Lua Cheatsheet

Lua is a powerful, efficient, lightweight, embeddable scripting language designed for extending applications.

Basic Syntax

Comments

-- Single-line comment

--[[
  Multi-line comment
  spans multiple lines
--]]

Function Call Shorthand

Parentheses can be omitted when a function receives exactly one string literal or table constructor:

print("Hello World")  <==>  print "Hello World"
dofile('app.lua')     <==>  dofile 'app.lua'
config({ x = 1 })     <==>  config{ x = 1 }

Tables & Arrays

Tables are Lua's sole data-structuring mechanism. Arrays use 1-based indexing:

-- Empty table
local t = {}

-- Key-value table (dictionary)
local user = { name = "Alice", age = 30 }
user.role = "admin"
user["email"] = "[email protected]"

-- Sequential array (1-based index)
local fruits = { "apple", "banana", "cherry" }
print(fruits[1])  -- prints "apple"
print(#fruits)    -- length operator (3)

---

Control Flow & Loops

Conditionals

Only false and nil are falsy in Lua. 0 and "" are truthy:

if score >= 90 then
  grade = "A"
elseif score >= 80 then
  grade = "B"
else
  grade = "C"
end

Loops

-- While loop
local count = 1
while count <= 5 do
  count = count + 1
end

-- Repeat...until loop (runs at least once)
repeat
  count = count - 1
until count == 0

-- Numeric for loop: for var = start, stop [, step]
for i = 1, 10, 2 do
  print(i) -- 1, 3, 5, 7, 9
end

-- Generic for loop: ipairs (array) and pairs (dictionary)
for index, value in ipairs(fruits) do
  print(index, value)
end

for key, value in pairs(user) do
  print(key, value)
end

---

Functions

Definitions & Multiple Return Values

-- Multiple returns
local function min_max(a, b)
  if a < b then
    return a, b
  else
    return b, a
  end
end

local minimum, maximum = min_max(10, 5)

-- Variable arguments (varargs)
local function sum(...)
  local s = 0
  for _, v in ipairs({...}) do
    s = s + v
  end
  return s
end

---

Metatables & Object-Oriented Programming

Metatables & Metamethods

local Vector = {}
Vector.__index = Vector

function Vector.new(x, y)
  local instance = setmetatable({}, Vector)
  instance.x = x
  instance.y = y
  return instance
end

-- Vector addition metamethod
function Vector.__add(a, b)
  return Vector.new(a.x + b.x, a.y + b.y)
end

function Vector:__tostring()
  return string.format("(%d, %d)", self.x, self.y)
end

local v1 = Vector.new(1, 2)
local v2 = Vector.new(3, 4)
local v3 = v1 + v2
print(v3) -- "(4, 6)"

Metamethod Reference

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Standard Library APIs

String Functions

string.len("hello")             -- 5 (#"hello" also works)
string.lower("HELLO")           -- "hello"
string.upper("hello")           -- "HELLO"
string.sub("hello", 2, 4)       -- "ell"
string.format("%s = %d", "x", 5)-- "x = 5"
string.gsub("banana", "a", "o") -- "bonono"
string.find("hello world", "wor") -- 7, 9

Table Functions

table.insert(fruits, "orange")     -- Append
table.insert(fruits, 1, "grape")   -- Insert at index 1
table.remove(fruits, 1)            -- Remove first item
table.concat(fruits, ", ")         -- Join strings: "apple, banana"
table.sort(fruits)                 -- Sort array in-place

Math Functions

math.abs(-10)         -- 10
math.floor(3.7)       -- 3
math.ceil(3.2)        -- 4
math.max(1, 9, 4)     -- 9
math.random(1, 100)   -- Random integer between 1 and 100

---

References

• Lua 5.4 Reference Manual
• Programming in Lua (First Edition)