package main //声明本文件的package名
 import "fmt" //import语言的fmt库——用于输出
 func main() {
   fmt.Println("hello world")
 }

解释执行（实际是编译成a.out再执行.
 $go run hello.go
 hello world
编译执行
 
 $go build hello.go
 $ls
 hello hello.go
 $./hello
 hello world

package main
 
import "fmt"
import "math"
 
func main() {
  fmt.Println("hello world")
 
  fmt.Printf("%tn", 1==2)
  fmt.Printf("二进制：%bn", 255)
  fmt.Printf("八进制：%on", 255)
  fmt.Printf("十六进制：%Xn", 255)
  fmt.Printf("十进制：%dn", 255)
  fmt.Printf("浮点数：%fn", math.Pi)
  fmt.Printf("字符串：%sn", "hello world")
}

//声明初始化一个变量
var x int = 100
var str string = "hello world"</pre>
//声明初始化多个变量
var i, j, k int = 1, 2, 3
 
//不用指明类型，通过初始化值来推导
var b = true //bool型

const s string = "hello world"
 const pi float32 = 3.1415926

func main() {
  var a [5]int
  fmt.Println("array a:", a)
 
  a[1] = 10
  a[3] = 30
  fmt.Println("assign:", a)
 
  fmt.Println("len:", len(a))
 
  b := [5]int{1, 2, 3, 4, 5}
  fmt.Println("init:", b)
 
  var c [2][3]int
  for i := 0; i < 2; i++ {
    for j := 0; j < 3; j++ {
      c[i][j] = i + j
    }
  }
  fmt.Println("2d: ", c)
}

a := [5]int{1, 2, 3, 4, 5}
b := a[2:4] // a[2] 和 a[3]，但不包括a[4]
fmt.Println(b)
b = a[:4] // 从 a[0]到a[4]，但不包括a[4]
fmt.Println(b)
b = a[2:] // 从 a[2]到a[4]，且包括a[2]
fmt.Println(b)

//if 语句
if x % 2 == 0 {
  //...
}
//if - else
if x % 2 == 0 {
  //偶数...
} else {
  //奇数...
}
 
//多分支
if num < 0 {
  //负数
} else if num == 10 {
  //零
} else {
  //正数
}

switch i {
  case 1:
    fmt.Println("one")
  case 2:
    fmt.Println("two")
  case 3:
    fmt.Println("three")
  case 4,5,6:
    fmt.Println("four, five, six")
  default:
    fmt.Println("invalid value!")
}

//经典的for语句 init; condition; post
for i := 0; i<10; i++{
   fmt.Println(i)
}
 
//精简的for语句 condition
i := 1
for i<10 {
  fmt.Println(i)
  i++
}
 
//死循环的for语句 相当于for(;;)
i :=1
for {
  if i>10 {
    break
  }
  i++
}

func main(){
  m := make(map[string]int) //使用make创建一个空的map
 
  m["one"] = 1
  m["two"] = 2
  m["three"] = 3
 
  fmt.Println(m) //输出 map[three:3 two:2 one:1] (顺序在运行时可能不一样)
  fmt.Println(len(m)) //输出 3
 
  v := m["two"] //从map里取值
  fmt.Println(v) // 输出 2
 
  delete(m, "two")
  fmt.Println(m) //输出 map[three:3 one:1]
 
  m1 := map[string]int{"one": 1, "two": 2, "three": 3}
  fmt.Println(m1) //输出 map[two:2 three:3 one:1] (顺序在运行时可能不一样)
 
  for key, val := range m1{
    fmt.Printf("%s => %d n", key, val)
    /*输出：(顺序在运行时可能不一样)
      three => 3
      one => 1
      two => 2*/
  }
}

var i int = 1
var pInt *int = &i
//输出：i=1   pInt=0xf8400371b0    *pInt=1
fmt.Printf("i=%dtpInt=%pt*pInt=%dn", i, pInt, *pInt)
 
*pInt = 2
//输出：i=2   pInt=0xf8400371b0    *pInt=2
fmt.Printf("i=%dtpInt=%pt*pInt=%dn", i, pInt, *pInt)
 
i = 3
//输出：i=3   pInt=0xf8400371b0    *pInt=3
fmt.Printf("i=%dtpInt=%pt*pInt=%dn", i, pInt, *pInt)

var p *[]int = new([]int)  // 为切片结构分配内存；*p == nil；很少使用
var v []int = make([]int, 10) // 切片v现在是对一个新的有10个整数的数组的引用
 
// 不必要地使问题复杂化：
var p *[]int = new([]int)
fmt.Println(p) //输出：&[]
*p = make([]int, 10, 10)
fmt.Println(p) //输出：&[0 0 0 0 0 0 0 0 0 0]
fmt.Println((*p)[2]) //输出： 0
 
// 习惯用法:
v := make([]int, 10)
fmt.Println(v) //输出：[0 0 0 0 0 0 0 0 0 0]

package main
import "fmt"
 
func max(a int, b int) int { //注意参数和返回值是怎么声明的
 
  if a > b {
    return a
  }
  return b
}
 
func main(){
  fmt.Println(max(4, 5))
}

package main
import "fmt"
 
func main(){
  v, e := multi_ret("one")
  fmt.Println(v,e) //输出 1 true
 
  v, e = multi_ret("four")
  fmt.Println(v,e) //输出 0 false
 
  //通常的用法(注意分号后有e)
  if v, e = multi_ret("four"); e {
    // 正常返回
  }else{
    // 出错返回
  }
}
 
func multi_ret(key string) (int, bool){
  m := map[string]int{"one": 1, "two": 2, "three": 3}
 
  var err bool
  var val int
 
  val, err = m[key]
 
  return val, err
}

func sum(nums ...int) {
  fmt.Print(nums, " ") //输出如 [1, 2, 3] 之类的数组
  total := 0
  for _, num := range nums { //要的是值而不是下标
    total += num
  }
  fmt.Println(total)
}
func main() {
  sum(1, 2)
  sum(1, 2, 3)
 
  //传数组
  nums := []int{1, 2, 3, 4}
  sum(nums...)
}

func nextNum() func() int {
  i,j := 1,1
  return func() int {
    var tmp = i+j
    i, j = j, tmp
    return tmp
  }
}
//main函数中是对nextNum的调用，其主要是打出下一个斐波拉契数
func main(){
  nextNumFunc := nextNum()
  for i:=0; i<10; i++ {
    fmt.Println(nextNumFunc())
  }
}

func fact(n int) int {
  if n == 0 {
    return 1
  }
  return n * fact(n-1)
}
 
func main() {
  fmt.Println(fact(7))
}

type Person struct {
  name string
  age int
  email string
}
 
func main() {
  //初始化
  person := Person{"Tom", 30, "tom@gmail.com"}
  person = Person{name:"Tom", age: 30, email:"tom@gmail.com"}
 
  fmt.Println(person) //输出 {Tom 30 tom@gmail.com}
 
  pPerson := &person
 
  fmt.Println(pPerson) //输出 &{Tom 30 tom@gmail.com}
 
  pPerson.age = 40
  person.name = "Jerry"
  fmt.Println(person) //输出 {Jerry 40 tom@gmail.com}
}

type rect struct {
  width, height int
}
 
func (r *rect) area() int { //求面积
  return r.width * r.height
}
 
func (r *rect) perimeter() int{ //求周长
  return 2*(r.width + r.height)
}
 
func main() {
  r := rect{width: 10, height: 15}
 
  fmt.Println("面积: ", r.area())
  fmt.Println("周长: ", r.perimeter())
 
  rp := &r
  fmt.Println("面积: ", rp.area())
  fmt.Println("周长: ", rp.perimeter())
}

//---------- 接 口 --------//
type shape interface {
  area() float64 //计算面积
  perimeter() float64 //计算周长
}
 
//--------- 长方形 ----------//
type rect struct {
  width, height float64
}
 
func (r *rect) area() float64 { //面积
  return r.width * r.height
}
 
func (r *rect) perimeter() float64 { //周长
  return 2*(r.width + r.height)
}
 
//----------- 圆 形 ----------//
type circle struct {
  radius float64
}
 
func (c *circle) area() float64 { //面积
  return math.Pi * c.radius * c.radius
}
 
func (c *circle) perimeter() float64 { //周长
  return 2 * math.Pi * c.radius
}
 
// ----------- 接口的使用 -----------//
func interface_test() {
  r := rect {width:2.9, height:4.8}
  c := circle {radius:4.3}
 
  s := []shape{&r, &c} //通过指针实现
 
  for _, sh := range s {
    fmt.Println(sh)
    fmt.Println(sh.area())
    fmt.Println(sh.perimeter())
  }
}

package main
 
import "fmt"
import "errors"
 
//自定义的出错结构
type myError struct {
  arg int
  errMsg string
}
//实现Error接口
func (e *myError) Error() string {
  return fmt.Sprintf("%d - %s", e.arg, e.errMsg)
}
 
//两种出错
func error_test(arg int) (int, error) {
  if arg < 0 {
     return -1, errors.New("Bad Arguments - negtive!")
   }else if arg >256 {
    return -1, &myError{arg, "Bad Arguments - too large!"}
  }
  return arg*arg, nil
}
 
//相关的测试
func main() {
  for _, i := range []int{-1, 4, 1000} {
    if r, e := error_test(i); e != nil {
      fmt.Println("failed:", e)
    } else {
      fmt.Println("success:", r)
    }
  }
}

func CopyFile(dstName, srcName string) (written int64, err error) {
  src, err := os.Open(srcName)
  if err != nil {
    return
  }
 
  dst, err := os.Create(dstName)
  if err != nil {
    return
  }
 
  written, err = io.Copy(dst, src)
  dst.Close()
  src.Close()
  return
}

func CopyFile(dstName, srcName string) (written int64, err error) {
  src, err := os.Open(srcName)
  if err != nil {
    return
  }
  defer src.Close()
 
  dst, err := os.Create(dstName)
  if err != nil {
    return
  }
  defer dst.Close()
 
  return io.Copy(dst, src)
}

var user = os.Getenv("USER")
 
func init() {
  if user == "" {
    panic("no value for $USER")
  }
}

func g(i int) {
  if i>1 {
    fmt.Println("Panic!")
    panic(fmt.Sprintf("%v", i))
  }
 
}
 
func f() {
  defer func() {
    if r := recover(); r != nil {
      fmt.Println("Recovered in f", r)
    }
  }()
 
  for i := 0; i < 4; i++ {
    fmt.Println("Calling g with ", i)
    g(i)
    fmt.Println("Returned normally from g.")
   }
}
 
func main() {
  f()
  fmt.Println("Returned normally from f.")
}

Calling g with 0
Returned normally from g.
Calling g with 1
Returned normally from g.
Calling g with 2
Panic!
Recovered in f 2
Returned normally from f