Variables and Scope

Status

This note is complete, reviewed, and considered stable.

Variables

A variable in Go is a storage location with a name, associated with a type, that holds a value. Variables can be declared using the var keyword or short declaration syntax.

Variable Declaration

Go supports three ways to declare variables:

1. Using var keyword (explicit type):

   var name string
   name = "John"

2. Using var keyword (implicit type inference):

   var age = 30

3. Short variable declaration (only inside functions):

   name := "Alice"

Multiple Variable Declarations

You can declare and initialize multiple variables at once:

var x, y int = 10, 20
a, b := 3.14, "Hello"

Default Values

If a variable is declared but not initialized, it gets a default value based on its type:

• Numeric types: 0
• Strings: ""
• Booleans: false
• Pointers, slices, maps, channels, interfaces: nil

Constants in Go

In addition to variables, Go also supports constants, which are similar to variables but, once assigned, cannot be changed. Constants can be typed or untyped.

Declaring Constants

1. Using const keyword:

   const pi = 3.14      // Untyped constant
   const radius = 10    // Untyped constant
   const isActive = true // Untyped constant

2. Typed Constants:

   const daysInWeek int = 7

Advantages of Constants

• Immutability – constants cannot be changed
• Performance – evaluated at compile-time
• Expressiveness – improves code readability

Example of Constants in Go

package main

import "fmt"

const Pi = 3.14159
const Greeting = "Hello, Go!"

func main() {
    fmt.Println("Pi:", Pi)
    fmt.Println(Greeting)
}

Constant Expressions

const (
    x = 5
    y = 10
    sum = x + y // evaluated at compile time
)

Data Types

Go is a statically typed language, meaning the type of a variable is known and checked at compile time.

Basic Data Types

1. Numeric Types

   • Integer: int, int8, int16, int32, int64
   • Unsigned: uint, uint8, uint16, uint32, uint64
   • Floating-point: float32, float64
   • Complex: complex64, complex128

2. Boolean

   • true or false

3. String

   • Immutable UTF-8 character sequence

Composite Data Types

1. Array

   • Fixed size and homogeneous

   var arr [3]int = [3]int{1, 2, 3}

2. Slice

   • Dynamically sized

   nums := []int{1, 2, 3}

3. Map

   • Key-value pairs

   m := map[string]int{"A": 1}

4. Struct

   • Custom type grouping multiple fields

   type Person struct {
       Name string
       Age  int
   }

5. Pointer

   • Stores memory address of a value

   var ptr *int

Special Types

1. Interface

   • Defines a set of method signatures

   type Shape interface {
       Area() float64
   }

2. Function Types

   • Functions can be assigned to variables

   var f func(int, int) int

Type Casting

Go is strongly typed and does not support implicit type conversions. Type casting must be done explicitly.

Syntax

var a int = 42
var b float64 = float64(a)

Common Conversions

• Integer to Float

  var i int = 10
  var f float64 = float64(i)

• Float to Integer (truncates decimal part)

  var f float64 = 3.14
  var i int = int(f) // i = 3

• Integer to String

  import "strconv"
  s := strconv.Itoa(42) // "42"

• String to Integer

  import "strconv"
  i, err := strconv.Atoi("42")

• Rune to String

  var r rune = 'A'
  var s string = string(r) // "A"

• Byte to String

  var b byte = 65
  s := string(b) // "A"

Type Assertion (for interfaces)

var i interface{} = "hello"
s := i.(string) // type assertion

Use the comma-ok idiom to avoid panic:

s, ok := i.(string)
if ok {
    fmt.Println("Conversion successful:", s)
}

Variable Scope

The scope of a variable defines where it can be accessed in a program.

Local Scope

Variables declared inside a function or block are accessible only within that block.

func main() {
    x := 10
    fmt.Println(x)
}

Package Scope

Variables declared outside any function are accessible to all functions in the same package.

var count int // package-scoped

Exported Scope (Cross-Package Access)

A variable is exported (accessible from other packages) if it starts with an uppercase letter.

var PublicVar = "Visible to other packages"

Shadowing

A local variable can shadow a package-level variable with the same name.

var x = 5

func main() {
    x := 10
    fmt.Println(x) // prints 10, not 5
}

Example Code

package main

import "fmt"

var packageVar = "I'm accessible within this package!"
const pi = 3.14159

func main() {
    var localVar int = 42
    shortVar := "I was declared using short syntax!"

    fmt.Println(packageVar)
    fmt.Println(localVar)
    fmt.Println(shortVar)
    fmt.Println("Pi constant:", pi)

    demonstrateScope()
    demonstrateCasting()
}

func demonstrateScope() {
    fmt.Println(packageVar)
}

func demonstrateCasting() {
    var i int = 42
    var f float64 = float64(i)
    fmt.Println("Int to Float:", f)

    s := fmt.Sprintf("%d", i)
    fmt.Println("Int to String:", s)
}