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Lua 5.1 Reference Manual

by Roberto Ierusalimschy, Luiz Henrique de Figueiredo, Waldemar Celes

Copyright &copy; 2006-2008 Lua.org, PUC-Rio.

Freely available under the terms of the

<a href="http://www.lua.org/license.html#5">Lua license</a>.

<a href="contents.html#contents">contents</A>

&middot;

<a href="contents.html#index">index</A>

<h1>1 - <a name="1">Introduction</a></h1>

Lua is an extension programming language designed to support

general procedural programming with data description

facilities.

It also offers good support for object-oriented programming,

functional programming, and data-driven programming.

Lua is intended to be used as a powerful, light-weight

scripting language for any program that needs one.

Lua is implemented as a library, written in <em>clean</em> C

(that is, in the common subset of ANSI&nbsp;C and C++).

Being an extension language, Lua has no notion of a "main" program:

it only works <em>embedded</em> in a host client,

called the <em>embedding program</em> or simply the <em>host</em>.

This host program can invoke functions to execute a piece of Lua code,

can write and read Lua variables,

and can register C&nbsp;functions to be called by Lua code.

Through the use of C&nbsp;functions, Lua can be augmented to cope with

a wide range of different domains,

thus creating customized programming languages sharing a syntactical framework.

The Lua distribution includes a sample host program called <code>lua</code>,

which uses the Lua library to offer a complete, stand-alone Lua interpreter.

Lua is free software,

and is provided as usual with no guarantees,

as stated in its license.

The implementation described in this manual is available

at Lua's official web site, <code>www.lua.org</code>.

Like any other reference manual,

this document is dry in places.

For a discussion of the decisions behind the design of Lua,

see the technical papers available at Lua's web site.

For a detailed introduction to programming in Lua,

see Roberto's book, <em>Programming in Lua (Second Edition)</em>.

<h1>2 - <a name="2">The Language</a></h1>

This section describes the lexis, the syntax, and the semantics of Lua.

In other words,

this section describes

which tokens are valid,

how they can be combined,

and what their combinations mean.

The language constructs will be explained using the usual extended BNF notation,

in which

{<em>a</em>}&nbsp;means&nbsp;0 or more <em>a</em>'s, and

[<em>a</em>]&nbsp;means an optional <em>a</em>.

Non-terminals are shown like non-terminal,

keywords are shown like <b>kword</b>,

and other terminal symbols are shown like `<b>=</b>&acute;.

The complete syntax of Lua can be found in <a href="#8">&sect;8</a>

at the end of this manual.

<h2>2.1 - <a name="2.1">Lexical Conventions</a></h2>

<em>Names</em>

(also called <em>identifiers</em>)

in Lua can be any string of letters,

digits, and underscores,

not beginning with a digit.

This coincides with the definition of names in most languages.

(The definition of letter depends on the current locale:

any character considered alphabetic by the current locale

can be used in an identifier.)

Identifiers are used to name variables and table fields.

The following <em>keywords</em> are reserved

and cannot be used as names:

and break do else elseif

end false for function if

in local nil not or

repeat return then true until while

Lua is a case-sensitive language:

<code>and</code> is a reserved word, but <code>And</code> and <code>AND</code>

are two different, valid names.

As a convention, names starting with an underscore followed by

uppercase letters (such as <a href="#pdf-_VERSION"><code>_VERSION</code></a>)

are reserved for internal global variables used by Lua.

The following strings denote other tokens:

+ - * / % ^ #

== ~= &lt;= &gt;= &lt; &gt; =

( ) { } [ ]

; : , . .. ...

<em>Literal strings</em>

can be delimited by matching single or double quotes,

and can contain the following C-like escape sequences:

'<code>\a</code>' (bell),

'<code>\b</code>' (backspace),

'<code>\f</code>' (form feed),

'<code>\n</code>' (newline),

'<code>\r</code>' (carriage return),

'<code>\t</code>' (horizontal tab),

'<code>\v</code>' (vertical tab),

'<code>\\</code>' (backslash),

'<code>\"</code>' (quotation mark [double quote]),

and '<code>\'</code>' (apostrophe [single quote]).

Moreover, a backslash followed by a real newline

results in a newline in the string.

A character in a string can also be specified by its numerical value

using the escape sequence <code>\<em>ddd</em></code>,

where <em>ddd</em> is a sequence of up to three decimal digits.

(Note that if a numerical escape is to be followed by a digit,

it must be expressed using exactly three digits.)

Strings in Lua can contain any 8-bit value, including embedded zeros,

which can be specified as '<code>\0</code>'.

Literal strings can also be defined using a long format

enclosed by <em>long brackets</em>.

We define an <em>opening long bracket of level <em>n</em></em> as an opening

square bracket followed by <em>n</em> equal signs followed by another

opening square bracket.

So, an opening long bracket of level&nbsp;0 is written as <code>[[</code>,

an opening long bracket of level&nbsp;1 is written as <code>[=[</code>,

and so on.

A <em>closing long bracket</em> is defined similarly;

for instance, a closing long bracket of level&nbsp;4 is written as <code>]====]</code>.

A long string starts with an opening long bracket of any level and

ends at the first closing long bracket of the same level.

Literals in this bracketed form can run for several lines,

do not interpret any escape sequences,

and ignore long brackets of any other level.

They can contain anything except a closing bracket of the proper level.

For convenience,

when the opening long bracket is immediately followed by a newline,

the newline is not included in the string.

As an example, in a system using ASCII

(in which '<code>a</code>' is coded as&nbsp;97,

newline is coded as&nbsp;10, and '<code>1</code>' is coded as&nbsp;49),

the five literal strings below denote the same string:

a = 'alo\n123"'

a = "alo\n123\""

a = '\97lo\10\04923"'

a = [[alo

123"]]

a = [==[

alo

123"]==]

A <em>numerical constant</em> can be written with an optional decimal part

and an optional decimal exponent.

Lua also accepts integer hexadecimal constants,

by prefixing them with <code>0x</code>.

Examples of valid numerical constants are

3 3.0 3.1416 314.16e-2 0.31416E1 0xff 0x56

A <em>comment</em> starts with a double hyphen (<code>--</code>)

anywhere outside a string.

If the text immediately after <code>--</code> is not an opening long bracket,

the comment is a <em>short comment</em>,

which runs until the end of the line.

Otherwise, it is a <em>long comment</em>,

which runs until the corresponding closing long bracket.

Long comments are frequently used to disable code temporarily.

<h2>2.2 - <a name="2.2">Values and Types</a></h2>

Lua is a <em>dynamically typed language</em>.

This means that

variables do not have types; only values do.

There are no type definitions in the language.

All values carry their own type.

All values in Lua are <em>first-class values</em>.

This means that all values can be stored in variables,

passed as arguments to other functions, and returned as results.

There are eight basic types in Lua:

<em>nil</em>, <em>boolean</em>, <em>number</em>,

<em>string</em>, <em>function</em>, <em>userdata</em>,

<em>thread</em>, and <em>table</em>.

<em>Nil</em> is the type of the value <b>nil</b>,

whose main property is to be different from any other value;

it usually represents the absence of a useful value.

<em>Boolean</em> is the type of the values <b>false</b> and <b>true</b>.

Both <b>nil</b> and <b>false</b> make a condition false;

any other value makes it true.

<em>Number</em> represents real (double-precision floating-point) numbers.

(It is easy to build Lua interpreters that use other

internal representations for numbers,

such as single-precision float or long integers;

see file <code>luaconf.h</code>.)

<em>String</em> represents arrays of characters.

Lua is 8-bit clean:

strings can contain any 8-bit character,

including embedded zeros ('<code>\0</code>') (see <a href="#2.1">&sect;2.1</a>).

Lua can call (and manipulate) functions written in Lua and

functions written in C

(see <a href="#2.5.8">&sect;2.5.8</a>).

The type <em>userdata</em> is provided to allow arbitrary C&nbsp;data to

be stored in Lua variables.

This type corresponds to a block of raw memory

and has no pre-defined operations in Lua,

except assignment and identity test.

However, by using <em>metatables</em>,

the programmer can define operations for userdata values

(see <a href="#2.8">&sect;2.8</a>).

Userdata values cannot be created or modified in Lua,

only through the C&nbsp;API.

This guarantees the integrity of data owned by the host program.

The type <em>thread</em> represents independent threads of execution

and it is used to implement coroutines (see <a href="#2.11">&sect;2.11</a>).

Do not confuse Lua threads with operating-system threads.

Lua supports coroutines on all systems,

even those that do not support threads.

The type <em>table</em> implements associative arrays,

that is, arrays that can be indexed not only with numbers,

but with any value (except <b>nil</b>).

Tables can be <em>heterogeneous</em>;

that is, they can contain values of all types (except <b>nil</b>).

Tables are the sole data structuring mechanism in Lua;

they can be used to represent ordinary arrays,

symbol tables, sets, records, graphs, trees, etc.

To represent records, Lua uses the field name as an index.

The language supports this representation by

providing <code>a.name</code> as syntactic sugar for <code>a["name"]</code>.

There are several convenient ways to create tables in Lua

(see <a href="#2.5.7">&sect;2.5.7</a>).

Like indices,

the value of a table field can be of any type (except <b>nil</b>).

In particular,

because functions are first-class values,

table fields can contain functions.

Thus tables can also carry <em>methods</em> (see <a href="#2.5.9">&sect;2.5.9</a>).

Tables, functions, threads, and (full) userdata values are <em>objects</em>:

variables do not actually <em>contain</em> these values,

only <em>references</em> to them.

Assignment, parameter passing, and function returns

always manipulate references to such values;

these operations do not imply any kind of copy.

The library function <a href="#pdf-type"><code>type</code></a> returns a string describing the type

of a given value.

<h3>2.2.1 - <a name="2.2.1">Coercion</a></h3>

Lua provides automatic conversion between

string and number values at run time.

Any arithmetic operation applied to a string tries to convert

this string to a number, following the usual conversion rules.

Conversely, whenever a number is used where a string is expected,

the number is converted to a string, in a reasonable format.

For complete control over how numbers are converted to strings,

use the <code>format</code> function from the string library

(see <a href="#pdf-string.format"><code>string.format</code></a>).

<h2>2.3 - <a name="2.3">Variables</a></h2>

Variables are places that store values.

There are three kinds of variables in Lua:

global variables, local variables, and table fields.

A single name can denote a global variable or a local variable

(or a function's formal parameter,

which is a particular kind of local variable):

var ::= Name

Name denotes identifiers, as defined in <a href="#2.1">&sect;2.1</a>.

Any variable is assumed to be global unless explicitly declared

as a local (see <a href="#2.4.7">&sect;2.4.7</a>).

Local variables are <em>lexically scoped</em>:

local variables can be freely accessed by functions

defined inside their scope (see <a href="#2.6">&sect;2.6</a>).

Before the first assignment to a variable, its value is <b>nil</b>.

Square brackets are used to index a table:

var ::= prefixexp `<b>[</b>&acute; exp `<b>]</b>&acute;

The meaning of accesses to global variables

and table fields can be changed via metatables.

An access to an indexed variable <code>t[i]</code> is equivalent to

a call <code>gettable_event(t,i)</code>.

(See <a href="#2.8">&sect;2.8</a> for a complete description of the

<code>gettable_event</code> function.

This function is not defined or callable in Lua.

We use it here only for explanatory purposes.)

The syntax <code>var.Name</code> is just syntactic sugar for

<code>var["Name"]</code>:

var ::= prefixexp `<b>.</b>&acute; Name

All global variables live as fields in ordinary Lua tables,

called <em>environment tables</em> or simply

<em>environments</em> (see <a href="#2.9">&sect;2.9</a>).

Each function has its own reference to an environment,

so that all global variables in this function

will refer to this environment table.

When a function is created,

it inherits the environment from the function that created it.

To get the environment table of a Lua function,

you call <a href="#pdf-getfenv"><code>getfenv</code></a>.

To replace it,

you call <a href="#pdf-setfenv"><code>setfenv</code></a>.

(You can only manipulate the environment of C&nbsp;functions

through the debug library; (see <a href="#5.9">&sect;5.9</a>).)

An access to a global variable <code>x</code>

is equivalent to <code>_env.x</code>,

which in turn is equivalent to

gettable_event(_env, "x")

where <code>_env</code> is the environment of the running function.

(See <a href="#2.8">&sect;2.8</a> for a complete description of the

<code>gettable_event</code> function.

This function is not defined or callable in Lua.

Similarly, the <code>_env</code> variable is not defined in Lua.

We use them here only for explanatory purposes.)

<h2>2.4 - <a name="2.4">Statements</a></h2>

Lua supports an almost conventional set of statements,

similar to those in Pascal or C.

This set includes

assignments, control structures, function calls,

and variable declarations.

<h3>2.4.1 - <a name="2.4.1">Chunks</a></h3>

The unit of execution of Lua is called a <em>chunk</em>.

A chunk is simply a sequence of statements,

which are executed sequentially.

Each statement can be optionally followed by a semicolon:

chunk ::= {stat [`<b>;</b>&acute;]}

There are no empty statements and thus '<code>;;</code>' is not legal.

Lua handles a chunk as the body of an anonymous function

with a variable number of arguments

(see <a href="#2.5.9">&sect;2.5.9</a>).

As such, chunks can define local variables,

receive arguments, and return values.

A chunk can be stored in a file or in a string inside the host program.

To execute a chunk,

Lua first pre-compiles the chunk into instructions for a virtual machine,

and then it executes the compiled code

with an interpreter for the virtual machine.

Chunks can also be pre-compiled into binary form;

see program <code>luac</code> for details.

Programs in source and compiled forms are interchangeable;

Lua automatically detects the file type and acts accordingly.

<h3>2.4.2 - <a name="2.4.2">Blocks</a></h3><p>

A block is a list of statements;

syntactically, a block is the same as a chunk:

block ::= chunk

A block can be explicitly delimited to produce a single statement:

stat ::= <b>do</b> block <b>end</b>

Explicit blocks are useful

to control the scope of variable declarations.

Explicit blocks are also sometimes used to

add a <b>return</b> or <b>break</b> statement in the middle

of another block (see <a href="#2.4.4">&sect;2.4.4</a>).

<h3>2.4.3 - <a name="2.4.3">Assignment</a></h3>

Lua allows multiple assignments.

Therefore, the syntax for assignment

defines a list of variables on the left side

and a list of expressions on the right side.

The elements in both lists are separated by commas:

stat ::= varlist `<b>=</b>&acute; explist

varlist ::= var {`<b>,</b>&acute; var}

explist ::= exp {`<b>,</b>&acute; exp}

Expressions are discussed in <a href="#2.5">&sect;2.5</a>.

Before the assignment,

the list of values is <em>adjusted</em> to the length of

the list of variables.

If there are more values than needed,

the excess values are thrown away.

If there are fewer values than needed,

the list is extended with as many <b>nil</b>'s as needed.

If the list of expressions ends with a function call,

then all values returned by that call enter the list of values,

before the adjustment

(except when the call is enclosed in parentheses; see <a href="#2.5">&sect;2.5</a>).

The assignment statement first evaluates all its expressions

and only then are the assignments performed.

Thus the code

i = 3

i, a[i] = i+1, 20

sets <code>a[3]</code> to 20, without affecting <code>a[4]</code>

because the <code>i</code> in <code>a[i]</code> is evaluated (to 3)

before it is assigned&nbsp;4.

Similarly, the line

x, y = y, x

exchanges the values of <code>x</code> and <code>y</code>,

and

x, y, z = y, z, x

cyclically permutes the values of <code>x</code>, <code>y</code>, and <code>z</code>.

The meaning of assignments to global variables

and table fields can be changed via metatables.

An assignment to an indexed variable <code>t[i] = val</code> is equivalent to

<code>settable_event(t,i,val)</code>.

(See <a href="#2.8">&sect;2.8</a> for a complete description of the

<code>settable_event</code> function.

This function is not defined or callable in Lua.

We use it here only for explanatory purposes.)

An assignment to a global variable <code>x = val</code>

is equivalent to the assignment

<code>_env.x = val</code>,

which in turn is equivalent to

settable_event(_env, "x", val)

where <code>_env</code> is the environment of the running function.

(The <code>_env</code> variable is not defined in Lua.

We use it here only for explanatory purposes.)

<h3>2.4.4 - <a name="2.4.4">Control Structures</a></h3><p>

The control structures

<b>if</b>, <b>while</b>, and <b>repeat</b> have the usual meaning and

familiar syntax:

stat ::= <b>while</b> exp <b>do</b> block <b>end</b>

stat ::= <b>repeat</b> block <b>until</b> exp

stat ::= <b>if</b> exp <b>then</b> block {<b>elseif</b> exp <b>then</b> block} [<b>else</b> block] <b>end</b>

Lua also has a <b>for</b> statement, in two flavors (see <a href="#2.4.5">&sect;2.4.5</a>).

The condition expression of a

control structure can return any value.

Both <b>false</b> and <b>nil</b> are considered false.

All values different from <b>nil</b> and <b>false</b> are considered true

(in particular, the number 0 and the empty string are also true).

In the <b>repeat</b>&ndash;<b>until</b> loop,

the inner block does not end at the <b>until</b> keyword,

but only after the condition.

So, the condition can refer to local variables

declared inside the loop block.

The <b>return</b> statement is used to return values

from a function or a chunk (which is just a function).

Functions and chunks can return more than one value,

and so the syntax for the <b>return</b> statement is

stat ::= <b>return</b> [explist]

The <b>break</b> statement is used to terminate the execution of a

<b>while</b>, <b>repeat</b>, or <b>for</b> loop,

skipping to the next statement after the loop:

stat ::= <b>break</b>

A <b>break</b> ends the innermost enclosing loop.

The <b>return</b> and <b>break</b>

statements can only be written as the <em>last</em> statement of a block.

If it is really necessary to <b>return</b> or <b>break</b> in the

middle of a block,

then an explicit inner block can be used,

as in the idioms

<code>do return end</code> and <code>do break end</code>,

because now <b>return</b> and <b>break</b> are the last statements in

their (inner) blocks.

<h3>2.4.5 - <a name="2.4.5">For Statement</a></h3>

The <b>for</b> statement has two forms:

one numeric and one generic.

The numeric <b>for</b> loop repeats a block of code while a

control variable runs through an arithmetic progression.

It has the following syntax:

stat ::= <b>for</b> Name `<b>=</b>&acute; exp `<b>,</b>&acute; exp [`<b>,</b>&acute; exp] <b>do</b> block <b>end</b>

The <em>block</em> is repeated for <em>name</em> starting at the value of

the first <em>exp</em>, until it passes the second <em>exp</em> by steps of the

third <em>exp</em>.

More precisely, a <b>for</b> statement like

for v = <em>e1</em>, <em>e2</em>, <em>e3</em> do <em>block</em> end

is equivalent to the code:

do

local <em>var</em>, <em>limit</em>, <em>step</em> = tonumber(<em>e1</em>), tonumber(<em>e2</em>), tonumber(<em>e3</em>)

if not (<em>var</em> and <em>limit</em> and <em>step</em>) then error() end

while (<em>step</em> &gt; 0 and <em>var</em> &lt;= <em>limit</em>) or (<em>step</em> &lt;= 0 and <em>var</em> &gt;= <em>limit</em>) do

local v = <em>var</em>

<em>block</em>

<em>var</em> = <em>var</em> + <em>step</em>

end

end

Note the following:

All three control expressions are evaluated only once,

before the loop starts.

They must all result in numbers.

<code><em>var</em></code>, <code><em>limit</em></code>, and <code><em>step</em></code> are invisible variables.

The names shown here are for explanatory purposes only.

If the third expression (the step) is absent,

then a step of&nbsp;1 is used.

You can use <b>break</b> to exit a <b>for</b> loop.

The loop variable <code>v</code> is local to the loop;

you cannot use its value after the <b>for</b> ends or is broken.

If you need this value,

assign it to another variable before breaking or exiting the loop.

The generic <b>for</b> statement works over functions,

called <em>iterators</em>.

On each iteration, the iterator function is called to produce a new value,

stopping when this new value is <b>nil</b>.

The generic <b>for</b> loop has the following syntax:

stat ::= <b>for</b> namelist <b>in</b> explist <b>do</b> block <b>end</b>

namelist ::= Name {`<b>,</b>&acute; Name}

A <b>for</b> statement like

for <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> in <em>explist</em> do <em>block</em> end

is equivalent to the code:

do

local <em>f</em>, <em>s</em>, <em>var</em> = <em>explist</em>

while true do

local <em>var_1</em>, &middot;&middot;&middot;, <em>var_n</em> = <em>f</em>(<em>s</em>, <em>var</em>)

<em>var</em> = <em>var_1</em>

if <em>var</em> == nil then break end

<em>block</em>

end

end

Note the following:

<code><em>explist</em></code> is evaluated only once.

Its results are an <em>iterator</em> function,

a <em>state</em>,

and an initial value for the first <em>iterator variable</em>.

<code><em>f</em></code>, <code><em>s</em></code>, and <code><em>var</em></code> are invisible variables.

The names are here for explanatory purposes only.

You can use <b>break</b> to exit a <b>for</b> loop.

The loop variables <code><em>var_i</em></code> are local to the loop;

you cannot use their values after the <b>for</b> ends.

If you need these values,

then assign them to other variables before breaking or exiting the loop.

<h3>2.4.6 - <a name="2.4.6">Function Calls as Statements</a></h3><p>

To allow possible side-effects,

function calls can be executed as statements:

stat ::= functioncall

In this case, all returned values are thrown away.

Function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>.

<h3>2.4.7 - <a name="2.4.7">Local Declarations</a></h3><p>

Local variables can be declared anywhere inside a block.

The declaration can include an initial assignment:

stat ::= <b>local</b> namelist [`<b>=</b>&acute; explist]

If present, an initial assignment has the same semantics

of a multiple assignment (see <a href="#2.4.3">&sect;2.4.3</a>).

Otherwise, all variables are initialized with <b>nil</b>.

A chunk is also a block (see <a href="#2.4.1">&sect;2.4.1</a>),

and so local variables can be declared in a chunk outside any explicit block.

The scope of such local variables extends until the end of the chunk.

The visibility rules for local variables are explained in <a href="#2.6">&sect;2.6</a>.

<h2>2.5 - <a name="2.5">Expressions</a></h2>

The basic expressions in Lua are the following:

exp ::= prefixexp

exp ::= <b>nil</b> | <b>false</b> | <b>true</b>

exp ::= Number

exp ::= String

exp ::= function

exp ::= tableconstructor

exp ::= `<b>...</b>&acute;

exp ::= exp binop exp

exp ::= unop exp

prefixexp ::= var | functioncall | `<b>(</b>&acute; exp `<b>)</b>&acute;

Numbers and literal strings are explained in <a href="#2.1">&sect;2.1</a>;

variables are explained in <a href="#2.3">&sect;2.3</a>;

function definitions are explained in <a href="#2.5.9">&sect;2.5.9</a>;

function calls are explained in <a href="#2.5.8">&sect;2.5.8</a>;

table constructors are explained in <a href="#2.5.7">&sect;2.5.7</a>.

Vararg expressions,

denoted by three dots ('<code>...</code>'), can only be used when

directly inside a vararg function;

they are explained in <a href="#2.5.9">&sect;2.5.9</a>.

Binary operators comprise arithmetic operators (see <a href="#2.5.1">&sect;2.5.1</a>),

relational operators (see <a href="#2.5.2">&sect;2.5.2</a>), logical operators (see <a href="#2.5.3">&sect;2.5.3</a>),

and the concatenation operator (see <a href="#2.5.4">&sect;2.5.4</a>).

Unary operators comprise the unary minus (see <a href="#2.5.1">&sect;2.5.1</a>),

the unary <b>not</b> (see <a href="#2.5.3">&sect;2.5.3</a>),

and the unary <em>length operator</em> (see <a href="#2.5.5">&sect;2.5.5</a>).

Both function calls and vararg expressions can result in multiple values.

If an expression is used as a statement

(only possible for function calls (see <a href="#2.4.6">&sect;2.4.6</a>)),

then its return list is adjusted to zero elements,

thus discarding all returned values.

If an expression is used as the last (or the only) element

of a list of expressions,

then no adjustment is made

(unless the call is enclosed in parentheses).

In all other contexts,

Lua adjusts the result list to one element,

discarding all values except the first one.

Here are some examples:

f() -- adjusted to 0 results

g(f(), x) -- f() is adjusted to 1 result

g(x, f()) -- g gets x plus all results from f()

a,b,c = f(), x -- f() is adjusted to 1 result (c gets nil)

a,b = ... -- a gets the first vararg parameter, b gets

-- the second (both a and b can get nil if there

-- is no corresponding vararg parameter)

a,b,c = x, f() -- f() is adjusted to 2 results

a,b,c = f() -- f() is adjusted to 3 results

return f() -- returns all results from f()

return ... -- returns all received vararg parameters

return x,y,f() -- returns x, y, and all results from f()

{f()} -- creates a list with all results from f()

{...} -- creates a list with all vararg parameters

{f(), nil} -- f() is adjusted to 1 result

Any expression enclosed in parentheses always results in only one value.

Thus,

<code>(f(x,y,z))</code> is always a single value,

even if <code>f</code> returns several values.

(The value of <code>(f(x,y,z))</code> is the first value returned by <code>f</code>

or <b>nil</b> if <code>f</code> does not return any values.)

<h3>2.5.1 - <a name="2.5.1">Arithmetic Operators</a></h3><p>

Lua supports the usual arithmetic operators:

the binary <code>+</code> (addition),

<code>-</code> (subtraction), <code>*</code> (multiplication),

<code>/</code> (division), <code>%</code> (modulo), and <code>^</code> (exponentiation);

and unary <code>-</code> (negation).

If the operands are numbers, or strings that can be converted to

numbers (see <a href="#2.2.1">&sect;2.2.1</a>),

then all operations have the usual meaning.

Exponentiation works for any exponent.

For instance, <code>x^(-0.5)</code> computes the inverse of the square root of <code>x</code>.

Modulo is defined as

a % b == a - math.floor(a/b)*b

That is, it is the remainder of a division that rounds

the quotient towards minus infinity.

<h3>2.5.2 - <a name="2.5.2">Relational Operators</a></h3><p>

The relational operators in Lua are

== ~= &lt; &gt; &lt;= &gt;=

These operators always result in <b>false</b> or <b>true</b>.

Equality (<code>==</code>) first compares the type of its operands.

If the types are different, then the result is <b>false</b>.

Otherwise, the values of the operands are compared.

Numbers and strings are compared in the usual way.

Objects (tables, userdata, threads, and functions)

are compared by <em>reference</em>:

two objects are considered equal only if they are the <em>same</em> object.

Every time you create a new object

(a table, userdata, thread, or function),

this new object is different from any previously existing object.

You can change the way that Lua compares tables and userdata

by using the "eq" metamethod (see <a href="#2.8">&sect;2.8</a>).