // Windows, BSD, ...
        fmt.Println("Other")
    }

    // as with for and if, you can have an assignment statement before the switch value
    switch os := runtime.GOOS; os {
    case "darwin": ...
    }

    // you can also make comparisons in switch cases
    number := 42
    switch {
        case number < 42:
            fmt.Println("Smaller")
        case number == 42:
            fmt.Println("Equal")
        case number > 42:
            fmt.Println("Greater")
    }

    // cases can be presented in comma-separated lists
    var char byte = '?'
    switch char {
        case ' ', '?', '&', '=', '#', '+', '%':
            fmt.Println("Should escape")
    }

Arrays, Slices, Ranges

Arrays

var a [10]int // declare an int array with length 10. Array length is part of the type!
a[3] = 42     // set elements
i := a[3]     // read elements

// declare and initialize
var a = [2]int{1, 2}
a := [2]int{1, 2} //shorthand
a := [...]int{1, 2} // elipsis -> Compiler figures out array length

Slices

var a []int                              // declare a slice - similar to an array, but length is unspecified
var a = []int {1, 2, 3, 4}               // declare and initialize a slice (backed by the array given implicitly)
a := []int{1, 2, 3, 4}                   // shorthand
chars := []string{0:"a", 2:"c", 1: "b"}  // ["a", "b", "c"]

var b = a[lo:hi]	// creates a slice (view of the array) from index lo to hi-1
var b = a[1:4]		// slice from index 1 to 3
var b = a[:3]		// missing low index implies 0
var b = a[3:]		// missing high index implies len(a)
a =  append(a,17,3)	// append items to slice a
c := append(a,b...)	// concatenate slices a and b

// create a slice with make
a = make([]byte, 5, 5)	// first arg length, second capacity
a = make([]byte, 5)	// capacity is optional

// create a slice from an array
x := [3]string{"Лайка", "Белка", "Стрелка"}
s := x[:] // a slice referencing the storage of x

Operations on Arrays and Slices

len(a) gives you the length of an array/a slice. It's a built-in function, not a attribute/method on the array.

// loop over an array/a slice
for i, e := range a {
    // i is the index, e the element
}

// if you only need e:
for _, e := range a {
    // e is the element
}

// ...and if you only need the index
for i := range a {
}

// In Go pre-1.4, you'll get a compiler error if you're not using i and e.
// Go 1.4 introduced a variable-free form, so that you can do this
for range time.Tick(time.Second) {
    // do it once a sec
}

Maps

m := make(map[string]int)
m["key"] = 42
fmt.Println(m["key"])

delete(m, "key")

elem, ok := m["key"] // test if key "key" is present and retrieve it, if so

// map literal
var m = map[string]Vertex{
    "Bell Labs": {40.68433, -74.39967},
    "Google":    {37.42202, -122.08408},
}

// iterate over map content
for key, value := range m {
}

Structs

There are no classes, only structs. Structs can have methods.

// A struct is a type. It's also a collection of fields

// Declaration
type Vertex struct {
    X, Y float64
}

// Creating
var v = Vertex{1, 2}
var v = Vertex{X: 1, Y: 2} // Creates a struct by defining values with keys
var v = []Vertex{{1,2},{5,2},{5,5}} // Initialize a slice of structs

// Accessing members
v.X = 4

// You can declare methods on structs. The struct you want to declare the
// method on (the receiving type) comes between the the func keyword and
// the method name. The struct is copied on each method call(!)
func (v Vertex) Abs() float64 {
    return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

// Call method
v.Abs()

// For mutating methods, you need to use a pointer (see below) to the Struct
// as the type. With this, the struct value is not copied for the method call.
func (v *Vertex) add(n float64) {
    v.X += n
    v.Y += n
}

Anonymous structs: Cheaper and safer than using map[string]interface{}.

point := struct {
	X, Y int
}{1, 2}

Pointers

p := Vertex{1, 2}  // p is a Vertex
q := &p            // q is a pointer to a Vertex
r := &Vertex{1, 2} // r is also a pointer to a Vertex

// The type of a pointer to a Vertex is *Vertex

var s *Vertex = new(Vertex) // new creates a pointer to a new struct instance

Interfaces

// interface declaration
type Awesomizer interface {
    Awesomize() string
}

// types do *not* declare to implement interfaces
type Foo struct {}

// instead, types implicitly satisfy an interface if they implement all required methods
func (foo Foo) Awesomize() string {
    return "Awesome!"
}

Embedding

There is no subclassing in Go. Instead, there is interface and struct embedding.

// ReadWriter implementations must satisfy both Reader and Writer
type ReadWriter interface {
    Reader
    Writer
}

// Server exposes all the methods that Logger has
type Server struct {
    Host string
    Port int
    *log.Logger
}

// initialize the embedded type the usual way
server := &Server{"localhost", 80, log.New(...)}

// methods implemented on the embedded struct are passed through
server.Log(...) // calls server.Logger.Log(...)

// the field name of the embedded type is its type name (in this case Logger)
var logger *log.Logger = server.Logger

Errors

There is no exception handling. Instead, functions that might produce an error just declare an additional return value of type error. This is the error interface:

// The error built-in interface type is the conventional interface for representing an error condition,
// with the nil value representing no error.
type error interface {
    Error() string
}

Here's an example:

func sqrt(x float64) (float64, error) {
	if x < 0 {
		return 0, errors.New("negative value")
	}
	return math.Sqrt(x), nil
}

func main() {
	val, err := sqrt(-1)
	if err != nil {
		// handle error
		fmt.Println(err) // negative value
		return
	}
	// All is good, use `val`.
	fmt.Println(val)
}

Concurrency

Goroutines

Goroutines are lightweight threads (managed by Go, not OS threads). go f(a, b) starts a new goroutine which runs f (given f is a function).

// just a function (which can be later started as a goroutine)
func doStuff(s string) {
}

func main() {
    // using a named function in a goroutine
    go doStuff("foobar")

    // using an anonymous inner function in a goroutine
    go func (x int) {
        // function body goes here
    }(42)
}

Channels

ch := make(chan int) // create a channel of type int
ch <- 42             // Send a value to the channel ch.
v := <-ch            // Receive a value from ch

// Non-buffered channels block. Read blocks when no value is available, write blocks until there is a read.

// Create a buffered channel. Writing to a buffered channels does not block if less than <buffer size> unread values have been written.
ch := make(chan int, 100)

close(ch) // closes the channel (only sender should close)

// read from channel and test if it has been closed
v, ok := <-ch

// if ok is false, channel has been closed

// Read from channel until it is closed
for i := range ch {
    fmt.Println(i)
}

// select blocks on multiple channel operations, if one unblocks, the corresponding case is executed
func doStuff(channelOut, channelIn chan int) {
    select {
    case channelOut <- 42:
        fmt.Println("We could write to channelOut!")
    case x := <- channelIn:
        fmt.Println("We could read from channelIn")
    case <-time.After(time.Second * 1):
        fmt.Println("timeout")
    }
}

Channel Axioms

• A send to a nil channel blocks forever

  var c chan string
  c <- "Hello, World!"
  // fatal error: all goroutines are asleep - deadlock!

• A receive from a nil channel blocks forever

  var c chan string
  fmt.Println(<-c)
  // fatal error: all goroutines are asleep - deadlock!

• A send to a closed channel panics

  var c = make(chan string, 1)
  c <- "Hello, World!"
  close(c)
  c <- "Hello, Panic!"
  // panic: send on closed channel

• A receive from a closed channel returns the zero value immediately

  var c = make(chan int, 2)
  c <- 1
  c <- 2
  close(c)
  for i := 0; i < 3; i++ {
      fmt.Printf("%d ", <-c)
  }
  // 1 2 0

Printing

fmt.Println("Hello, 你好, नमस्ते, Привет, ᎣᏏᏲ") // basic print, plus newline
p := struct { X, Y int }{ 17, 2 }
fmt.Println( "My point:", p, "x coord=", p.X ) // print structs, ints, etc
s := fmt.Sprintln( "My point:", p, "x coord=", p.X ) // print to string variable

fmt.Printf("%d hex:%x bin:%b fp:%f sci:%e",17,17,17,17.0,17.0) // c-ish format
s2 := fmt.Sprintf( "%d %f", 17, 17.0 ) // formatted print to string variable

hellomsg := `
 "Hello" in Chinese is 你好 ('Ni Hao')
 "Hello" in Hindi is नमस्ते ('Namaste')
` // multi-line string literal, using back-tick at beginning and end

Reflection

Type Switch

A type switch is like a regular switch statement, but the cases in a type switch specify types (not values) which are compared against the type of the value held by the given interface value.

func do(i interface{}) {
	switch v := i.(type) {
	case int:
		fmt.Printf("Twice %v is %v\n", v, v*2)
	case string:
		fmt.Printf("%q is %v bytes long\n", v, len(v))
	default:
		fmt.Printf("I don't know about type %T!\n", v)
	}
}

func main() {
	do(21)
	do("hello")
	do(true)
}

Snippets

Files Embedding

Go programs can embed static files using the "embed" package as follows:

package main

import (
	"embed"
	"log"
	"net/http"
)

// content holds the static content (2 files) for the web server.
//go:embed a.txt b.txt
var content embed.FS

func main() {
	http.Handle("/", http.FileServer(http.FS(content)))
	log.Fatal(http.ListenAndServe(":8080", nil))
}

Full Playground Example

HTTP Server

package main

import (
    "fmt"
    "net/http"
)

// define a type for the response
type Hello struct{}

// let that type implement the ServeHTTP method (defined in interface http.Handler)
func (h Hello) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, "Hello!")
}

func main() {
    var h Hello
    http.ListenAndServe("localhost:4000", h)
}

// Here's the method signature of http.ServeHTTP:
// type Handler interface {
//     ServeHTTP(w http.ResponseWriter, r *http.Request)
// }

golang-cheat-sheet's Issues

Operator Precedence

All non-built in types can have methods

https://github.com/basti1302/go-lang-cheat-sheet/blame/master/README.markdown#L345
and
https://github.com/basti1302/go-lang-cheat-sheet/blame/master/README.markdown#L361-L363

Seem to imply that functions can only be assigned to structs, however all user defined types can have methods. There is nothing unique about structs with regard to method/function declaration.

see example:
https://play.golang.org/p/DPFZz0mCbi

Example of Anonymous structs "Cheaper and safer than using map[string]interface{}"

Hello,

Please explain how one can replace map[string]interface{} functionality with an anonymous struct

Adding common Go packages?

Would you be looking to add common Go modules to this guide, or is it no longer being maintained? I'd be happy to contribute.

Hello add this cheat sheet to awesome-cheatsheet

Hello, we have indexd this cheat sheet to awesome-cheatsheet

Thank You !!! 🚀 🚀 🚀

Generic type in Golang 1.18

Could you add information about generic of Golang 1.18 for cheat sheet. Tks!

Add comment section

Personally, I think adding a "comment" section will be better for this cheat sheet.

https://www.youtube.com/channel/UCKpfLD-ULmi62n2AJ5badjw link my acc pless Folower ME

threads implementation specifics

Goroutines are lightweight threads (managed by Go, not OS threads).

Actually, the way goroutines are implemented are subject to the specific implementation. With gccgo, it does (or did?) use an OS thread for each goroutine.
from http://www.airs.com/blog/archives/448 :
"
On the functionality side, the gccgo library uses a single operating system thread for each goroutine. That is not what 6g/8g do: they multiplex goroutines onto operating system threads. Multiplexing is more efficient for a language like Go, and I need to change gccgo to work that way.
"

When to use pointers?

There is an example regarding pointers, but there is no comment that describes when to use pointers.

add testing to the cheat-sheet

Closures do modify outer variables

Hi,

I believe the example about Closures is not quite correct. The comment says that the returned outer_var is still 2 because inner() has only modified a copy of it. However, when outer() returns, inner() has not been called yet, and this is why outer_var is 2 at that time. When inner is executed before outer() returns, then outer_var is returned as 101. See this Playground test code: https://play.golang.org/p/e_wktwN6me

Thanks,
Christoph

goto and labels

Page numbers and release version of cheat-sheet

I have printed the pages, but there are no page numbers. In my opinion, the pdf and odt should be part of a release rather than being stored in git.

break & continue

Please add break & continue that controls the for loop. Thx.

Add an example for iota identifier in const declaration

An example can be found here

https://www.youtube.com/channel/UCKpfLD-ULmi62n2AJ5badjw

panic, recover and defer

How about adding other languages' translation version to this repo?

This repo is really useful when I want to check the syntax of Go.

I have translated a chinese version of this repo.
golang-cheat-sheet-cn

If you'd like to to add other languages's translation into your repo, I can open a PR.

Thanks.

When to use interfaces?

Update your golang_refcard.pdf, please

outer_var in the outer func wasn't changed because the anonymous inner function hadn't been called yet.

// Closures: don't mutate outer vars, instead redefine them!
func outer() (func() int, int) {
	outer_var := 2 // NOTE outer_var is outside inner's scope
	inner := func() int {
		outer_var += 99  // attempt to mutate outer_var
		return outer_var // => 101 (but outer_var is a newly redefined
		//  variable visible only inside inner)
	}
	return inner, outer_var // => 101, 2 (still!)
}

I saw you made updates on the README file, but some people may also read your pdf file.

Add a section for strings

I wish there was a section on strings. How to index them, some formatting tricks, etc.

Hello world example can be shorter

You don't need to include fmt package for printing "hello world"

package main

func main(){
  println("Hello world")
}

Functions As Values And Closures gives out whole bunch of errors

Hi, I'm new to go, but I can't compile source code from 'Functions As Values And Closures' chapter.
I had to change:
outer_var = 444 to outer_var := 444,
func outer() func() int, int{ to func outer() (func() int, int){
and in both another_scope and outer functions I'm facing undefined foo error. Am I doing smth wrong?
My go version output is go version go1.4.2 linux/amd64

typo in maps example

elem, ok = m["key"] // test if key "key" is present and retrieve it, if so

should actually be

elem, ok := m["key"] // test if key "key" is present and retrieve it, if so

You are missing a :

ESP translation

Is it ok if I begin to translate the document to Spanish, or is someone else already working on it?

a8m/golang-cheat-sheet