What does := mean in Go?

Question

I'm following this tutorial, specifically exercise 8:

http://tour.golang.org/#8

package main

import "fmt"

func swap(x, y string) (string, string) {
    return y, x
}

func main() {
    a, b := swap("hello", "world")
    fmt.Println(a, b)
}

Specifically what does the := mean? Searching for Go documentation is very hard, ironically.

@BenjaminGruenbaum I guess it's a case of knowing what to search for. I tried `go := meaning`, `golang :=` and nothing relevant came up. — sergserg, May 16 '13 at 01:29
For what it's worth, it wasn't where I usually looked for Go resources/specification — Benjamin Gruenbaum, May 16 '13 at 01:30
If you run into something in the language that you don't understand, instead of googling (which is definitely not going to work well for punctuation), just pop open [golang.org/ref/spec](http://golang.org/ref/spec) and search there. That's the actual spec for the language, and it's not very large. Second occurrence of ":=" on the page is precisely what you want. — Lily Ballard, May 16 '13 at 01:39
It's explained [in the tutorial](http://tour.golang.org/#12) — , May 16 '13 at 01:50
I came here from google, so the question might not be entirely invalid. — Doug Molineux, Nov 07 '15 at 17:53
It's just a shorthand for declaring and initializing a variable (or multiple variables when using a comma). The types are inferred from the values passed in. So in your case, you would have two variables `a` and `b`, both of type `string`, initialized with the values returned from `swap("hello", "world")` — pjmonk, Jun 02 '20 at 19:04

score 21 · Accepted Answer · edited May 16 '13 at 01:30

21

A short variable declaration uses the syntax:

ShortVarDecl = IdentifierList ":=" ExpressionList .

It is a shorthand for a regular variable declaration with initializer expressions but no types:

edited May 16 '13 at 01:30

NullPoiиteя

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answered May 16 '13 at 01:30

Benjamin Gruenbaum

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peterSO · Answer 2 · 2013-05-16T01:53:23.107

8

Keep on going to page 12 of the tour!

A Tour of Go

Short variable declarations

Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type.

(Outside a function, every construct begins with a keyword and the := construct is not available.)

edited May 16 '13 at 01:53

answered May 16 '13 at 01:47

peterSO

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score 7 · Answer 3 · answered Feb 12 '21 at 16:50

7

As others have explained already, := is for both declaration, and assignment, whereas = is for assignment only.

For example, var abc int = 20 is the same as abc := 20.

It's useful when you don't want to fill up your code with type or struct declarations.

answered Feb 12 '21 at 16:50

javeria

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9

Take my comment as a little joke: no aswer should start with "As others have explained already...", right? – Ferran Maylinch Feb 15 '21 at 18:17

score 5 · Answer 4 · edited Mar 28 '22 at 14:32

5

The := syntax is shorthand for declaring and initializing a variable, example f := "car" is the short form of var f string = "car"

The short variable declaration operator(:=) can only be used for declaring local variables. If you try to declare a global variable using the short declaration operator, you will get an error.

Refer official documentation for more details

edited Mar 28 '22 at 14:32

MeanwhileInHell

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answered Feb 10 '21 at 15:57

Arun

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1

You could add a link to the official doc, like the accepted answer. I also think that this symbol can only be used in local variables, right? If that's true, you could add that. – Ferran Maylinch Feb 11 '21 at 18:47
@FerranMaylinch Thanks for the suggestions to make it more accurate .. – Arun Feb 12 '21 at 16:55
1

It's funny that you got a negative vote, and then someone wrote the same answer – Ferran Maylinch Feb 15 '21 at 18:14

score 1 · Answer 5 · answered Nov 13 '18 at 13:21

1

:= is not an operator. It is a part of the syntax of the Short variable declarations clause.

more on this: https://golang.org/ref/spec#Short_variable_declarations

answered Nov 13 '18 at 13:21

Md. Tazbir Ur Rahman Bhuiyan

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liam · Answer 6 · 2016-05-13T22:22:03.537

0

According to my book on Go, it is just a short variable declaration statement exactly the same as

var s = ""

But it is more easy to declare, and the scope of it is less expansive. A := var decleration also can't have a type of interface{}. This is something that you will probably run into years later though

edited May 13 '16 at 22:22

answered Nov 25 '15 at 14:53

liam

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The only difference between the two is that var can be used in a global scope. You can also use `var` in functions, and it'll behave exactly the same as `:=`. Also, you can use the `interface{}` type with `var` declarations. – pjmonk Jun 02 '20 at 19:08

score -2 · Answer 7 · answered Nov 23 '15 at 08:39

-2

:= represents a variable, we can assign a value to a variable using :=.

answered Nov 23 '15 at 08:39

user5594344

1

What does := mean in Go?

7 Answers7