TopolearnKnowing the types and the local/global rule prevents the most common Lua bugs.
Lua is dynamically typed: values have types, variables don't. There are eight types, and type(v) returns the name as a string: nil, boolean, number, string, function, table, thread (coroutines) and userdata (C data).
Since 5.3 a number has two subtypes: integers (64-bit) and floats. 3 is an integer and 3.0 a float; math.type tells them apart. / always gives a float, // is floor division and ^ is exponentiation (always a float). Integer arithmetic wraps around on overflow instead of turning into a float.
Strings are immutable byte sequences, written with single quotes, double quotes, or long brackets [[ ... ]] for multi-line text. Concatenate with ..; #s is the length in bytes.
Variables are global by default. Assigning to an undeclared name creates a global, and reading a misspelled name silently gives nil. Always write local x = 1: locals are faster, scoped to their block, and cannot leak.
Lua supports multiple assignment, local a, b = 1, 2, and swapping with a, b = b, a. Extra values are discarded and missing ones become nil.
Example
local n, f = 10, 10.0
print(type(n), math.type(n), math.type(f)) --> number integer float
print(n == f) --> true (same mathematical value)
print(7 / 2, 7 // 2) --> 3.5 3
print(2 ^ 10) --> 1024.0
print(math.maxinteger + 1 == math.mininteger) --> true (wraps)
local s = "Lua" .. " " .. 5.4
print(s, #s) --> Lua 5.4 7
local a, b, c = 1, 2
print(a, b, c) --> 1 2 nil
a, b = b, a
print(a, b) --> 2 1
function leak() counter = 1 end -- oops: creates a global
leak()
print(counter) --> 1Remember
- Eight types;
type(v)returns the name as a string - Numbers are integers or floats;
/gives a float,//floors - Declare everything
local; undeclared names are globals
Watch out for
- A typo in a variable name silently reads
nilinstead of raising an error.