title: Go date: 2020-12-17 21:51:44 background: bg-[#4ba4cc] tags: - Go categories: - Programming intro: |
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
s1 := "Learn Go!" // string
b, c := 1, 2 // int
d := true // bool
See: Basic types
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
// Single line comment
/* Multi-
line comment */
if true {
fmt.Println("Yes!")
}
See: Flow control
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
.
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
x := 5
x++
fmt.Println("x + 4 =", x + 4)
fmt.Println("x * 4 =", x * 4)
See: More Operators
isTrue := true
isFalse := false
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
┌────┬────┬────┬────┬─────┬─────┐
| 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]
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)
func main () {
b := *getPointer()
fmt.Println("Value is", b)
}
func getPointer () (myPointer *int) {
a := 234
return &a
}
a := new(int)
*a = 234
See: Pointers
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
const s string = "constant"
const Phi = 1.618
const n = 500000000
const d = 3e20 / n
fmt.Println(d)
i := 90
f := float64(i)
u := uint(i)
// Will be equal to the character Z
s := string(i)
i := 90
// need import "strconv"
s := strconv.Itoa(i)
fmt.Println(s) // Outputs: 90
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]))
}
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
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 |
a := 10
if a > 20 {
fmt.Println(">")
} else if a < 20 {
fmt.Println("<")
} else {
fmt.Println("=")
}
x := "hello go!"
if count := len(x); count > 0 {
fmt.Println("Yes")
}
if _, err := doThing(); err != nil {
fmt.Println("Uh oh")
}
```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)
}
```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++
}
for i := 0; i <= 5; i++ {
if i % 2 == 0 {
continue
}
fmt.Println(i)
}
for {
fmt.Println("loop")
break
}
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
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.
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{1, 2}
v.X = 2
Doing v.X
is the same as doing (*v).X
, when v
is a pointer.
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))
func vals() (int, int) {
return 3, 7
}
a, b := vals()
fmt.Println(a) // => 3
fmt.Println(b) // => 7
r1, r2 := func() (string, string) {
x := []string{"hello", "quickref.me"}
return x[0], x[1]
}()
// => hello quickref.me
fmt.Println(r1, r2)
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.
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
import --> const --> var --> init()
var num = setNumber()
func setNumber() int {
return 42
}
func init() {
num = 0
}
func main() {
fmt.Println(num) // => 0
}
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
}
func scope() func() int{
outer_var := 2
foo := func() int {return outer_var}
return foo
}
// Outpus: 2
fmt.Println(scope()())
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
import "fmt"
import "math/rand"
import (
"fmt" // gives fmt.Println
"math/rand" // gives rand.Intn
)
See: Importing
import r "math/rand"
import (
"fmt"
r "math/rand"
)
r.Intn()
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
// Begin with a capital letter
func Hello () {
···
}
See: Exported names
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
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
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
ch := make(chan int, 2)
ch <- 1
ch <- 2
ch <- 3
// fatal error:
// all goroutines are asleep - deadlock
See: Buffered channels
func main() {
defer func() {
fmt.Println("Done")
}()
fmt.Println("Working...")
}
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 fmt.Println("Done")
fmt.Println("Working...")
}
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 (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 Shape interface {
Area() float64
Perimeter() float64
}
type Rectangle struct {
Length, Width float64
}
Struct Rectangle
implicitly implements interface Shape
by implementing all of its 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
.
{.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())
}
+ | & | += | &= | && | == | != | ( | ) |
- | | | -= | |= | || | < | <= | [ | ] |
* | ^ | *= | ^= | <- | > | >= | { | } |
/ | << | /= | <<= | ++ | = | := | , | ; |
% | >> | %= | >>= | -- | ! | ... | . | : |
&^ | &^= |