Learning notes for Golang

Go is an open source programming language that makes it easy to build simple, reliable, and efficient software.
Go is a general purpose programming language with advanced features and a clean syntax. Because of its wide availability on a variety of platforms, its robust well-documented common library, and its focus on good software engineering principles, Go is an ideal language to learn as your first programming language.
This is my learning note of GO Tour(https://tour.golang.org), just for reference and memory.

basics

Basic types

bool

string

int int8 int16 int32 int64
uint uint8 uint16 uint32 uint64 uintptr

byte // alias for uint8

rune // alias for int32
// represents a Unicode code point

float32 float64

complex64 complex128

More types

Pointers

var p *int
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
}

Struct

package main

import "fmt"

type Vertex struct {
X, Y int
}

var (
v1 = Vertex{1, 2} // has type Vertex
v2 = Vertex{X: 1} // Y:0 is implicit
v3 = Vertex{} // X:0 and Y:0
p = &Vertex{1, 2} // has type *Vertex
)

func main() {
fmt.Println(v1, p, v2, v3)
}

Array And Slice

var a [10]int   //array
var s []int //slice
s[lo:hi] //evaluates to a slice of the elements from lo through hi-1


//Making slices
a := make([]int, 5) //len(a) = 5
b := make([]int, 5, 10) //len(b) = 5, cap(b) = 10

Range

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)
}
}

Maps

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)
}

Function

Function

package main

import "fmt"

func add(x int, y int) int {
return x + y
}

func main() {
fmt.Println(add(42, 13))
}

Multiple result in function

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)
}

Named return values in function

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))
}

Function values

Functions are values too. They can be passed around just like other values.

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))
}

Function closure

A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense the function is “bound” to the variables.

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),
)
}
}

Flowcontrol

defer

A defer statement defers the execution of a function until the surrounding function returns.
Deferred function calls are pushed onto a stack. When a function returns, its deferred calls are executed in last-in-first-out order.

package main

import "fmt"

func main() {
fmt.Println("counting")

for i := 0; i < 10; i++ {
defer fmt.Println(i)
}

fmt.Println("done")
}

Methods

Methods with pointer receviers

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())
}

Interface

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 implement Abser
a = &v // a *Vertex implement Abser

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)
}

Concurrency

Goroutines

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")
}

Channels

ch <- v    // Send v to channel ch.
v := <-ch // Receive from ch, and
// assign value to 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 // send sum to 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 // receive from c

fmt.Println(x, y, x+y)
}

Buffered Channels

ch := make(chan int, 100)

Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.

Range and Close

A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression: after

v, ok := <-ch

ok is false if there are no more values to receive and the channel is closed.

The loop for i := range c receives values from the channel repeatedly until it is closed.

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)
}
}

Select

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)
}

Default Selection

The default case in a select is run if no other case is ready.

Use a default case to try a send or receive without blocking:

select {
case i := <-c:
// use i
default:
// receiving from c would block
}