Variables and Data Types

In this tutorial, we will learn about variables. We will also learn about the different data types that Go provides us.

Variables

Let's start with declaring a variable.

This is also known as declaration without initialization:

var foo string

Declaration with initialization:

var foo string = "Go is awesome"

Multiple declarations:

var foo, bar string = "Hello", "World"
// OR
var (
	foo string = "Hello"
	bar string  = "World"
)

Type is omitted but will be inferred:

var foo = "What's my type?"

Shorthand declaration, here we omit var keyword and type is always implicit. This is how we will see variables being declared most of the time. We also use the := for declaration plus assignment.

foo := "Shorthand!"

Note: Shorthand only works inside function bodies.

Constants

We can also declare constants with the const keyword. Which as the name suggests, are fixed values that cannot be reassigned.

const constant = "This is a constant"

It is also important to note that, only constants can be assigned to other constants.

const a = 10
const b = a // ✅ Works

var a = 10
const b = a // ❌ a (variable of type int) is not constant (InvalidConstInit)

Data Types

Perfect! Now let's look at some basic data types available in Go. Starting with string.

String

In Go, a string is a sequence of bytes. They are declared either using double quotes or backticks which can span multiple lines.

var name string = "My name is Go"

var bio string = `I am statically typed.
									I was designed at Google.`

Bool

Next is bool which is used to store boolean values. It can have two possible values - true or false.

var value bool = false
var isItTrue bool = true

Operators

We can use the following operators on boolean types

Numeric types

Now, let's talk about numeric types.

Signed and Unsigned integers

Go has several built-in integer types of varying sizes for storing signed and unsigned integers

The size of the generic int and uint types are platform-dependent. This means it is 32-bits wide on a 32-bit system and 64-bits wide on a 64-bit system.

var i int = 404                     // Platform dependent
var i8 int8 = 127                   // -128 to 127
var i16 int16 = 32767               // -2^15 to 2^15 - 1
var i32 int32 = -2147483647         // -2^31 to 2^31 - 1
var i64 int64 = 9223372036854775807 // -2^63 to 2^63 - 1

Similar to signed integers, we have unsigned integers.

var ui uint = 404                     // Platform dependent
var ui8 uint8 = 255                   // 0 to 255
var ui16 uint16 = 65535               // 0 to 2^16
var ui32 uint32 = 2147483647          // 0 to 2^32
var ui64 uint64 = 9223372036854775807 // 0 to 2^64
var uiptr uintptr                     // Integer representation of a memory address

If you noticed, there's also an unsigned integer pointer uintptr type, which is an integer representation of a memory address. It is not recommended to use this, so we don't have to worry about it.

So which one should we use?

It is recommended that whenever we need an integer value, we should just use int unless we have a specific reason to use a sized or unsigned integer type.

Byte and Rune

Golang has two additional integer types called byte and rune that are aliases for uint8 and int32 data types respectively.

type byte = uint8
type rune = int32

A rune represents a unicode code point.

var b byte = 'a'
var r rune = '🍕'

Floating point

Next, we have floating point types which are used to store numbers with a decimal component.

Go has two floating point types float32 and float64. Both type follows the IEEE-754 standard.

The default type for floating point values is float64.

var f32 float32 = 1.7812 // IEEE-754 32-bit
var f64 float64 = 3.1415 // IEEE-754 64-bit

Operators

Go provides several operators for performing operations on numeric types.

Complex

There are 2 complex types in Go, complex128 where both real and imaginary parts are float64 and complex64 where real and imaginary parts are float32.

We can define complex numbers either using the built-in complex function or as literals.

var c1 complex128 = complex(10, 1)
var c2 complex64 = 12 + 4i

Zero Values

Now let's discuss zero values. So in Go, any variable declared without an explicit initial value is given its zero value. For example, let's declare some variables and see:

var i int
var f float64
var b bool
var s string

fmt.Printf("%v %v %v %q\n", i, f, b, s)

$ go run main.go
0 0 false ""

So, as we can see int and float are assigned as 0, bool as false, and string as an empty string. This is quite different from how other languages do it. For example, most languages initialize unassigned variables as null or undefined.

This is great, but what are those percent symbols in our Printf function? As you've already guessed, they are used for formatting and we will learn about them later.

Type Conversion

Moving on, now that we have seen how data types work, let's see how to do type conversion.

i := 42
f := float64(i)
u := uint(f)

fmt.Printf("%T %T", f, u)

$ go run main.go
float64 uint

And as we can see, it prints the type as float64 and uint.

Note that this is different from parsing.

Alias types

Alias types were introduced in Go 1.9. They allow developers to provide an alternate name for an existing type and use it interchangeably with the underlying type.

package main

import "fmt"

type MyAlias = string

func main() {
	var str MyAlias = "I am an alias"

	fmt.Printf("%T - %s", str, str) // Output: string - I am an alias
}

Defined types

Lastly, we have defined types that unlike alias types do not use an equals sign.

package main

import "fmt"

type MyDefined string

func main() {
	var str MyDefined = "I am defined"

	fmt.Printf("%T - %s", str, str) // Output: main.MyDefined - I am defined
}

But wait...what's the difference?

So, defined types do more than just give a name to a type.

It first defines a new named type with an underlying type. However, this defined type is different from any other type, including its underline type.

Hence, it cannot be used interchangeably with the underlying type like alias types.

It's a bit confusing at first, hopefully, this example will make things clear.

package main

import "fmt"

type MyAlias = string

type MyDefined string

func main() {
	var alias MyAlias
	var def MyDefined

	// ✅ Works
	var copy1 string = alias

	// ❌ Cannot use def (variable of type MyDefined) as string value in variable
	var copy2 string = def

	fmt.Println(copy1, copy2)
}

As we can see, we cannot use the defined type interchangeably with the underlying type, unlike alias types.