Learn Go lang (https://golang.org/) in one file

tour-go-lang.go

// every go program is package
package main

// factored import modules
import (
  "fmt"
  "time"
  "runtime"
  "math"
  // last name become package name mostly
  "math/rand"
  "math/cmplx"
)

// function with arguments and return value
func add(x int, y int) int {
	return x + y
}

// multile return values
func swap(x, y string) (string, string) {
	return y, x
}

// naked return or named return values
func split(sum int) (x, y int) {
	x = sum * 4 / 9
	y = sum - x
	return
}

// package level variables
var c, python, java bool

// variables with initializers
var i, j int = 1, 2

// factored variables declaration
var (
// 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
	ToBe   bool       = false
	MaxInt uint64     = 1<<64 - 1
	z      complex128 = cmplx.Sqrt(-5 + 12i)
)

// numeric constants are high precision values
const (
	// Create a huge number by shifting a 1 bit left 100 places.
	// In other words, the binary number that is 1 followed by 100 zeroes.
	Big = 1 << 100
	// Shift it right again 99 places, so we end up with 1<<1, or 2.
	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("Hello World")
  fmt.Println("The time is", time.Now())
  fmt.Println("My favourite number is", rand.Intn(10))
  fmt.Printf("Now you have %g problems.\n", math.Sqrt(7))

  // only function starts with capital letter can be import/called of module
  fmt.Println("Pi is ", math.Pi)
  fmt.Println("Sum of 1 and 2 is ", add(1, 2))

  a, b := swap("hello", "world")
	fmt.Println(a, b)

  fmt.Println(split(17))

  // function level variable
  var i int
	fmt.Println(i, c, python, java)

  // If an initializer is present, the type can be omitted
  var c, python, java = true, false, "no!"
	fmt.Println(i, j, c, python, java)

  var j int = 2
  // short variable declaration inside a function only with implicit type
	k := 3
	c1, python1, java1 := true, false, "no!"

	fmt.Println(i, j, k, c1, python1, java1)

  // constants cannot be declared using the := syntax
  const f = "%T(%v)\n"
	fmt.Printf(f, ToBe, ToBe)
	fmt.Printf(f, MaxInt, MaxInt)
	fmt.Printf(f, z, z)

  // zero values variable declaration
  var i1 int
	var f1 float64
	var b1 bool
	var s1 string
	fmt.Printf("%v %v %v %q\n", i1, f1, b1, s1)

  // type conversions
  var x, y int = 3, 4
	var f2 float64 = math.Sqrt(float64(x*x + y*y))
	var z uint = uint(f2)
	fmt.Println(x, y, z)

  // type inference
  v := 42 // change me!
	fmt.Printf("v is of type %T\n", v)

  // numeric constants
  fmt.Println(needInt(Small))
	fmt.Println(needFloat(Small))
	fmt.Println(needFloat(Big))

  // for loop (only one looping construct in Go lang)
  sum := 0
	for i := 0; i < 10; i++ {
		sum += i
	}
	fmt.Println(sum)

  // init and post statement are optional
  sum1 := 1
	for ; sum1 < 1000; {
		sum1 += sum1
	}
	fmt.Println(sum1)

  // for loop is Go's "while loop"
  sum2 := 1
	for sum2 < 1000 {
		sum2 += sum2
	}
	fmt.Println(sum2)

  // infinite for loop
  // for {
	// }

  // if
  x1 := float64(-4)
  if x1 < 0 {
		fmt.Println(fmt.Sprint(math.Sqrt(-x1)) + "i")
	}
	fmt.Println(fmt.Sprint(math.Sqrt(2)))

  // if with a short statement to execute before the condition
  var x2, n, lim float64 = 3, 2, 10
  if v := math.Pow(x2, n); v < lim {
	   fmt.Println(v)
	}

  // if and else
  if v := math.Pow(x2, n); v < lim {
		fmt.Println(v)
	} else {
		fmt.Printf("%g >= %g\n", v, lim)
	}
	// can't use v here, though
	fmt.Println(lim)

  // switch of Go
  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)
	}

  // switch with no condition is the same as switch true
  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.")
	}

  // defer keyword defers the execution of a function until the surrounding
  // function returns
  defer fmt.Println("world")
	fmt.Println("hello")

  // stacking defers
  // when a function returns, its deferred calls are executed in
  // last-in-first-out order
  fmt.Println("counting")
	for i := 0; i < 10; i++ {
		defer fmt.Println(i)
	}
  fmt.Println("done")

  // Pointers (holds the memory address of a variable)
  i2, j2 := 42, 2701
	p := &i2         // point to i2
	fmt.Println(*p) // read i2 through the pointer
	*p = 21         // set i2 through the pointer
	fmt.Println(i2)  // see the new value of i2
	p = &j         // point to j2
	*p = *p / 37   // divide j2 through the pointer
	fmt.Println(j2) // see the new value of j2

  // Structs (collection of fields)
  type Vertex struct {
    X int
    Y int
    // X, Y int (this is also right syntax of declaration)
  }
  fmt.Println(Vertex{1, 2})

  // Struct fields are accessed using a dot.
  v1 := Vertex{1, 2}
	v1.X = 4
	fmt.Println(v1.X)

  // Pointers to Structs
  p1 := &v1
  // or you can access field as (*p).X
	p1.X = 1e9
	fmt.Println(v1)

  // Struct Literals
  var (
  	v2 = Vertex{1, 2}  // has type Vertex
  	v3 = Vertex{X: 1}  // Y:0 is implicit
  	v4 = Vertex{}      // X:0 and Y:0
  	p2  = &Vertex{1, 2} // has type *Vertex
  )
  fmt.Println(v2, p2, v3, v4)

  // Arrays
  var a1 [2]string
	a1[0] = "Hello"
	a1[1] = "World"
	fmt.Println(a1[0], a1[1])
	fmt.Println(a1)

	primes := [6]int{2, 3, 5, 7, 11, 13}
	fmt.Println(primes)

  // Array Slice
  var s []int = primes[1:4]
	fmt.Println(s)

  // Slices are like references to Arrays
  names := [4]string{
		"John",
		"Paul",
		"George",
		"Ringo",
	}
	fmt.Println(names)
	a3 := names[0:2]
	b3 := names[1:3]
	fmt.Println(a3, b3)
	b3[0] = "XXX"
	fmt.Println(a3, b3)
	fmt.Println(names)

  // Slice Literals
  q := []int{2, 3, 5, 7, 11, 13}
	fmt.Println(q)

	r := []bool{true, false, true, true, false, true}
	fmt.Println(r)

	s2 := []struct {
		i int
		b bool
	}{
		{2, true},
		{3, false},
		{5, true},
		{7, true},
		{11, false},
		{13, true},
	}
	fmt.Println(s2)

  // Slice defaults (omit lower or upper bound takes defaults)
  s3 := []int{2, 3, 5, 7, 11, 13}

	s3 = s3[1:4]
	fmt.Println(s3)

	s = s3[:2]
	fmt.Println(s3)

	s3 = s3[1:]
	fmt.Println(s3)

  // Slice length and capacity
  s4 := []int{2, 3, 5, 7, 11, 13}
	fmt.Println(s4)

	// Slice the slice to give it zero length.
	s4 = s4[:0]
	fmt.Println(s4)

	// Extend its length.
	s4 = s4[:4]
	fmt.Println(s4)

	// Drop its first two values.
	s4 = s4[2:]
	fmt.Println(s4)

  // Nil Slice (length=0, capacity=0)
  var s5 []int
	fmt.Println(s5, len(s5), cap(s5))
	if s5 == nil {
		fmt.Println("nil!")
	}

}