Golang, what does the following do []

Question

I'm new to golang and have a basic question. I have the following code taken from an example from the web

func (d Direction) String() string {
    return [...]string{"North", "East", "South", "West"}[d]
}

I'm confused what does the [d] do in the method body?

Answer 1

[d] is simply an index expression , it indexes the array created with a composite literal preceding it.

This:

[...]string{"North", "East", "South", "West"}

Is an array composite literal, it creates an array of element type string with the listed elements, and the subsequent [d] indexes this array. The method returns the d ^th element of this 4-sized array.

Note that the ... means we want the compiler to determine the array size automatically:

The notation ... specifies an array length equal to the maximum element index plus one.

Arrays in Go is not to be mistaken with slices . For a good introduction on arrays and slices, read the official blog posts:

The Go Blog: Go Slices: usage and internals

The Go Blog: Arrays, slices (and strings): The mechanics of 'append'

Answer 2

This part declares an array literal with four strings:

[...]string{"North", "East", "South", "West"}

Then this part gets the d th element from the array:

[...]string{"North", "East", "South", "West"}[d]

Direction has to be int for this to work.

Answer 3

@icza and @Burak Serdar have mentioned that [d] is an index expression.

Following is just a working example to see the output

package main

import "fmt"

type Direction int

func (d Direction) String() string {
    return [...]string{"North", "East", "South", "West"}[d]
}

func main() {
    n:=Direction(0)  // d=0
    fmt.Println(n)
    w:=Direction(3)  // d=3
    fmt.Println(w)
}

Output:

North
West

To be more clear,

return [...]string{"North", "East", "South", "West"}[d]

can be expanded as

func (d Direction) String() string {
    var directions = [...]string{"North", "East", "South", "West"}
    return directions[d]
}