出处---Go编程语言
Go基础语法,方便查阅
package main
import (
"fmt"
"math/rand"
)
func main() {
fmt.Println("My favorite number is", rand.Intn(10))
}
My favorite number is 1
import "fmt"
import "math"
package main
import (
"fmt"
"math"
)
func main() {
fmt.Printf("Now you have %g problems.", math.Sqrt(7))
}
Now you have 2.6457513110645907 problems.
package main
import (
"fmt"
"math"
)
func main() {
fmt.Println(math.pi)
}
tmp/sandbox583763709/main.go:9: cannot refer to unexported name math.pi
tmp/sandbox583763709/main.go:9: undefined: math.pi
package main
import "fmt"
func add(x int, y int) int {
return x + y
}
func main() {
fmt.Println(add(42, 13))
}
55
x int, y int
被缩写为
x, y int
package main
import "fmt"
func add(x, y int) int {
return x + y
}
func main() {
fmt.Println(add(42, 13))
}
55
swap
函数返回了两个字符串。package main
import "fmt"
func swap(x, y string) (string, string) {
return y, x
}
func main() {
a, b := swap("hello", "world")
fmt.Println(a, b)
}
world hello
package main
import "fmt"
func split(sum int) (x, y int) {
x = sum * 4 / 9
y = sum - x
return
}
func main() {
fmt.Println(split(17))
}
7 10
package main
import "fmt"
var c, python, java bool
func main() {
var i int
fmt.Println(i, c, python, java)
}
0 false false false
package main
import "fmt"
var i, j int = 1, 2
func main() {
var c, python, java = true, false, "no!"
fmt.Println(i, j, c, python, java)
}
1 2 true false no!
package main
import "fmt"
func main() {
var i, j int = 1, 2
k := 3
c, python, java := true, false, "no!"
fmt.Println(i, j, k, c, python, java)
}
1 2 3 true false no!
package main
import (
"fmt"
"math/cmplx"
)
var (
ToBe bool = false
MaxInt uint64 = 1<<64 - 1
z complex128 = cmplx.Sqrt(-5 + 12i)
)
func main() {
const f = "%T(%v)\n"
fmt.Printf(f, ToBe, ToBe)
fmt.Printf(f, MaxInt, MaxInt)
fmt.Printf(f, z, z)
}
bool(false)
uint64(18446744073709551615)
complex128((2+3i))
package main
import (
"fmt"
"math"
)
func main() {
var x, y int = 3, 4
var f float64 = math.Sqrt(float64(x*x + y*y))
var z uint = uint(f)
fmt.Println(x, y, z)
}
3 4 5
package main
import "fmt"
func main() {
var i int
var f float64
var b bool
var s string
fmt.Printf("%v %v %v %q\n", i, f, b, s)
}
0 0 false ""
var i int
j := i // j 也是一个 int
i := 42 // int
f := 3.142 // float64
g := 0.867 + 0.5i // complex128
package main
import "fmt"
func main() {
v := 42 // change me!
fmt.Printf("v is of type %T\n", v)
}
v is of type int
package main
import "fmt"
const Pi = 3.14
func main() {
const World = "世界"
fmt.Println("Hello", World)
fmt.Println("Happy", Pi, "Day")
const Truth = true
fmt.Println("Go rules?", Truth)
}
Hello 世界
Happy 3.14 Day
Go rules? true
needInt(Big)
吧。package main
import "fmt"
const (
Big = 1 << 100
Small = Big >> 99
)
func needInt(x int) int { return x*10 + 1 }
func needFloat(x float64) float64 {
return x * 0.1
}
func main() {
fmt.Println(needInt(Small))
fmt.Println(needFloat(Small))
fmt.Println(needFloat(Big))
}
21
0.2
1.2676506002282295e+29
package main
import "fmt"
func main() {
sum := 0
for i := 0; i < 10; i++ {
sum += i
}
fmt.Println(sum)
}
45
package main
import "fmt"
func main() {
sum := 1
for ; sum < 1000; {
sum += sum
}
fmt.Println(sum)
}
1024
package main
import "fmt"
func main() {
sum := 1
for sum < 1000 {
sum += sum
}
fmt.Println(sum)
}
1024
package main
func main() {
for {
}
}
process took too long
package main
import (
"fmt"
"math"
)
func sqrt(x float64) string {
if x < 0 {
return sqrt(-x) + "i"
}
return fmt.Sprint(math.Sqrt(x))
}
func main() {
fmt.Println(sqrt(2), sqrt(-4))
}
1.4142135623730951 2i
package main
import (
"fmt"
"math"
)
func pow(x, n, lim float64) float64 {
if v := math.Pow(x, n); v < lim {
return v
}
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}
9 20
package main
import (
"fmt"
"math"
)
func pow(x, n, lim float64) float64 {
if v := math.Pow(x, n); v < lim {
return v
} else {
fmt.Printf("%g >= %g\n", v, lim)
}
// 这里开始就不能使用 v 了
return lim
}
func main() {
fmt.Println(
pow(3, 2, 10),
pow(3, 3, 20),
)
}
27 >= 20
9 20
package main
import (
"fmt"
"runtime"
)
func main() {
fmt.Print("Go runs on ")
switch os := runtime.GOOS; os {
case "darwin":
fmt.Println("OS X.")
case "linux":
fmt.Println("Linux.")
default:
// freebsd, openbsd,
// plan9, windows...
fmt.Printf("%s.", os)
}
}
Go runs on nacl.
package main
import (
"fmt"
"time"
)
func main() {
fmt.Println("When's Saturday?")
today := time.Now().Weekday()
switch time.Saturday {
case today + 0:
fmt.Println("Today.")
case today + 1:
fmt.Println("Tomorrow.")
case today + 2:
fmt.Println("In two days.")
default:
fmt.Println("Too far away.")
}
}
When's Saturday?
Too far away.
package main
import (
"fmt"
"time"
)
func main() {
t := time.Now()
switch {
case t.Hour() < 12:
fmt.Println("Good morning!")
case t.Hour() < 17:
fmt.Println("Good afternoon.")
default:
fmt.Println("Good evening.")
}
}
Good evening.
package main
import "fmt"
func main() {
defer fmt.Println("world")
fmt.Println("hello")
}
hello
world
package main
import "fmt"
func main() {
fmt.Println("counting")
for i := 0; i < 10; i++ {
defer fmt.Println(i)
}
fmt.Println("done")
}
counting
done
9
8
7
6
5
4
3
2
1
0
*T
是指向类型 T的值的指针。其零值是 nil。var p *int
&
符号会生成一个指向其作用对象的指针。i := 42
p = &i
*
符号表示指针指向的底层的值。fmt.Println(*p) // 通过指针 p 读取 i
*p = 21 // 通过指针 p 设置 i
package main
import "fmt"
func main() {
i, j := 42, 2701
p := &i // point to i
fmt.Println(*p) // read i through the pointer
*p = 21 // set i through the pointer
fmt.Println(i) // see the new value of i
p = &j // point to j
*p = *p / 37 // divide j through the pointer
fmt.Println(j) // see the new value of j
}
42
21
73
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
fmt.Println(Vertex{1, 2})
}
{1 2}
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
v.X = 4
fmt.Println(v.X)
}
4
package main
import "fmt"
type Vertex struct {
X int
Y int
}
func main() {
v := Vertex{1, 2}
p := &v
p.X = 1e9
fmt.Println(v)
}
{1000000000 2}
&
返回一个指向结构体的指针。package main
import "fmt"
type Vertex struct {
X, Y int
}
var (
v1 = Vertex{1, 2} // 类型为 Vertex
v2 = Vertex{X: 1} // Y:0 被省略
v3 = Vertex{} // X:0 和 Y:0
p = &Vertex{1, 2} // 类型为 *Vertex
)
func main() {
fmt.Println(v1, p, v2, v3)
}
{1 2} &{1 2} {1 0} {0 0}
var a [10]int
package main
import "fmt"
func main() {
var a [2]string
a[0] = "Hello"
a[1] = "World"
fmt.Println(a[0], a[1])
fmt.Println(a)
}
Hello World
[Hello World]
[]T
是一个元素类型为 T的 slice。len(s)
返回 slice s 的长度。package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
fmt.Println("s ==", s)
for i := 0; i < len(s); i++ {
fmt.Printf("s[%d] == %d\n", i, s[i])
}
}
s == [2 3 5 7 11 13]
s[0] == 2
s[1] == 3
s[2] == 5
s[3] == 7
s[4] == 11
s[5] == 13
package main
import (
"fmt"
"strings"
)
func main() {
// Create a tic-tac-toe board.
game := [][]string{
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
[]string{"_", "_", "_"},
}
// The players take turns.
game[0][0] = "X"
game[2][2] = "O"
game[2][0] = "X"
game[1][0] = "O"
game[0][2] = "X"
printBoard(game)
}
func printBoard(s [][]string) {
for i := 0; i < len(s); i++ {
fmt.Printf("%s\n", strings.Join(s[i], " "))
}
}
X _ X
O _ _
X _ O
s[lo:hi]
s[lo:lo]
s[lo:lo+1]
package main
import "fmt"
func main() {
s := []int{2, 3, 5, 7, 11, 13}
fmt.Println("s ==", s)
fmt.Println("s[1:4] ==", s[1:4])
// 省略下标代表从 0 开始
fmt.Println("s[:3] ==", s[:3])
// 省略上标代表到 len(s) 结束
fmt.Println("s[4:] ==", s[4:])
}
s == [2 3 5 7 11 13]
s[1:4] == [3 5 7]
s[:3] == [2 3 5]
s[4:] == [11 13]
make
创建。这会分配一个全是零值的数组并且返回一个 slice 指向这个数组:a := make([]int, 5) // len(a)=5
b := make([]int, 0, 5) // len(b)=0, cap(b)=5
b = b[:cap(b)] // len(b)=5, cap(b)=5
b = b[1:] // len(b)=4, cap(b)=4
package main
import "fmt"
func main() {
a := make([]int, 5)
printSlice("a", a)
b := make([]int, 0, 5)
printSlice("b", b)
c := b[:2]
printSlice("c", c)
d := c[2:5]
printSlice("d", d)
}
func printSlice(s string, x []int) {
fmt.Printf("%s len=%d cap=%d %v\n",
s, len(x), cap(x), x)
}
a len=5 cap=5 [0 0 0 0 0]
b len=0 cap=5 []
c len=2 cap=5 [0 0]
d len=3 cap=3 [0 0 0]
package main
import "fmt"
func main() {
var z []int
fmt.Println(z, len(z), cap(z))
if z == nil {
fmt.Println("nil!")
}
}
[] 0 0
nil!
append
。 内建函数的文档对 append
有详细介绍。func append(s []T, vs ...T) []T
append
的第一个参数 s是一个元素类型为 T的 slice ,其余类型为 T的值将会附加到该 slice 的末尾。append
的结果是一个包含原 slice 所有元素加上新添加的元素的 slice。s
的底层数组太小,而不能容纳所有值时,会分配一个更大的数组。 返回的 slice 会指向这个新分配的数组。package main
import "fmt"
func main() {
var a []int
printSlice("a", a)
// append works on nil slices.
a = append(a, 0)
printSlice("a", a)
// the slice grows as needed.
a = append(a, 1)
printSlice("a", a)
// we can add more than one element at a time.
a = append(a, 2, 3, 4)
printSlice("a", a)
}
func printSlice(s string, x []int) {
fmt.Printf("%s len=%d cap=%d %v\n",
s, len(x), cap(x), x)
}
a len=0 cap=0 []
a len=1 cap=2 [0]
a len=2 cap=2 [0 1]
a len=5 cap=8 [0 1 2 3 4]
引言
数组
var a [4]int
a[0] = 1
i := a[0]
// i == 1
// a[2] == 0, int 类型的零值
类型 [4]int对应内存中四个连续的整数
b := [2]string{"Penn", "Teller"}
b := [...]string{"Penn", "Teller"}
切片
letters := []string{"a", "b", "c", "d"}
func make([]T, len, cap) []T
var s []bytes = make([]byte, 5, 5)// s == []byte{0, 0, 0, 0, 0}
s := make([]byte, 5)
len
和cap
获取切片的长度和容量信息。 len(s) == 5
cap(s) == 5
nil
。对于切片的零值, len
和cap
都将返回0。b[1:4]
创建的切片引用数组 b
的第1到3个元素空间(对应切片的索引为0到2)。b := []byte{'g', 'o', 'l', 'a', 'n', 'g'}
// b[1:4] == []byte{'o', 'l', 'a'}, sharing the same storage as b
// b[:2] == []byte{'g', 'o'}
// b[2:] == []byte{'l', 'a', 'n', 'g'}
// b[:] == b
x := [3]string{"Лайка", "Белка", "Стрелка"}
s := x[:] // a slice referencing the storage of x
切片的内幕
切片是一个数组片段的描述
s的结构
s = s[2:4]
数据结构和它引用的底层数组
d := []byte{'r', 'o', 'a', 'd'}
e := d[2:]
// e == []byte{'a', 'd'}
e[1] = 'm'
// e == []byte{'a', 'm'}
// d == []byte{'r', 'o', 'a', 'm'}
s = s[:cap(s)]
切片的生长(copy and append 函数)
t := make([]byte, len(s), (cap(s)+1)*2) // +1 in case cap(s) == 0
for i := range s {
t[i] = s[i]
}
s = t
func copy(dst, src []T) int
copy
函数支持不同长度的切片之间的复制(它只复制较短切片的长度个元素)。 此外,copy
函数可以正确处理源和目的切片有重叠的情况。t := make([]byte, len(s), (cap(s)+1)*2)
copy(t, s)
s = t
func AppendByte(slice []byte, data ...byte) []byte {
m := len(slice)
n := m + len(data)
if n > cap(slice) { // if necessary, reallocate
// allocate double what's needed, for future growth.
newSlice := make([]byte, (n+1)*2)
copy(newSlice, slice) slice = newSlice
}
slice = slice[0:n]
copy(slice[m:n], data)
return slice
}
p := []byte{2, 3, 5}
p = AppendByte(p, 7, 11, 13)
// p == []byte{2, 3, 5, 7, 11, 13}
func append(s []T, x ...T) []T
a := make([]int, 1)
// a == []int{0}
a = append(a, 1, 2, 3)
// a == []int{0, 1, 2, 3}
a := []string{"John", "Paul"}
b := []string{"George", "Ringo", "Pete"}
a = append(a, b...) // equivalent to "append(a, b[0], b[1], b[2])"
// a == []string{"John", "Paul", "George", "Ringo", "Pete"}
// Filter returns a new slice holding only
// the elements of s that satisfy f()
func Filter(s []int, fn func(int) bool) []int {
var p []int // == nil
for _, v := range s {
if fn(v) { p = append(p, v)
}
}
return p
}
可能的“陷阱”
FindDigits
函数加载整个文件到内存,然后搜索第一个连续的数字,最后结果以切片方式返回。var digitRegexp = regexp.MustCompile("[0-9]+")
func FindDigits(filename string) []byte {
b, _ := ioutil.ReadFile(filename)
return digitRegexp.Find(b)
}
[]byte
指向保存整个文件的数组。因为切片引用了原始的数组, 导致 GC 不能释放数组的空间;只用到少数几个字节却导致整个文件的内容都一直保存在内存里。func CopyDigits(filename string) []byte {
b, _ := ioutil.ReadFile(filename)
b = digitRegexp.Find(b)
c := make([]byte, len(b))
copy(c, b) return c
}
延伸阅读
package main
import "fmt"
var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}
func main() {
for i, v := range pow {
fmt.Printf("2**%d = %d\n", i, v)
}
}
2**0 = 1
2**1 = 2
2**2 = 4
2**3 = 8
2**4 = 16
2**5 = 32
2**6 = 64
2**7 = 128
package main
import "fmt"
func main() {
pow := make([]int, 10)
for i := range pow {
pow[i] = 1 << uint(i)
}
for _, value := range pow {
fmt.Printf("%d\n", value)
}
}
1
2
4
8
16
32
64
128
256
512
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m map[string]Vertex
func main() {
m = make(map[string]Vertex)
m["Bell Labs"] = Vertex{
40.68433, -74.39967,
}
fmt.Println(m["Bell Labs"])
}
{40.68433 -74.39967}
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m = map[string]Vertex{
"Bell Labs": Vertex{
40.68433, -74.39967,
},
"Google": Vertex{
37.42202, -122.08408,
},
}
func main() {
fmt.Println(m)
}
map[Bell Labs:{40.68433 -74.39967} Google:{37.42202 -122.08408}]
package main
import "fmt"
type Vertex struct {
Lat, Long float64
}
var m = map[string]Vertex{
"Bell Labs": {40.68433, -74.39967},
"Google": {37.42202, -122.08408},
}
func main() {
fmt.Println(m)
}
map[Bell Labs:{40.68433 -74.39967} Google:{37.42202 -122.08408}]
m[key] = elem
elem = m[key]
delete(m, key)
elem, ok = m[key]
package main
import "fmt"
func main() {
m := make(map[string]int)
m["Answer"] = 42
fmt.Println("The value:", m["Answer"])
m["Answer"] = 48
fmt.Println("The value:", m["Answer"])
delete(m, "Answer")
fmt.Println("The value:", m["Answer"])
v, ok := m["Answer"]
fmt.Println("The value:", v, "Present?", ok)
}
The value: 42
The value: 48
The value: 0
The value: 0 Present? false
package main
import (
"fmt"
"math"
)
func compute(fn func(float64, float64) float64) float64 {
return fn(3, 4)
}
func main() {
hypot := func(x, y float64) float64 {
return math.Sqrt(x*x + y*y)
}
fmt.Println(hypot(5, 12))
fmt.Println(compute(hypot))
fmt.Println(compute(math.Pow))
}
13
5
81
adder
返回一个闭包。每个返回的闭包都被绑定到其各自的sum
变量上。package main
import "fmt"
func adder() func(int) int {
sum := 0
return func(x int) int {
sum += x
return sum
}
}
func main() {
pos, neg := adder(), adder()
for i := 0; i < 10; i++ {
fmt.Println(
pos(i),
neg(-2*i),
)
}
}
1 -2
3 -6
6 -12
10 -20
15 -30
21 -42
28 -56
36 -72
45 -90
package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := &Vertex{3, 4}
fmt.Println(v.Abs())
}
5
package main
import (
"fmt"
"math"
)
type MyFloat float64
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
func main() {
f := MyFloat(-math.Sqrt2)
fmt.Println(f.Abs())
}
1.4142135623730951
*Vertex
指针类型上,而另一个在 MyFloat
值类型上。 有两个原因需要使用指针接收者。首先避免在每个方法调用中拷贝值(如果值类型是大的结构体的话会更有效率)。其次,方法可以修改接收者指向的值。Scale
方法使用 Vertex
代替*Vertex
作为接收者。Vertex
的时候Scale
方法没有任何作用。Scale
修改 v。当 v是一个值(非指针),方法看到的是 Vertex的副本,并且无法修改原始值。package main
import (
"fmt"
"math"
)
type Vertex struct {
X, Y float64
}
func (v *Vertex) Scale(f float64) {
v.X = v.X * f
v.Y = v.Y * f
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
func main() {
v := &Vertex{3, 4}
fmt.Printf("Before scaling: %+v, Abs: %v\n", v, v.Abs())
v.Scale(5)
fmt.Printf("After scaling: %+v, Abs: %v\n", v, v.Abs())
}
Before scaling: &{X:3 Y:4}, Abs: 5
After scaling: &{X:15 Y:20}, Abs: 25
*Vertex
(指针类型)上, 所以 Vertex
(值类型)不满足 Abser。package main
import (
"fmt"
"math"
)
type Abser interface {
Abs() float64
}
func main() {
var a Abser
f := MyFloat(-math.Sqrt2)
v := Vertex{3, 4}
a = f // a MyFloat 实现了 Abser
a = &v // a *Vertex 实现了 Abser
// 下面一行,v 是一个 Vertex(而不是 *Vertex)
// 所以没有实现 Abser。
a = v
fmt.Println(a.Abs())
}
type MyFloat float64
func (f MyFloat) Abs() float64 {
if f < 0 {
return float64(-f)
}
return float64(f)
}
type Vertex struct {
X, Y float64
}
func (v *Vertex) Abs() float64 {
return math.Sqrt(v.X*v.X + v.Y*v.Y)
}
5
package main
import (
"fmt"
"os"
)
type Reader interface {
Read(b []byte) (n int, err error)
}
type Writer interface {
Write(b []byte) (n int, err error)
}
type ReadWriter interface {
Reader
Writer
}
func main() {
var w Writer
// os.Stdout 实现了 Writer
w = os.Stdout
fmt.Fprintf(w, "hello, writer\n")
}
hello, writer
type Stringer interface {
String() string
}
fmt
包 (还有许多其他包)使用这个来进行输出。package main
import "fmt"
type Person struct {
Name string
Age int
}
func (p Person) String() string {
return fmt.Sprintf("%v (%v years)", p.Name, p.Age)
}
func main() {
a := Person{"Arthur Dent", 42}
z := Person{"Zaphod Beeblebrox", 9001}
fmt.Println(a, z)
}
Arthur Dent (42 years) Zaphod Beeblebrox (9001 years)
type error interface {
Error() string
}
i, err := strconv.Atoi("42")
if err != nil {
fmt.Printf("couldn't convert number: %v\n", err)
return}
fmt.Println("Converted integer:", i)
package main
import (
"fmt"
"time"
)
type MyError struct {
When time.Time
What string
}
func (e *MyError) Error() string {
return fmt.Sprintf("at %v, %s",
e.When, e.What)
}
func run() error {
return &MyError{
time.Now(),
"it didn't work",
}
}
func main() {
if err := run(); err != nil {
fmt.Println(err)
}
}
at 2009-11-10 23:00:00 +0000 UTC, it didn't work
func (T) Read(b []byte) (n int, err error)
package main
import (
"fmt"
"io"
"strings"
)
func main() {
r := strings.NewReader("Hello, Reader!")
b := make([]byte, 8)
for {
n, err := r.Read(b)
fmt.Printf("n = %v err = %v b = %v\n", n, err, b)
fmt.Printf("b[:n] = %q\n", b[:n])
if err == io.EOF {
break
}
}
}
n = 8 err = <nil> b = [72 101 108 108 111 44 32 82]
b[:n] = "Hello, R"
n = 6 err = <nil> b = [101 97 100 101 114 33 32 82]
b[:n] = "eader!"
n = 0 err = EOF b = [101 97 100 101 114 33 32 82]
b[:n] = ""
package http
type Handler interface {
ServeHTTP(w ResponseWriter, r *Request)
}
package main
import (
"fmt"
"log"
"net/http"
)
type Hello struct{}
func (h Hello) ServeHTTP(
w http.ResponseWriter,
r *http.Request) {
fmt.Fprint(w, "Hello!")
}
func main() {
var h Hello
err := http.ListenAndServe("localhost:4000", h)
if err != nil {
log.Fatal(err)
}
}
2009/11/10 23:00:00 listen tcp: Protocol not available
package image
type Image interface {
ColorModel() color.Model
Bounds() Rectangle
At(x, y int) color.Color
}
package main
import (
"fmt"
"image"
)
func main() {
m := image.NewRGBA(image.Rect(0, 0, 100, 100))
fmt.Println(m.Bounds())
fmt.Println(m.At(0, 0).RGBA())
}
(0,0)-(100,100)
0 0 0 0
go f(x, y, z)
f(x, y, z)
package main
import (
"fmt"
"time"
)
func say(s string) {
for i := 0; i < 5; i++ {
time.Sleep(100 * time.Millisecond)
fmt.Println(s)
}
}
func main() {
go say("world")
say("hello")
}
hello
hello
world
world
hello
hello
world
world
hello
ch <- v // 将 v 送入 channel ch。
v := <-ch // 从 ch 接收,并且赋值给 v。
ch := make(chan int)
package main
import "fmt"
func sum(a []int, c chan int) {
sum := 0
for _, v := range a {
sum += v
}
c <- sum // 将和送入 c
}
func main() {
a := []int{7, 2, 8, -9, 4, 0}
c := make(chan int)
go sum(a[:len(a)/2], c)
go sum(a[len(a)/2:], c)
x, y := <-c, <-c // 从 c 中获取
fmt.Println(x, y, x+y)
}
-5 17 12
ch := make(chan int, 100)
package main
import "fmt"
func main() {
ch := make(chan int, 2)
ch <- 1
ch <- 2
fmt.Println(<-ch)
fmt.Println(<-ch)
}
1
2
-----------------
-----------------
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [chan send]:
main.main()
/tmp/sandbox156608315/main.go:9 +0x100
v, ok := <-ch
for i := range c
会不断从 channel 接收值,直到它被关闭。package main
import (
"fmt"
)
func fibonacci(n int, c chan int) {
x, y := 0, 1
for i := 0; i < n; i++ {
c <- x
x, y = y, x+y
}
close(c)
}
func main() {
c := make(chan int, 10)
go fibonacci(cap(c), c)
for i := range c {
fmt.Println(i)
}
}
0
1
1
2
3
5
8
13
21
34
package main
import "fmt"
func fibonacci(c, quit chan int) {
x, y := 0, 1
for {
select {
case c <- x:
x, y = y, x+y
case <-quit:
fmt.Println("quit")
return
}
}
}
func main() {
c := make(chan int)
quit := make(chan int)
go func() {
for i := 0; i < 10; i++ {
fmt.Println(<-c)
}
quit <- 0
}()
fibonacci(c, quit)
}
0
1
1
2
3
5
8
13
21
34
quit
select {
case i := <-c:
// 使用 idefault:
// 从 c 读取会阻塞
}
package main
import (
"fmt"
"time"
)
func main() {
tick := time.Tick(100 * time.Millisecond)
boom := time.After(500 * time.Millisecond)
for {
select {
case <-tick:
fmt.Println("tick.")
case <-boom:
fmt.Println("BOOM!")
return
default:
fmt.Println(" .")
time.Sleep(50 * time.Millisecond)
}
}
}
.
.
tick.
.
.
tick.
.
.
tick.
.
.
tick.
.
.
tick.
BOOM!
package main
import (
"fmt"
"sync"
"time"
)
// SafeCounter 的并发使用是安全的。
type SafeCounter struct {
v map[string]int
mux sync.Mutex
}
// Inc 增加给定 key 的计数器的值。
func (c *SafeCounter) Inc(key string) {
c.mux.Lock()
// Lock 之后同一时刻只有一个 goroutine 能访问 c.v
c.v[key]++
c.mux.Unlock()
}
// Value 返回给定 key 的计数器的当前值。
func (c *SafeCounter) Value(key string) int {
c.mux.Lock()
// Lock 之后同一时刻只有一个 goroutine 能访问 c.v
defer c.mux.Unlock()
return c.v[key]
}
func main() {
c := SafeCounter{v: make(map[string]int)}
for i := 0; i < 1000; i++ {
go c.Inc("somekey")
}
time.Sleep(time.Second)
fmt.Println(c.Value("somekey"))
}
1000
package main
import (
"fmt"
)
type Fetcher interface {
// Fetch 返回 URL 的 body 内容,并且将在这个页面上找到的 URL 放到一个 slice 中。
Fetch(url string) (body string, urls []string, err error)
}
// Crawl 使用 fetcher 从某个 URL 开始递归的爬取页面,直到达到最大深度。
func Crawl(url string, depth int, fetcher Fetcher) {
// TODO: 并行的抓取 URL。
// TODO: 不重复抓取页面。
// 下面并没有实现上面两种情况:
if depth <= 0 {
return
}
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("found: %s %q\n", url, body)
for _, u := range urls {
Crawl(u, depth-1, fetcher)
}
return
}
func main() {
Crawl("http://golang.org/", 4, fetcher)
}
// fakeFetcher 是返回若干结果的 Fetcher。
type fakeFetcher map[string]*fakeResult
type fakeResult struct {
body string
urls []string
}
func (f fakeFetcher) Fetch(url string) (string, []string, error) {
if res, ok := f[url]; ok {
return res.body, res.urls, nil
}
return "", nil, fmt.Errorf("not found: %s", url)
}
// fetcher 是填充后的 fakeFetcher。
var fetcher = fakeFetcher{
"http://golang.org/": &fakeResult{
"The Go Programming Language",
[]string{
"http://golang.org/pkg/",
"http://golang.org/cmd/",
},
},
"http://golang.org/pkg/": &fakeResult{
"Packages",
[]string{
"http://golang.org/",
"http://golang.org/cmd/",
"http://golang.org/pkg/fmt/",
"http://golang.org/pkg/os/",
},
},
"http://golang.org/pkg/fmt/": &fakeResult{
"Package fmt",
[]string{
"http://golang.org/",
"http://golang.org/pkg/",
},
},
"http://golang.org/pkg/os/": &fakeResult{
"Package os",
[]string{
"http://golang.org/",
"http://golang.org/pkg/",
},
},
}
found: http://golang.org/ "The Go Programming Language"
found: http://golang.org/pkg/ "Packages"
found: http://golang.org/ "The Go Programming Language"
found: http://golang.org/pkg/ "Packages"
not found: http://golang.org/cmd/
not found: http://golang.org/cmd/
found: http://golang.org/pkg/fmt/ "Package fmt"
found: http://golang.org/ "The Go Programming Language"
found: http://golang.org/pkg/ "Packages"
found: http://golang.org/pkg/os/ "Package os"
found: http://golang.org/ "The Go Programming Language"
found: http://golang.org/pkg/ "Packages"
not found: http://golang.org/cmd/