cos

var x uint8 = 1<<1 | 1<<5

fmt.Printf("%08b\n", x) // "00100010", the set {1, 5}

o := 0666
fmt.Printf("%d %[1]o %#[1]o\n", o) // "438 666 0666"

x := int64(0xdeadbeef)
fmt.Printf("%d %[1]x %#[1]x %#[1]X\n", x)
// Output:
// 3735928559 deadbeef 0xdeadbeef 0XDEADBEEF

ascii := 'a'
unicode := '國'
newline := '\n'
fmt.Printf("%d %[1]c %[1]q\n", ascii)   // "97 a 'a'"
fmt.Printf("%d %[1]c %[1]q\n", unicode) // "22269 國 '國'"

medals := []string{"gold", "silver", "bronze"}
for i := len(medals) - 1; i >= 0; i-- {
    fmt.Println(medals[i]) // "bronze", "silver", "gold"
}

for x := 0; x < 8; x++ {
    fmt.Printf("x = %d e^x = %8.3f\n", x, math.Exp(float64(x)))
}
// x = 0       e^x =    1.000
// x = 1       e^x =    2.718
// x = 2       e^x =    7.389
// x = 3       e^x =   20.086
// x = 4       e^x =   54.598
// x = 5       e^x =  148.413
// x = 6       e^x =  403.429
// x = 7       e^x = 1096.633

var z float64
fmt.Println(z, -z, 1/z, -1/z, z/z) // "0 -0 +Inf -Inf NaN"

nan := math.NaN()
fmt.Println(nan == nan, nan < nan, nan > nan) // "false false false"

var x complex128 = complex(1, 2) // 1+2i
var y complex128 = complex(3, 4) // 3+4i
fmt.Println(x*y)                 // "(-5+10i)"
fmt.Println(real(x*y))           // "-5"
fmt.Println(imag(x*y))           // "10"

fmt.Println(1i * 1i) // "(-1+0i)", i^2 = -1

fmt.Println(1i * 1i) // "(-1+0i)", i^2 = -1

fmt.Println(cmplx.Sqrt(-1)) // "(0+1i)"

fmt.Println(s[0:5]) // "hello"
fmt.Println(s[:5]) // "hello"
fmt.Println(s[7:]) // "world"
fmt.Println(s[:])  // "hello, world"

fmt.Println("goodbye" + s[5:]) // "goodbye, world"

const GoUsage = `Go is a tool for managing Go source code.
Usage:
    go command [arguments]
...`

package main

import (
	"fmt"
	"strings"
)

func main() {
	a := "hello"
	fmt.Println(strings.Contains(a, "ll"))                // true
	fmt.Println(strings.Count(a, "l"))                    // 2
	fmt.Println(strings.HasPrefix(a, "he"))               // true
	fmt.Println(strings.HasSuffix(a, "llo"))              // true
	fmt.Println(strings.Index(a, "ll"))                   // 2
	fmt.Println(strings.Join([]string{"he", "llo"}, "-")) // he-llo
	fmt.Println(strings.Repeat(a, 2))                     // hellohello
	fmt.Println(strings.Replace(a, "e", "E", -1))         // hEllo
	fmt.Println(strings.Split("a-b-c", "-"))              // [a b c]
	fmt.Println(strings.ToLower(a))                       // hello
	fmt.Println(strings.ToUpper(a))                       // HELLO
	fmt.Println(len(a))                                   // 5
	b := "你好"
	fmt.Println(len(b)) // 6
}

package main

import "fmt"

type point struct {
	x, y int
}

func main() {
	s := "hello"
	n := 123
	p := point{1, 2}
	fmt.Println(s, n) // hello 123
	fmt.Println(p)    // {1 2}

	fmt.Printf("s=%v\n", s)  // s=hello
	fmt.Printf("n=%v\n", n)  // n=123
	fmt.Printf("p=%v\n", p)  // p={1 2}
	fmt.Printf("p=%+v\n", p) // p={x:1 y:2}
	fmt.Printf("p=%#v\n", p) // p=main.point{x:1, y:2}

	f := 3.141592653
	fmt.Println(f)          // 3.141592653
	fmt.Printf("%.2f\n", f) // 3.14
}

package main

import (
	"fmt"
	"strconv"
)

func main() {
	f, _ := strconv.ParseFloat("1.234", 64)
	fmt.Println(f) // 1.234

	n, _ := strconv.ParseInt("111", 10, 64)
	fmt.Println(n) // 111

	n, _ = strconv.ParseInt("0x1000", 0, 64)
	fmt.Println(n) // 4096

	n2, _ := strconv.Atoi("123")
	fmt.Println(n2) // 123

	n2, err := strconv.Atoi("AAA")
	fmt.Println(n2, err) // 0 strconv.Atoi: parsing "AAA": invalid syntax

	n3 := strconv.Itoa(123) // 這個不返回error
	fmt.Println(n3) // 123
}

const pi = 3.14159 // approximately; math.Pi is a better approximation
const (
    e  = 2.71828182845904523536028747135266249775724709369995957496696763
    pi = 3.14159265358979323846264338327950288419716939937510582097494459
)
const (
    a = 1
    b
    c = 2
    d
)

type Weekday int
const (
    Sunday Weekday = iota
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday
)
// Sunday 對應 0 
// Monday 對應 1 
// ....
// Saturday 對應 6

type Flags uint

const (
    FlagUp Flags = 1 << iota // is up
    FlagBroadcast            // supports broadcast access capability
    FlagLoopback             // is a loopback interface
    FlagPointToPoint         // belongs to a point-to-point link
    FlagMulticast            // supports multicast access capability
)

fmt.Println(FlagUp, FlagBroadcast, FlagLoopback, FlagPointToPoint, FlagMulticast)
// 1 2 4 8 16

var 變量名 類型 = 表達式

var i, j, k int     // int int int
var b, f, s = true, 2.3, "hello" // bool float64 string

var f, err = os.Open(name) // os.Open returns a file and an error

i := 100                  // int
i, j := 0, 1              // int int
var boiling float64 = 100 // a float64
var names []string
var err error

x := 1
p := &x         // p, of type *int, points to x
fmt.Println(*p) // "1"
*p = 2          // equivalent to x = 2
fmt.Println(x)  // "2"

p := new(int)   // p, *int 類型, 指向匿名的 int 變量
fmt.Println(*p) // "0"
*p = 2          // 設置 int 匿名變量的值為 2
fmt.Println(*p) // "2"

type 類型名 底層類型

import "fmt"

type Celsius float64    // 攝氏溫度
type Fahrenheit float64 // 華氏溫度

const (
    AbsoluteZeroC Celsius = -273.15 // 絕對零度
    FreezingC     Celsius = 0       // 結冰點溫度
    BoilingC      Celsius = 100     // 沸水溫度
)

func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }

func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }

var c Celsius
var f Fahrenheit
fmt.Println(c == 0)          // "true"
fmt.Println(f >= 0)          // "true"
fmt.Println(c == f)          // compile error: type mismatch
fmt.Println(c == Celsius(f)) // "true"! 類型轉換操作不會改變值

func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }

c := FToC(212.0)
fmt.Println(c.String()) // "100°C"
fmt.Printf("%v\n", c)   // "100°C"; no need to call String explicitly
fmt.Printf("%s\n", c)   // "100°C"
fmt.Println(c)          // "100°C"
fmt.Printf("%g\n", c)   // "100"; does not call String
fmt.Println(float64(c)) // "100"; does not call String

for initialization; condition; post {
    // zero or more statements
}

i := 1
for {
        fmt.Println("loop")
        break
}
for j := 7; j < 9; j++ {
        fmt.Println(j)
}

for n := 0; n < 5; n++ {
        if n%2 == 0 {
                continue
        }
        fmt.Println(n)
}
for i <= 3 {
        fmt.Println(i)
        i = i + 1
}

if 7%2 == 0 {
        fmt.Println("7 is even")
} else {
        fmt.Println("7 is odd")
}

if 8%4 == 0 {
        fmt.Println("8 is divisible by 4")
}

if num := 9; num < 0 {
        fmt.Println(num, "is negative")
} else if num < 10 {
        fmt.Println(num, "has 1 digit")
} else {
        fmt.Println(num, "has multiple digits")
}

package main

import (
	"fmt"
	"time"
)

func main() {
	a := 2
	switch a {
	case 1:
		fmt.Println("one")
	case 2:
		fmt.Println("two")
	case 3:
		fmt.Println("three")
	case 4, 5:
		fmt.Println("four or five")
	default:
		fmt.Println("other")
	}

	t := time.Now()
	switch {
	case t.Hour() < 12:
		fmt.Println("It's before noon")
	default:
		fmt.Println("It's after noon")
	}
}

package main

import (
	"fmt"
	"os"
	"os/exec"
)

func main() {
	// go run example/20-env/main.go a b c d
	fmt.Println(os.Args)           // [/var/folders/8p/n34xxfnx38dg8bv_x8l62t_m0000gn/T/go-build3406981276/b001/exe/main a b c d]
	fmt.Println(os.Getenv("PATH")) // /usr/local/go/bin...
	fmt.Println(os.Setenv("AA", "BB"))

	buf, err := exec.Command("grep", "127.0.0.1", "/etc/hosts").CombinedOutput()
	if err != nil {
		panic(err)
	}
	fmt.Println(string(buf)) // 127.0.0.1       localhost
}

package main

import "fmt"

func main() {

	var a [5]int
	a[4] = 100
	fmt.Println("get:", a[2])
	fmt.Println("len:", len(a))

	b := [5]int{1, 2, 3, 4, 5}
	fmt.Println(b)

	var twoD [2][3]int
	for i := 0; i < 2; i++ {
		for j := 0; j < 3; j++ {
			twoD[i][j] = i + j
		}
	}
	fmt.Println("2d: ", twoD)
}

package main

import "fmt"

func main() {
	s := make([]string, 3)
	s[0] = "a"
	s[1] = "b"
	s[2] = "c"
	fmt.Println("get:", s[2])   // c
	fmt.Println("len:", len(s)) // 3

	s = append(s, "d")
	s = append(s, "e", "f")
	fmt.Println(s) // [a b c d e f]

	c := make([]string, len(s))
	copy(c, s)
	fmt.Println(c) // [a b c d e f]

	fmt.Println(s[2:5]) // [c d e]
	fmt.Println(s[:5])  // [a b c d e]
	fmt.Println(s[2:])  // [c d e f]

	good := []string{"g", "o", "o", "d"}
	fmt.Println(good) // [g o o d]
}

package main

import "fmt"

func main() {
	m := make(map[string]int)
	m["one"] = 1
	m["two"] = 2
	fmt.Println(m)           // map[one:1 two:2]
	fmt.Println(len(m))      // 2
	fmt.Println(m["one"])    // 1
	fmt.Println(m["unknow"]) // 0

	r, ok := m["unknow"]
	fmt.Println(r, ok) // 0 false

	delete(m, "one")

	m2 := map[string]int{"one": 1, "two": 2}
	var m3 = map[string]int{"one": 1, "two": 2}
	fmt.Println(m2, m3)
}

package main

import "fmt"

func main() {
	nums := []int{2, 3, 4}
	sum := 0
	for i, num := range nums {
		sum += num
		if num == 2 {
			fmt.Println("index:", i, "num:", num) // index: 0 num: 2
		}
	}
	fmt.Println(sum) // 9

	m := map[string]string{"a": "A", "b": "B"}
	for k, v := range m {
		fmt.Println(k, v) // b 8; a A
	}
	for k := range m {
		fmt.Println("key: ", k) // key:  a; key:  b
	}
	for _, v := range m {
		fmt.Println("value:", v) // value: A; value: B
	}
}

package main

import "fmt"

type user struct {
	name     string
	password string
}

func main() {
	a := user{name: "wang", password: "1024"}
	b := user{"wang", "1024"}
	c := user{name: "wang"}
	c.password = "1024"
	var d user
	d.name = "wang"
	d.password = "1024"

	fmt.Println(a, b, c, d)                 // {wang 1024} {wang 1024} {wang 1024} {wang 1024}
	fmt.Println(checkPassword(a, "haha"))   // false
	fmt.Println(checkPassword2(&a, "haha")) // false
}

func checkPassword(u user, password string) bool {
	return u.password == password
}

func checkPassword2(u *user, password string) bool {
	return u.password == password
}

package main

import (
	"encoding/json"
	"fmt"
)

type userInfo struct {
	Name  string
	Age   int `json:"age"`
	Hobby []string
}

func main() {
	a := userInfo{Name: "wang", Age: 18, Hobby: []string{"Golang", "TypeScript"}}
	buf, err := json.Marshal(a)
	if err != nil {
		panic(err)
	}
	fmt.Println(buf)         // [123 34 78 97...]
	fmt.Println(string(buf)) // {"Name":"wang","age":18,"Hobby":["Golang","TypeScript"]}

	buf, err = json.MarshalIndent(a, "", "\t")
	if err != nil {
		panic(err)
	}
	fmt.Println(string(buf))

	var b userInfo
	err = json.Unmarshal(buf, &b)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%#v\n", b) // main.userInfo{Name:"wang", Age:18, Hobby:[]string{"Golang", "TypeScript"}}
}

package main

import (
	"fmt"
	"time"
)

func main() {
	now := time.Now()
	fmt.Println(now) // 2022-05-07 13:12:03.7190528 +0800 CST m=+0.004990401
	t := time.Date(2022, 5, 7, 13, 25, 36, 0, time.UTC)
	t2 := time.Date(2022, 8, 12, 12, 30, 36, 0, time.UTC)
	fmt.Println(t)                                                  // 2022-05-07 13:25:36 +0000 UTC
	fmt.Println(t.Year(), t.Month(), t.Day(), t.Hour(), t.Minute()) // 2022 March 27 1 25
	fmt.Println(t.Format("2006-01-02 15:04:05"))                    // 2022-05-07 13:25:36
	diff := t2.Sub(t)
	fmt.Println(diff)                           // 2327h5m0s
	fmt.Println(diff.Minutes(), diff.Seconds()) // 139625 8.3775e+06
	t3, err := time.Parse("2006-01-02 15:04:05", "2022-05-07 13:25:36")
	if err != nil {
		panic(err)
	}
	fmt.Println(t3 == t)    // true
	fmt.Println(now.Unix()) // 1651900531
}

package main

import "fmt"

func add(a int, b int) int {
	return a + b
}

func add2(a, b int) int {
	return a + b
}

func exists(m map[string]string, k string) (v string, ok bool) {
	v, ok = m[k]
	return v, ok
}

func main() {
	res := add(1, 2)
	fmt.Println(res) // 3

	v, ok := exists(map[string]string{"a": "A"}, "a")
	fmt.Println(v, ok) // A True
}

package main

import (
	"errors"
	"fmt"
)

type user struct {
	name     string
	password string
}

func findUser(users []user, name string) (v *user, err error) {
	for _, u := range users {
		if u.name == name {
			return &u, nil
		}
	}
	return nil, errors.New("not found")
}

func main() {
	u, err := findUser([]user{{"wang", "1024"}}, "wang")
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(u.name) // wang

	if u, err := findUser([]user{{"wang", "1024"}}, "li"); err != nil {
		fmt.Println(err) // not found
		return
	} else {
		fmt.Println(u.name)
	}
}

package main

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

func main() {
	maxNum := 100
	rand.Seed(time.Now().UnixNano())
	secretNumber := rand.Intn(maxNum)
	// fmt.Println("The secret number is ", secretNumber)

	fmt.Println("Please input your guess")
	//reader := bufio.NewReader(os.Stdin)
	for {
		//input, err := reader.ReadString('\n')
		var guess int
		_, err := fmt.Scanf("%d", &guess)
		fmt.Scanf("%*c")    // 吃回車
		if err != nil {
			fmt.Println("An error occured while reading input. Please try again", err)
			continue
		}
		//input = strings.TrimSuffix(input, "\n")
		if err != nil {
			fmt.Println("Invalid input. Please enter an integer value")
			continue
		}
		fmt.Println("You guess is", guess)
		if guess > secretNumber {
			fmt.Println("Your guess is bigger than the secret number. Please try again")
		} else if guess < secretNumber {
			fmt.Println("Your guess is smaller than the secret number. Please try again")
		} else {
			fmt.Println("Correct, you Legend!")
			break
		}
	}
}