basic.rst

The basic things

In the previous chapter, we saw that Go programs are organized using packages and that Go natively supports UTF-8 for strings and identifiers. In this chapter we will see how to declare and use variables and constants and the different Go built-in types.

How to declare a variable?

There are several ways to declare a variable in Go.

The basic form is:

// declare a variable named "variable_name" of type "type"
var variable_name type

You can declare several variables of the same type in a single line, by separating them with commas.

// declare variables var1, var2, and var3 all of type type
var var1, var2, var3 type

And you can initialize a variable when declaring it too

/* declare a variable named "variable_name" of type "type" and initialize it
    to value*/
    var variable_name type = value

You can even initialize many variables that are declared in the same statement

/* declare a var1, var2, var3 of type "type" and initialize them to value1,
value2, and value3 respectively*/
var var1, var2, var3 type = value1, value2, value3

Guess what? You can omit the type and it will be inferred from the initializers

/* declare and initialize var1, var2 and var3 and initialize them
respectively to value1, value2, and value3. /
var var1, var2, var3 = value1, value2, value3

Even shorter, inside a function body (let me repeat that: only inside a function body) you can even drop the keyword var and use the := instead of =

// omit var and type, and use ':=' instead of '=' inside the function body
func test(){
    var1, var2, var3 := value1, value2, value3
}

Don't worry. It's actually easy. The examples with the builtin types, below, will illustrate both of these forms. Just remember that, unlike the C way, in Go the type is put at the end of the declaration and -should I repeat it?- that the := operator can only be used inside a function body.

The builtin types

Boolean

For boolean truth values, Go has the type bool (like the C++ one) that takes one of the values: true or false.

//Example snippet
var active bool //basic form
var enabled, disabled = true, false //type omitted, variables initialized
func test(){
    var available bool //general form
    valid := false //type and var omitted, and variable initialized
    available = true //normal assignation
}

Numeric types

For integer values, signed and unsigned, Go has int and uint both having the appropriate length for your machine (32 or 64 bits) But there's also explicit sized ints: int8, int16, int32, int64 and byte, uint8, uint16, uint32, uint64. With byte being an alias for uint8.

For floating point values, we have float32 and float64.

Wait that's not all, Go has native support for complex numbers too! In fact, you can use complex64 for numbers with 32 bits for the real part and 32 bits for the imaginary part, and there is complex128 for numbers with 64 bits for the real part and 64 bits for the imaginary part.

Table of numeric types

From the Go Programming Language Specification

Type	Values
uint8	the set of all unsigned 8-bit integers (0 to 255)
uint16	the set of all unsigned 16-bit integers (0 to 65535)
uint32	the set of all unsigned 32-bit integers (0 to 4294967295)
uint64	the set of all unsigned 64-bit integers (0 to 18446744073709551615)
int8	the set of all signed 8-bit integers (-128 to 127)
int16	the set of all signed 16-bit integers (-32768 to 32767)
int32	the set of all signed 32-bit integers (-2147483648 to 2147483647)
int64	the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
float32	the set of all IEEE-754 32-bit floating-point numbers
float64	the set of all IEEE-754 64-bit floating-point numbers
complex64	the set of all complex numbers with float32 real and imaginary parts
complex128	the set of all complex numbers with float64 real and imaginary parts
byte familiar alias for uint8

//Example snippet
var i int32 //basic form with a int32
var x, y, z = 1, 2, 3 //type omitted, variables initialized
func test(){
    var pi  float32 //basic form
    one, two, thre := 1, 2, 3 //type and var omitted, variables initialized
    c := 10+3i // a complex number, type infered and keyword 'var' omitted.
    pi = 3.14 // normal assignation
}

Strings

As seen in the previous chapter, strings are in UTF-8 and they are enclosed between two double quotes (") and their type is -you bet!- string.

//Example snippet
var french_hello string //basic form
var empty_string string = "" // here empty_string (like french_hello) equals ""
func test(){
    no, yes, maybe := "no", "yes", "maybe" //var and type omitted
    japanese_hello := "Ohaiou"  //type inferred, var keyword omitted
    french_hello = "Bonjour" //normal assignation
}

Constants

In Go, constants are -uh- constant values created at compile time, and they can be: numbers, boolean or strings.

The syntax to declare a constant is:

const constant_name = value

Some examples:

//example snippet
const i = 100
const pi = 3.14
const prefix = "go_"

Facilities

Grouping declarations

Multiple var, const and import declarations can be grouped using parenthesis.

So instead of writing:

//example snippet
import "fmt"
import "os"

const i = 100
const pi = 3.14
const prefix = "go_"

var i int
var pi = float32
var prefix string

You can write:

//example snippet with grouping
import(
    "fmt"
    "os"
)

const(
    i = 100
    pi = 3.14
    prefix = "go_"
)

var(
    i int
    pi = float32
    prefix string
)

Of course, you group consts with consts, vars with vars and imports with imports but you can not mix them in the same group!

iota and enumerations

Go provides the keyword iota that can be used when declaring enumerated constants, This keyword yelds an incremented value by 1, starting from 0, each time it is used.

Example:

//example snippet
const(
    x = iota //x == 0
    y = iota //y == 1
    z = iota //z == 2
    w // implicitely w == iota, therefore: w == 3
)

Well, that's it for this chapter. I told you, it won't be hard. In fact, Go eases variable declarations a lot. You'd almost feel like coding with a scripting language like python --and it's even better.