3.constants

/*
常量不变量。
它们只存在于编译中。
非类型化常量可以被隐含地转换为类型常量和变量不能的地方。
把非类型化的常量看作是一种类型，而不是一种类型。
了解显式和隐式转换。
参见常量及其在标准库中的使用。
*/
package main

func main() {
	// Constants live within the compiler.
	// They have a parallel type system.
	// Compiler can perform implicit conversions of untyped constants.

	// Untyped Constants.
	const ui = 12345    // kind: integer
	const uf = 3.141592 // kind: floating-point

	// Typed Constants still use the constant type system but their precision
	// is restricted.
	const ti int = 12345        // type: int
	const tf float64 = 3.141592 // type: float64

	// ./constants.go:XX: constant 1000 overflows uint8
	// const myUint8 uint8 = 1000

	// Constant arithmetic supports different kinds.
	// Kind Promotion is used to determine kind in these scenarios.

	// Variable answer will of type float64.
	var answer = 3 * 0.333 // KindFloat(3) * KindFloat(0.333)
	println(answer)

	// Constant third will be of kind floating point.
	const third = 1 / 3.0 // KindFloat(1) / KindFloat(3.0)

	// Constant zero will be of kind integer.
	const zero = 1 / 3 // KindInt(1) / KindInt(3)

	// This is an example of constant arithmetic between typed and
	// untyped constants. Must have like types to perform math.
	const one int8 = 1
	const two = 2 * one // int8(2) * int8(1)

}

...

// Sample program to show how constants do have a parallel type system.
// 示例程序，以显示常量如何具有并行类型系统。
package main

import "fmt"

const (
	// Max integer value on 64 bit architecture.
	maxInt = 9223372036854775807

	// Much larger value than int64.
	bigger = 9223372036854775808543522345

	// Will NOT compile
	// biggerInt int64 = 9223372036854775808543522345
)

func main() {
	fmt.Println("will compile")
}

iota

// Sample program to show how iota works.
package main

import "fmt"

func main() {
	const (
		A1 = iota // 0 : Start at 0
		B1 = iota // 1 : Increment by 1
		C1 = iota // 2 : Increment by 1
	)
	fmt.Println("1:", A1, B1, C1)

	const (
		A2 = iota // 0 : Start at 0
		B2        // 1 : Increment by 1
		C2        // 2 : Increment by 1
	)

	fmt.Println("2:", A2, B2, C2)

	const (
		A3 = iota + 1 // 1 : Start at 0 + 1
		B3            // 2 : Increment by 1
		C3            // 3 : Increment by 1
	)

	fmt.Println("3:", A3, B3, C3)

	const (
		Ldate         = 1 << iota //  1 : Shift 1 to the left 0.  0000 0001
		Ltime                     //  2 : Shift 1 to the left 1.  0000 0010
		Lmicroseconds             //  4 : Shift 1 to the left 2.  0000 0100
		Llongfile                 //  8 : Shift 1 to the left 3.  0000 1000
		Lshortfile                // 16 : Shift 1 to the left 4.  0001 0000
		LUTC                      // 32 : Shift 1 to the left 5.  0010 0000
	)

	fmt.Println("Log:", Ldate, Ltime, Lmicroseconds, Llongfile, Lshortfile, LUTC)
	/*
	   Log: 1 2 4 8 16 32
	   左移1位（二进制）
	*/

}

package main

import (
	"time"
	"fmt"
)

// All material is licensed under the Apache License Version 2.0, January 2004
// http://www.apache.org/licenses/LICENSE-2.0

/*
// A Duration represents the elapsed time between two instants as
// an int64 nanosecond count. The representation limits the largest
// representable duration to approximately 290 years.


// Common durations. There is no definition for units of Day or larger
// to avoid confusion across daylight savings time zone transitions.

/*
//持续时间表示两个瞬间之间的运行时间。
//一个int64纳秒计数。表示限制最大。
//可表示的持续时间约为290年。


/ /共同的持续时间。对于白天或更大的单位，没有定义。
//为了避免混淆日光节约时区的过渡。
*/
 */
type Duration int64

const (
	Nanosecond  Duration = 1
	Microsecond          = 1000 * Nanosecond
	Millisecond          = 1000 * Microsecond
	Second               = 1000 * Millisecond
	Minute               = 60 * Second
	Hour                 = 60 * Minute
)
// Add returns the time t+d.

//func (t Time) Add(d Duration) Time

func main() {
	// Use the time package to get the current date/time.
	now := time.Now()

	// Subtract 5 nanoseconds from now using a literal constant.
	literal := now.Add(-5)

	// Subtract 5 seconds from now using a declared constant.
	const timeout = 5 * time.Second // time.Duration(5) * time.Duration(1000000000)
	constant := now.Add(-timeout)


	// Subtract 5 nanoseconds from now using a variable of type int64.
	minusFive := int64(-5)
	variable := now.Add(minusFive)

	// example4.go:50: cannot use minusFive (type int64) as type time.Duration in argument to now.Add

	// Display the values.
	fmt.Printf("Now     : %v\n", now)
	fmt.Printf("Literal : %v\n", literal)
	fmt.Printf("Constant: %v\n", constant)
	fmt.Printf("Variable: %v\n", variable)
}

练习

// Declare an untyped and typed constant and display their values.
//
// Multiply two literal constants into a typed variable and display the value.
package main

import "fmt"

const (
	// server is the IP address for connecting.
	server = "124.53.24.123"

	// port is the port to make that connection.
	port int16 = 9000
)

func main() {

	// Display the server information.
	fmt.Println(server)
	fmt.Println(port)

	// Calculate the number of minutes in 5320 seconds.
	minutes := 5320 / 60.0
	fmt.Println(minutes)
}