Go: Working with iota

The iota's keyword in golang is used to autoincrement a int or float variable inside de Const clause. It also can be used to declare an enum.

Declaring a iota

In this example you can see the behavior when assigning iota in an untyped variable "a" and a typed variable "b" and "c".

package main
import "fmt"
const (
   a         = iota
   b int     = iota
   c float64 = iota
)
func main() {
  fmt.Printf("%v — %T\n", a, a)
  fmt.Printf("%v — %T\n", b, b)
  fmt.Printf("%v — %T\n", c, c)
}

Result

0 — int
1 — int
2 — float64

Ignoring first generated value

It's possible to ignore the first generated value using iota, all you need to do, is assign iota to a underscore "_" and next const element will be assigned to 1. See the example:

package main
import "fmt"
const (
    _     = iota
    a     = iota
    b int = iota
    c float64 = iota
)
func main() {
   fmt.Printf("%v — %T\n", a, a)
   fmt.Printf("%v — %T\n", b, b)
   fmt.Printf("%v — %T\n", c, c)
}

Result

1 — int
2 — int
3 — float64

Operations with iota

It's possible to make operation when using iotas

package main
import "fmt"
const (
    _         = iota
    a         = iota + 10
    b int     = iota * 10
    c float64 = iota + 5.1
)
func main() {
   fmt.Printf("%v — %T\n", a, a)
   fmt.Printf("%v — %T\n", b, b)
   fmt.Printf("%v — %T\n", c, c)
}

Result

11 — int
20 — int
8.1 — float64

Reseting iota increment

Everytime a keywork Const appears in the code, iota's increment is reseted

package main
import "fmt"
const (
   a = iota
   b int = iota
)
const (
   c = iota
)
func main() {
   fmt.Printf("%v — %T\n", a, a)
   fmt.Printf("%v — %T\n", b, b)
   fmt.Printf("%v — %T\n", c, c)
}

Result

0 — int
1 — int
0 — int

Declaring enum with iota

Another good way to use iota is to declare a enum

package main
import "fmt"
// Size int — using enum type
type Size int
// Const for enum
const (
   Small Size = iota
   Medium
   Big
   Huge
)
func main() {
   var s Size = Medium
   fmt.Println("Size: ", s)
}
func (d Size) String() string {
   return […]string{"Small", "Medium", "Big", "Huge"}[d]
}

Result

Size: Medium

Full Source Code

filipemunhoz/golang-study

Thanks for reading.