常量(Constants)

常量(Constants)

Go支持字符、字符串、布尔和数值的常量。

Go supports constants of character, string, boolean, and numeric values.

const关键字用于声明常量。

const declares a constant value.

package main

import "fmt"

const s string = "constant" 

func main() {
	fmt.Println(s) 
}

package main

import "fmt"

const s string = "constant" 

func main() {
	fmt.Println(s) 
}

const语句可以出现在任何var语句可以出现的地方。

A const statement can appear anywhere a var statement can.

常量表达式可以执行具有任意精度的算术运算。

Constant expressions perform arithmetic with arbitrary precision.

package main

import "fmt"

func main() {
	const n = 500000000  
	const d = 3e20 / n  
	fmt.Println(d)  
}

package main

import "fmt"

func main() {
	const n = 500000000  
	const d = 3e20 / n  
	fmt.Println(d)  
}

数值常量在没有给定类型之前是没有类型的，例如通过显式转换给定类型。

A numeric constant has no type until it’s given one, such as by an explicit conversion.

package main

import "fmt"

func main() {
	const n = 500000000
	const d = 3e20 / n
	fmt.Println(d)

	fmt.Println(int64(d)) 
}

package main

import "fmt"

func main() {
	const n = 500000000
	const d = 3e20 / n
	fmt.Println(d)

	fmt.Println(int64(d)) 
}

通过在需要类型的上下文中使用数值，例如变量赋值或函数调用，可以给数字赋予类型。例如，这里的math.Sin期望一个float64类型的参数。

A number can be given a type by using it in a context that requires one, such as a variable assignment or function call. For example, here math.Sin expects a float64.

package main

import (
	"fmt"
	"math"
)

func main() {
	const n = 500000000 
	fmt.Println(math.Sin(n)) 
}

package main

import (
	"fmt"
	"math"
)

func main() {
	const n = 500000000 
	fmt.Println(math.Sin(n)) 
}

运行

package main

import (
	"fmt"
	"math"
)

const s string = "constant"

func main() {
	fmt.Println(s)

	const n = 500000000

	const d = 3e20 / n
	fmt.Println(d)

	fmt.Println(int64(d))

	fmt.Println(math.Sin(n))
}

package main

import (
	"fmt"
	"math"
)

const s string = "constant"

func main() {
	fmt.Println(s)

	const n = 500000000

	const d = 3e20 / n
	fmt.Println(d)

	fmt.Println(int64(d))

	fmt.Println(math.Sin(n))
}

bash

go run 04-constant.go 
# constant
# 6e+11
# 600000000000
# -0.28470407323754404

go run 04-constant.go 
# constant
# 6e+11
# 600000000000
# -0.28470407323754404

运行 ​

运行