Lua Types Tutorial

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Please look at TutorialExamples for notes about running the examples here. We'll use the print() function to print out values or calculations on those values. The parentheses around the arguments are important and will cause an error if omitted.

!

> print("hello") -- print the string hello.

hello

Numbers

The number type represents a floating point (fractional) number. There is no separate integer (non-fractional) type.

Lua allows simple arithmetic on numbers using the usual operators to add, subtract, multiply and divide.

> print(2+2)

4

> print(2-7)

-5

> print(7*8)

56

> print(7/8)

0.875

Notice that the numbers are not rounded into integers. They are floating point, or real numbers. We can assign values to variables using the = operator.

> x = 7

> print(x)

7

> x = x * 9

> print(x)

63

> print(x*2) -- will not change the value of x

126

> print(x)

63

For more information on Lua's number type you can look at the NumbersTutorial.

Strings

Lua also uses strings (i.e. text) types. To create strings, wrap text in "double quotes" or 'single quotes':

> print("hello")

hello

> who = "Lua user"

> print(who)

Lua user

> print("hello ")

hello 

> print("hello " .. who) -- the variable "who" was assigned above

hello Lua user

> print(who)

Lua user

Unlike some other languages, you cannot use the + operator to concatenate strings. i.e.:

> message = "hello " + who

stdin:1: attempt to perform arithmetic on a string value

stack traceback:

        stdin:1: in main chunk

        [C]: ?

For more information on Lua's string type you can look at the StringsTutorial.

Boolean

Boolean values have either the value true or false. If a value is not true, it must be false and vice versa. The not operator can be placed before a boolean value to invert it. i.e. not true is equal to false.

> x = true

> print(x)

true

> print(not x)

false

> print(not false)

true

> print(1 == 0) -- test whether two numbers are equal

false

> print(1 == 1)

true

> print(1 ~= 0) -- test whether two numbers are not equal

true

> print(true ~= false) -- is true not equal to false?

true

Note that for assignment you use a single equals sign (=), but for comparison you use a double equals sign (==). These two operators have different meanings but look similar, it's a common mistake to write one where you meant the other.

For more information on Lua's Boolean type you can look at the ExpressionsTutorial.

Tables

Lua has a general-purpose aggregate datatype called a table. Aggregate data types are used for storing collections (such as lists, sets, arrays, and associative arrays) containing other objects (including numbers, strings, or even other aggregates). Lua is a unique language in that tables (which are associative arrays) are used for representing all other aggregate types.

Tables are created using a pair of curly brackets {} . Let's create an empty table:

> x = {}

> print(x)

table: 0035C910

The TablesTutorial will later explain how to use tables.

Functions

In Lua, functions are assigned to variables, just like numbers and strings. Functions are created using the function keyword. Here we create a simple function which will print a friendly message.

> foo = function () print("hello") end -- declare the function

> foo() -- call the function

hello

> print(foo) -- get the value of the variable "foo"

function: 0035D6E8

> x = function() print("hello") end

> x()

hello

> print(x)

function: 0035EA20

A function can be part of a table:

> a = "aeiou" -- a string

> b = 13      -- a number

> c = function()  -- a function

>  print ("\n\n\tAin't it grand")

> end

> d = { a, b ,c} -- put them in a table

> function printit(tata)  -- print their types.

> table.unpack(tata) -- unpack the table

> for key, value in ipairs(tata) do print(key, type(value)) end

> end

> printit(d)

1       string

2       number

3       function

        

The FunctionsTutorial will later explain how to use functions.

nil values

nil is a special value which indicates the lack of a useful value. If you try getting a variable that doesn't exist you will get nil:

> print(x)

nil

> x = 2.5

> print(x)

2.5

The other places where nil is used will be shown in other tutorials.

Userdata

Userdata values are objects foreign to Lua, such as objects implemented in C. These typically come about when an object in a C library is exposed to Lua. You cannot do anything with a userdata value in Lua other than pass it around, it's only useful for giving to functions exposed by the same C library that made the userdata. But using metamethods (explained in a later tutorial), it's possible to make userdata work with operators and act similar to tables. Userdata is a more advanced topic discussed further in the [Lua Reference Manual].

Thread

A thread value represents an independent (cooperative) thread of execution. These are discussed further in CoroutinesTutorial.

Dynamic typing

You might have noticed that whilst we created the above variables, we did not have to specify which type of variable we were creating. For example,

a = 1

b = "hello"

c = {}

a = 1

a = "hello"

a = {}

Querying type

As Lua is a reflective language, we can use the Lua function type() to get a description of the type of a particular object.

> x = "123"  -- a string

> print(x, type(x)) -- show the value of x and its type

123     string

> x = x + 7  -- add a number to the string which forces coercion

> print(x, type(x)) -- again show the value and type

130     number