---
title: Go
date: 2020-12-17 21:51:44
background: bg-[#4ba4cc]
tags:
- Go
categories:
- Programming
intro: |
This cheat sheet provided basic syntax and methods to help you using [Go](https://go.dev/).
plugins:
- copyCode
- runCode
---
## Getting Started
### hello.go
```go
package main
import "fmt"
func main() {
fmt.Println("Hello, world!")
}
```
Run directly
```shell script
$ go run hello.go
Hello, world!
```
Or try it out in the [Go repl](https://repl.it/languages/go)
### Variables
```go
var s1 string
s1 = "Learn Go!"
// declare multiple variables at once
var b, c int = 1, 2
var d = true
```
Short declaration
```go
s1 := "Learn Go!" // string
b, c := 1, 2 // int
d := true // bool
```
See: [Basic types](#go-basic-types)
### Functions
```go
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](#go-functions)
### Comments
```go
// Single line comment
/* Multi-
line comment */
```
### If statement
```go
if true {
fmt.Println("Yes!")
}
```
See: [Flow control](#go-flow-control)
## Go Basic types
### Strings
```go
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`.
### Numbers
```go
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
```go
x := 5
x++
fmt.Println("x + 4 =", x + 4)
fmt.Println("x * 4 =", x * 4)
```
See: [More Operators](#go-operators-and-punctuation)
### Booleans
```go
isTrue := true
isFalse := false
```
#### Operators
```go
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](#go-operators-and-punctuation)
### Arrays {.row-span-2}
```go
┌────┬────┬────┬────┬─────┬─────┐
| 2 | 3 | 5 | 7 | 11 | 13 |
└────┴────┴────┴────┴─────┴─────┘
0 1 2 3 4 5
```
---
```go
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])
```
---
```go
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
```go
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
```go
func main () {
b := *getPointer()
fmt.Println("Value is", b)
}
```
```go
func getPointer () (myPointer *int) {
a := 234
return &a
}
```
```go
a := new(int)
*a = 234
```
See: [Pointers](https://tour.go.dev/moretypes/1)
### Slices
```go
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](https://gobyexample.com/slices)
### Constants
```go
const s string = "constant"
const Phi = 1.618
const n = 500000000
const d = 3e20 / n
fmt.Println(d)
```
### Type conversions
```go
i := 90
f := float64(i)
u := uint(i)
// Will be equal to the character Z
s := string(i)
```
#### How to get int string?
```go
i := 90
// need import "strconv"
s := strconv.Itoa(i)
fmt.Println(s) // Outputs: 90
```
## Go Strings
### Strings function
```go
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 {.row-span-2 .col-span-2}
```go
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](https://go.dev/pkg/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 |
## Go Flow control
### Conditional
```go
a := 10
if a > 20 {
fmt.Println(">")
} else if a < 20 {
fmt.Println("<")
} else {
fmt.Println("=")
}
```
### Statements in if
```go
x := "hello go!"
if count := len(x); count > 0 {
fmt.Println("Yes")
}
```
---
```go
if _, err := doThing(); err != nil {
fmt.Println("Uh oh")
}
```
### Switch
```go {.wrap}
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](https://github.com/golang/go/wiki/Switch)
### For loop
```go
for i := 0; i <= 10; i++ {
fmt.Println("i: ", i)
}
```
### For-Range loop
```go {.wrap}
nums := []int{2, 3, 4}
sum := 0
for _, num := range nums {
sum += num
}
fmt.Println("sum:", sum)
```
### While loop
```go
i := 1
for i <= 3 {
fmt.Println(i)
i++
}
```
### Continue keyword
```go
for i := 0; i <= 5; i++ {
if i % 2 == 0 {
continue
}
fmt.Println(i)
}
```
### Break keyword
```go
for {
fmt.Println("loop")
break
}
```
## Go Structs & Maps
### Defining {.row-span-2}
```go
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](https://tour.go.dev/moretypes/2)
### Literals
```go
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 {.row-span-2}
```go
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
```go
v := &Vertex{1, 2}
v.X = 2
```
Doing `v.X` is the same as doing `(*v).X`, when `v` is a pointer.
## Go Functions
### Multiple arguments
```go
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
```go
func vals() (int, int) {
return 3, 7
}
a, b := vals()
fmt.Println(a) // => 3
fmt.Println(b) // => 7
```
### Function literals
```go
r1, r2 := func() (string, string) {
x := []string{"hello", "cheatsheets.zip"}
return x[0], x[1]
}()
// => hello cheatsheets.zip
fmt.Println(r1, r2)
```
### Naked returns
```go
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.
### Variadic functions
```go
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
```go
import --> const --> var --> init()
```
---
```go
var num = setNumber()
func setNumber() int {
return 42
}
func init() {
num = 0
}
func main() {
fmt.Println(num) // => 0
}
```
### Functions as values
```go
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
```go
func scope() func() int{
outer_var := 2
foo := func() int {return outer_var}
return foo
}
// Outpus: 2
fmt.Println(scope()())
```
### Closures 2
```go
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
```
## Go Packages
### Importing {.row-span-2}
```go
import "fmt"
import "math/rand"
```
#### Same as
```go
import (
"fmt" // gives fmt.Println
"math/rand" // gives rand.Intn
)
```
See: [Importing](https://tour.go.dev/basics/1)
### Aliases {.row-span-2}
```go
import r "math/rand"
```
---
```go
import (
"fmt"
r "math/rand"
)
```
---
```go
r.Intn()
```
### Packages
```go
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](https://go.dev/doc/go1.4#internalpackages)
### Exporting names
```go
// Begin with a capital letter
func Hello () {
···
}
```
See: [Exported names](https://tour.go.dev/basics/3)
## Go Concurrency
### Goroutines {.row-span-2}
```go
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](https://tour.go.dev/concurrency/1), [Channels](https://tour.go.dev/concurrency/2)
### WaitGroup {.row-span-2}
```go
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](https://go.dev/pkg/sync/#WaitGroup)
### Closing channels
```go
ch <- 1
ch <- 2
ch <- 3
close(ch) // Closes a channel
```
---
```go
// Iterate the channel until closed
for i := range ch {
···
}
```
---
```go
// Closed if `ok == false`
v, ok := <- ch
```
See: [Range and close](https://tour.go.dev/concurrency/4)
### Buffered channels
```go
ch := make(chan int, 2)
ch <- 1
ch <- 2
ch <- 3
// fatal error:
// all goroutines are asleep - deadlock
```
See: [Buffered channels](https://tour.go.dev/concurrency/3)
## Go Error control
### Deferring functions
```go
func main() {
defer func() {
fmt.Println("Done")
}()
fmt.Println("Working...")
}
```
### Lambda defer
```go
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
```go
func main() {
defer fmt.Println("Done")
fmt.Println("Working...")
}
```
See: [Defer, panic and recover](https://blog.go.dev/defer-panic-and-recover)
## Go Methods {.cols-2}
### Receivers
```go
type Vertex struct {
X, Y float64
}
```
```go
func (v Vertex) Abs() float64 {
return math.Sqrt(v.X * v.X + v.Y * v.Y)
}
```
```go
v := Vertex{1, 2}
v.Abs()
```
See: [Methods](https://tour.go.dev/methods/1)
### Mutation
```go
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
```
```go
v := Vertex{6, 12}
v.Scale(0.5)
// `v` is updated
```
See: [Pointer receivers](https://tour.go.dev/methods/4)
## Go Interfaces {.cols-2}
### A basic interface
```go
type Shape interface {
Area() float64
Perimeter() float64
}
```
### Struct
```go
type Rectangle struct {
Length, Width float64
}
```
Struct `Rectangle` implicitly implements interface `Shape` by implementing all of its methods.
### Methods
```go
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
```go {.wrap}
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())
}
```
## Go generics {.cols-2}
### example 1
```go
// comparable represents types that can be compared.
type comparable interface {
int | float64 | string
}
// Max returns the maximum of two comparable values.
func Max[T comparable](a, b T) T {
if a > b {
return a
}
return b
}
func main() {
// Find the maximum of two integers.
maxInt := Max(10, 20)
fmt.Println("Max integer:", maxInt)
// Find the maximum of two floats.
maxFloat := Max(3.14, 2.71)
fmt.Println("Max float:", maxFloat)
// Find the maximum of two strings.
maxString := Max("apple", "banana")
fmt.Println("Max string:", maxString)
}
```
### example 2
```go
// Pair[T, U] represents a generic pair of values.
type Pair[T, U any] struct {
First T
Second U
}
func main() {
pair := Pair[int, string]{First: 42, Second: "hello"}
fmt.Println("First:", pair.First)
fmt.Println("Second:", pair.Second)
// Print the types of the values in the pair.
fmt.Println("Type of First:", reflect.TypeOf(pair.First))
fmt.Println("Type of Second:", reflect.TypeOf(pair.Second))
}
```
## Go File I/O {.cols-2}
### Reading a File
Read the contents of a file into memory.
```go
data, err := os.ReadFile("file.txt")
if err != nil {
log.Fatal(err)
}
```
### Writing to a File
Write data to a file with specific permissions (e.g., 0644 for read-write).
```go
err := os.WriteFile("file.txt", []byte("Hello, Go!"), 0644)
```
## Go Testing {.cols-2}
### Unit Test
Unit Test: A basic unit test follows the below naming convention and uses t.Errorf() to report failures.
```go
func TestAdd(t *testing.T) {
result := add(2, 3)
if result != 5 {
t.Errorf("Expected 5, got %d", result)
}
}
```
### Unit Test
Benchmark: Benchmark tests measure performance, using b.N to control iterations.
```go
func BenchmarkAdd(b *testing.B) {
for i := 0; i < b.N; i++ {
add(1, 2)
}
}
```
## Go JSON Handling {.cols-2}
### JSON Encoding
Convert a Go struct to JSON.
```go
data, _ := json.Marshal(person)
```
### JSON Decoding
Parse JSON data into a Go struct.
```go
json.Unmarshal(data, &person)
```
## Go Server {.cols-2}
### net/http (Standard Library)
```go
package main
import (
"fmt"
"net/http"
)
func handler(w http.ResponseWriter, r *http.Request) {
fmt.Fprint(w, "Hello from net/http!")
}
func main() {
http.HandleFunc("/", handler)
http.ListenAndServe(":8080", nil)
}
```
### gorilla/mux
```go
package main
import (
"fmt"
"net/http"
"github.com/gorilla/mux"
)
func main() {
r := mux.NewRouter()
r.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
fmt.Fprint(w, "Hello from Gorilla Mux!")
})
http.ListenAndServe(":8081", r)
}
```
### chi
```go
package main
import (
"fmt"
"net/http"
"github.com/go-chi/chi/v5"
)
func main() {
r := chi.NewRouter()
r.Get("/", func(w http.ResponseWriter, r *http.Request) {
fmt.Fprint(w, "Hello from Chi!")
})
http.ListenAndServe(":8082", r)
}
```
### gin
```go
package main
import (
"github.com/gin-gonic/gin"
)
func main() {
r := gin.Default()
r.GET("/", func(c *gin.Context) {
c.String(200, "Hello from Gin!")
})
r.Run(":8083")
}
```
### fiber
```go
package main
import (
"github.com/gofiber/fiber/v3"
)
func main() {
app := fiber.New()
app.Get("/", func(c *fiber.Ctx) error {
return c.SendString("Hello from Fiber!")
})
app.Listen(":8084")
}
```
### beego
```go
package main
import (
"github.com/beego/beego/v2/server/web"
)
func main() {
web.Get("/", func(ctx *web.Context) {
ctx.Output.Body([]byte("Hello from Beego!"))
})
web.Run()
}
```
## 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`
- `iota`
{.cols-3 .marker-none}
### Operators and punctuation
| | | | | | | | | |
| --------------- | --------------- | ---------------- | ---------------- | ----------------- | ---- | ----- | --- | --- |
| `+` | `&` | `+=` | `&=` | `&&` | `==` | `!=` | `(` | `)` |
| `-` | \| | `-=` | \|= | \|\| | `<` | `<=` | `[` | `]` |
| \* | `^` | \*= | `^=` | `<-` | `>` | `>=` | `{` | `}` |
| `/` | `<<` | `/=` | `<<=` | `++` | `=` | `:=` | `,` | `;` |
| `%` | `>>` | `%=` | `>>=` | `--` | `!` | `...` | `.` | `:` |
| | `&^` | `&^=` | | | | | | |
## Also see {.cols-1}
- [Devhints](https://devhints.io/go) _(devhints.io)_
- [A tour of Go](https://tour.go.dev/welcome/1) _(tour.go.dev)_
- [Go wiki](https://github.com/golang/go/wiki/) _(github.com)_
- [Effective Go](https://go.dev/doc/effective_go) _(go.dev)_
- [Go by Example](https://gobyexample.com/) _(gobyexample.com)_
- [Awesome Go](https://awesome-go.com/) _(awesome-go.com)_
- [JustForFunc Youtube](https://www.youtube.com/channel/UC_BzFbxG2za3bp5NRRRXJSw) _(youtube.com)_
- [Style Guide](https://github.com/golang/go/wiki/CodeReviewComments) _(github.com)_