Difference between := and = operators in Go

What is the difference between the = and := operators, and what are the use cases for them? They both seem to be for an assignment?

In Go, := is for declaration + assignment, whereas = is for assignment only.

For example, var foo int = 10 is the same as foo := 10.

Only = is the assignment operator.

:= is a part of the syntax of the Short variable declarations clause.

👉 There are some rules though. See this other answer for more details.

As others have explained already, := is for both declaration, assignment, and also for redeclaration; and it guesses (infers) the variable's type automatically.

For example, foo := 32 is a short-hand form of:

var foo int
foo = 32

// OR:
var foo int = 32

// OR:
var foo = 32

/* There are some rules: */

★ 1st Rule:

You can't use := out of funcs. It's because, out of any func, a statement should start with a keyword.

// no keywords below, illegal.
illegal := 42

// `var` keyword makes this statement legal.
var legal = 42

func foo() {
  alsoLegal := 42
  // reason: it's in a func scope.
}

★ 2nd Rule:

You can't use them twice (in the same scope):

legal := 42
legal := 42 // <-- error

Because, := introduces "a new variable", hence using it twice does not redeclare a second variable, so it's illegal.

★ 3rd Rule:

You can use them for multi-variable declarations and assignments:

foo, bar   := 42, 314
jazz, bazz := 22, 7

★ 4th Rule (Redeclaration):

You can use them twice in "multi-variable" declarations, if one of the variables is new:

foo, bar  := someFunc()
foo, jazz := someFunc()  // <-- jazz is new
baz, foo  := someFunc()  // <-- baz is new

This is legal, because, you're not declaring all the variables, you're just reassigning new values to the existing variables, and declaring new variables at the same time. This is called redeclaration.

★ 5th Rule:

You can use the short declaration to declare a variable in a newer scope even that variable is already declared with the same name before:

var foo int = 34

func some() {
  // because foo here is scoped to some func
  foo := 42  // <-- legal
  foo = 314  // <-- legal
}

Here, foo := 42 is legal, because, it declares foo in some() func's scope. foo = 314 is legal, because, it just assigns a new value to foo.

★ 6th Rule:

You can declare the same name in short statement blocks like: if, for, switch:

foo := 42
if foo := someFunc(); foo == 314 {
  // foo is scoped to 314 here
  // ...
}
// foo is still 42 here

Because, foo in if foo := ..., only belongs to that if clause and it's in a different scope.

So, as a general rule: If you want to easily declare a variable you can use :=, or, if you only want to overwrite an existing variable, you can use =.

References:

Short Variable Declaration Rules
A Visual Guide to Go Variables

:= is a short-hand for declaration.

a := 10
b := "gopher"

a will be declared as an int and initialized with value 10 where as b will be declared as a string and initialized with value gopher.

Their equivalents using = would be

var a = 10
var b = "gopher"

= is assignment operator. It is used the same way you would use it in any other language.

You can omit the type when you declare the variable and an initializer is present (http://tour.golang.org/#11).

Difference between := and = operators in Go

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