Constants · Tuck

Go supports constants of character, string, boolean, and numeric values. Constants are evaluated at compile time and cannot change.

package main

import (
	"fmt"
	"math"
)

const declares a constant value. It can appear anywhere a var can.

const s string = "constant"

iota counts up from zero inside a const block, which makes enumerations easy.

const (
	small = iota
	medium
	large
)

func main() {
	fmt.Println(s)

A numeric constant has arbitrary precision until it is given a type.

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

An untyped constant takes the type its context needs, here an int64 and then the float64 that math.Sin expects.

	fmt.Println(int64(d))
	fmt.Println(math.Sin(n))

	fmt.Println(small, medium, large)

Constant arithmetic is exact, unlike float variables.

	a, b := 0.1, 0.2
	fmt.Println(0.1+0.2 == 0.3, a+b == 0.3)
}

$ go run ./04_constants
constant
6e+11
600000000000
-0.28470407323754404
0 1 2
true false