Short Variable Declaration Operator(:=) in Go

Short Variable Declaration Operator(:=) in Golang is used to create the variables having a proper name and initial value. The main purpose of using this operator to declare and initialize the local variables inside the functions and to narrowing the scope of the variables. The type of the variable is determined by the type of the expression. var keyword is also used to create the variables of a specific type. So you can say that there are two ways to create the variables in Golang as follows:

• Using the var keyword
• Using the short variable declaration operator(:=)

In this article, we will only discuss the short variable declaration operator. To know about var keyword you can refer var keyword in Go. You can also read the difference between var keyword and short variable declaration operator to get a proper idea of using both.

Syntax of using short variable declaration operator:

variable_name := expression or value

Here, you must initialize the variable just after declaration. But using var keyword you can avoid initialization at the time of declaration. There is no need to mention the type of the variable. Expression or value on the right-hand side is used to evaluate the type of the variable.

Example: Here, we are declaring the variables using short declaration operator and we are not specifying the type of the variable. The type of the variable is determined by the type of the expression on the right-hand side of := operator. 

Go

// Go program to illustrate the use
// of := (short declaration
// operator)
package main

import "fmt"

func main() {

    // declaring and initializing the variable
    a := 30

    // taking a string variable
    Language: = "Go Programming"

    fmt.Println("The Value of a is: ", a)
    fmt.Println("The Value of Language is: ", Language)

}

Output:

The Value of a is:  30
The Value of Language is:  Go Programming

Declaring Multiple Variables Using Short Declaration Operator (:=)

Short Declaration operator can also be used to declare multiple variables of the same type or different types in the single declaration. The type of these variables is evaluated by the expression on the right-hand side of := operator. 

Example:

Go

// Go program to illustrate how to use := short
// declaration operator to declare multiple
// variables into a single declaration statement
package main

import "fmt"

func main() {

// multiple variables of same type(int)
geek1, geek2, geek3 := 117, 7834, 5685

// multiple variables of different types
geek4, geek5, geek6 := "GFG", 859.24, 1234

// Display the value and
// type of the variables
fmt.Printf("The value of geek1 is : %d\n", geek1)
fmt.Printf("The type of geek1 is : %T\n", geek1)

fmt.Printf("\nThe value of geek2 is : %d\n", geek2)
fmt.Printf("The type of geek2 is : %T\n", geek2)

fmt.Printf("\nThe value of geek3 is : %d\n", geek3)
fmt.Printf("The type of geek3 is : %T\n", geek3)

fmt.Printf("\nThe value of geek4 is : %s\n", geek4)
fmt.Printf("The type of geek4 is : %T\n", geek4)


fmt.Printf("\nThe value of geek5 is : %f\n", geek5)
fmt.Printf("The type of geek5 is : %T\n", geek5)

fmt.Printf("\nThe value of geek6 is : %d\n", geek6)
fmt.Printf("The type of geek6 is : %T\n", geek6)

}

Output:

The value of geek1 is : 117
The type of geek1 is : int

The value of geek2 is : 7834
The type of geek2 is : int

The value of geek3 is : 5685
The type of geek3 is : int

The value of geek4 is : GFG
The type of geek4 is : string

The value of geek5 is : 859.240000
The type of geek5 is : float64

The value of geek6 is : 1234
The type of geek6 is : int

Important Points:

• Short declaration operator can be used when at least one of the variable in the left-hand side of := operator is newly declared. A short variable declaration operator behaves like an assignment for those variables which are already declared in the same lexical block. To get a better idea about this concept, let’s take an example.
  Example 1: Below program will give an error as there are no new variables in the left-hand side of the := operator.

Go

// Go program to illustrate the concept
// of short variable declaration
package main

import "fmt"

func main() {

    // taking two variables
    p, q := 100, 200

    fmt.Println("Value of p ", p, "Value of q ", q)

    // this will give an error as
    // there are no new variable
    // on the left-hand side of :=
    p, q := 500, 600

    fmt.Println("Value of p ", p, "Value of q ", q)
}

Error:

./prog.go:17:10: no new variables on left side of := 

Example 2: In the below program, you can see that the line of code geek3, geek2 := 456, 200 will work fine without any error as there is at least a new variable i.e. geek3 on the left-hand side of := operator.

Go

// Go program to show how to use
// short variable declaration operator
package main

import "fmt"

func main() {

// Here, short variable declaration acts
// as an assignment for geek1 variable
// because same variable present in the same block
// so the value of geek2 is changed from 100 to 200
geek1, geek2 := 78, 100

// here, := is used as an assignment for geek2
// as it is already declared. Also, this line
// will work fine as geek3 is newly created
// variable
geek3, geek2 := 456, 200

// If you try to run the commented lines,
// then compiler will gives error because
// these variables are already defined
// geek1, geek2 := 745, 956
// geek3 := 150

// Display the values of the variables
fmt.Printf("The value of geek1 and geek2 is : %d %d\n", geek1, geek2)

fmt.Printf("The value of geek3 and geek2 is : %d %d\n", geek3, geek2)
}

Output:

The value of geek1 and geek2 is : 78 200
The value of geek3 and geek2 is : 456 200

• Go is a strongly typed language as you cannot assign a value of another type to the declared variable. 
  Example:

Go

// Go program to show how to use
// short variable declaration operator
package main

import "fmt"

func main() {

    // taking a variable of int type
    z := 50

    fmt.Printf("Value of z is %d", z)

    // reassigning the value of string type
    // it will give an error
    z := "Golang"
}

Error:

./prog.go:16:4: no new variables on left side of := 
./prog.go:16:7: cannot use “Golang” (type string) as type int in assignment 

• In a short variable declaration, it is allowed to initializing a set of variables by the calling function which returns multiple values. Or you can say variables can also be assigned values that are evaluated during run time. 
  Example:

// Here, math.Max function return 
// the maximum number in i variable
i := math.Max(x, y)

Local Variables or Global Variables?

With the help of short variable declaration operator(:=) you can only declare the local variable which has only block-level scope. Generally, local variables are declared inside the function block. If you will try to declare the global variables using the short declaration operator then you will get an error.

Example 1:

Go

// Go program to show the use of := operator
// to declare local variables
package main

import "fmt"

// using var keyword to declare
// and initialize the variable
// it is package or you can say
// global level scope
var geek1 = 900

// using short variable declaration
// it will give an error
geek2 := 200

func main() {

// accessing geek1 inside the function
fmt.Println(geek1)

// accessing geek2 inside the function
fmt.Println(geek2)

}

Error:

./prog.go:15:1: syntax error: non-declaration statement outside function body 

Example 2:

Go

// Go program to show the use of := operator
// to declare local variables
package main

import "fmt"

// using var keyword to declare
// and initialize the variable
// it is package or you can say
// global level scope
var geek1 = 900


func main() {

// using short variable declaration
// inside the main function
// it has local scope i.e. can't
// accessed outside the main function
geek2 := 200

// accessing geek1 inside the function
fmt.Println(geek1)

// accessing geek2 inside the function
fmt.Println(geek2)

}

Output:

900
200