Go 语言学习之常量

概念

常量是一种表达式，常量的计算可以在编译时完成，如果有错误，在编译时就会报错，常量的值不可以修改。

声明常量

常量声明可以同时指定类型和值，如果没有显示指定类型，则类型根据右边的表达式推断。需要特别注意的是，声明常量不同于声明变量可以不赋值，声明常量必须赋值。

const name string = "golang"
const PI = 3.14

func main() {
  fmt.Printf("name = %s type:%Tn", name, name)
  fmt.Printf("PI = %g type:%Tn", PI, PI)
}

声明一组常量，每个常量类型可以不同。

const (
  x = 1
  y = "hello"
  z = true
)

func main() {
  fmt.Printf("x = %d type:%Tn", x, x)
  fmt.Printf("y = %s type:%Tn", y, y)
  fmt.Printf("z = %t type:%Tn", z, z)
}

声明一组常量，除了第一个常量之外，其他常量都可以省略表达式，其他常量会复用其前面一个常量的类型和表达式。

const (
  a = 1
  b
  c = 2
  d
)

func main() {
  fmt.Printf("a = %d type:%Tn", a, a)
  fmt.Printf("b = %d type:%Tn", b, b)
  fmt.Printf("c = %d type:%Tn", c, c)
  fmt.Printf("d = %d type:%Tn", d, d)
}

常量生成器 iota

声明一组常量，可以使用常量生成器 iota，它可以不用逐个常量显示指定类型和赋值表达式，常量声明中，iota 从 0 开始取值，后续常量如果没有显示指定类型和赋值表达式，其值是其前面一个常量的值加 1。直到遇到赋值常量停止加 1，后续如果再遇到赋值 iota 的常量，则继续逐个加 1。

const (
  Sunday int = iota
  Monday
  Tuesday
  Wednesday
  Thursday
  Friday
  Saturday
)

const (
  aa int = iota
  bb
  cc
  dd = 10
  ee
  ff = iota
  gg
  hh
)

func main() {
  fmt.Printf("Sunday = %d type:%Tn", Sunday, Sunday)
  fmt.Printf("Monday = %d type:%Tn", Monday, Monday)
  fmt.Printf("Tuesday = %d type:%Tn", Tuesday, Tuesday)
  fmt.Printf("Wednesday = %d type:%Tn", Wednesday, Wednesday)
  fmt.Printf("Thursday = %d type:%Tn", Thursday, Tuesday)
  fmt.Printf("Friday = %d type:%Tn", Friday, Friday)
  fmt.Printf("Saturday = %d type:%Tn", Saturday, Saturday)
  fmt.Printf("aa = %d type:%Tn", aa, aa)
  fmt.Printf("bb = %d type:%Tn", bb, bb)
  fmt.Printf("cc = %d type:%Tn", cc, cc)
  fmt.Printf("dd = %d type:%Tn", dd, dd)
  fmt.Printf("ee = %d type:%Tn", ee, ee)
  fmt.Printf("ff= %d type:%Tn", ff, ff)
  fmt.Printf("gg = %d type:%Tn", gg, gg)
  fmt.Printf("hh = %d type:%Tn", hh, hh)
}

无类型常量

无类型常量，可以直接赋值给其他常量，显示指定类型的常量，赋值给其他常量时，会做强类型检查。

const username, age = "lucy", 17
const user = username

func main() {
  fmt.Printf("username = %s, age = %dn", username, age)
  fmt.Printf("user = %sn", user)
}

完整代码：

输出结果：

name = golang type:string
PI = 3.14 type:float64
x = 1 type:int
y = hello type:string
z = true type:bool
a = 1 type:int
b = 1 type:int
c = 2 type:int
d = 2 type:int
Sunday = 0 type:int
Monday = 1 type:int
Tuesday = 2 type:int
Wednesday = 3 type:int
Thursday = 4 type:int
Friday = 5 type:int
Saturday = 6 type:int
aa = 0 type:int
bb = 1 type:int
cc = 2 type:int
dd = 10 type:int
ee = 10 type:int
ff= 5 type:int
gg = 6 type:int
hh = 7 type:int
username = lucy, age = 17
user = lucy