Topic 01-types.md
Types
The standard function type returns the type of any Lua value:
type("") == "string" type(0) == "number" type({})) == "table" type(print) == "function" type(nil) == "nil"
nil
nil is a special case; other languages call it NULL, nil,Nothing,null,etc.
It is however a valid value and has a definite type, which is ‘nil’.
Only nil and the boolean false values make conditions fail.
Otherwise, nil is special when used with tables; table elements cannot be nil.
Be particularly careful when putting nil values into an array; these will become
‘holes’ and the value of #arr will become invalid.
Booleans
A boolean value is either true or false. The conditional operators, like >
and == return this value. The standard boolean operators and and or will
usually also return ‘boolean’.
Only false and nil make a condition fail.
a = 5 print(a > 4) --> true print(a == 4) --> false print(a > 4 and a < 10) --> true
Numbers
Lua has only one type of number, which is usually double-precision floating-point on desktop machines. This means you do not usually have to worry about integers and floats; this is one of the simplifications which makes Lua so small and fast. If you worry about storing integers as double-precision floats, remember that integers can be exactly represented up to about 1e52.
There are two rounding functions, math.floor and math.ceil ; so given 3.1, the
first gives the integer part (3), and the second gives the first integer that is
larger (4). math.ceil(x)==x is only true if x is an integer.
function is_integer (x) return math.ceil(x) == x end
0 does not evaluate as false, unlike C or Python.
Strings
A Lua string is usually quoted text, but can actually include any byte value, including the so-called null byte. (You can for instance read a binary file into a Lua string without any corruption.)
Both single and double quotes can be used, with no change in meaning – this is useful if you wish to quote a string inside a string:
message = “cannot find: ‘dolly’”
The best advice is: be consistent in your quoting.
Strings compare as you would expect, using the locale, so that s1==s2 behaves
sensibly, unlike in Java. In Lua, identical strings are identical objects.
There are cases where strings will be automatically considered to be numbers. For
instance, "2"+"3" will be the number 5. But if any of the strings cannot be
converted into a number, then there will be an error: “attempt to perform arithmetic
on a string value”. It isn’t a good idea to depend on this default behaviour,
because your program will crash on bad input.
The length operator # returns the number of bytes in a string, which is not
necessarily the number of characters. You may read UTF-8 text into a string, but then
anything that isn’t plain ASCII will occupy at least two bytes.
!ref:utf8' Later we will discuss how to handle UTF-8 correctly.
The string library provides string operations.
Tables
Lua tables are associative arrays; indexing a table is giving it a key and getting a value.
Any Lua value can be stored in a table, except for nil.
They are very efficient at behaving like arrays, that is, consecutive integer keys starting
at one. The length operator # only gives the number of these integer keys, not the total
number of keys.
The table library provides some useful operations on tables, mostly on array-like tables.
Functions
In Lua, functions are first-class values – that is, they can be passed around like any other value. Writing out a function as an explicit assignment makes this clearer:
sqr = function(x) return x^2 end
Coroutines
These have type ‘thread’ but are best understood as resumable functions.
Userdata
This type represents a wrapped C object. Although you cannot modify their internal contents (by
design) they can be given useful behaviour with metatables. For instance, the
standard function io.open returns a userdata object which has methods like read and close,
but you cannot access the internal representation of this file object.