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.

Stickle

@BenjaminGruenbaum I guess it's a case of knowing what to search for. I tried go := meaning, golang := and nothing relevant came up. – Stickle

For what it's worth, it wasn't where I usually looked for Go resources/specification – Terpsichore

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 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. – Spermatozoid

It's explained in the tutorial – Blend

I came here from google, so the question might not be entirely invalid. – Crossover

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") – Nkrumah

Answer 1

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:

Terpsichore

Answer 2

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.)

Amundsen

Answer 3

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.

Bath

Take my comment as a little joke: no aswer should start with "As others have explained already...", right? 😉 – Espalier

Answer 4

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

Magness

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. – Espalier

@FerranMaylinch Thanks for the suggestions to make it more accurate .. – Magness

It's funny that you got a negative vote, and then someone wrote the same answer 😁 – Espalier

Answer 5

:= 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

Rework

Answer 6

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

Terrapin

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. – Nkrumah

Answer 7

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

Bulldoze