Lua Cheat Sheet

print("Hello, World!")

-- Single line comment

--[[
  Multi-line
  comment
--]]

lua script.lua
lua -e 'print("Hello")'  -- One-liner

print("Hello")
print("World")  -- No semicolons needed

local x = 10      -- Local variable
y = 20            -- Global variable
local name = "Lua"

local x = nil     -- Nil/null value
if x == nil then
  print("x is nil")
end

local int = 42
local float = 3.14
local exp = 1.5e-3

local s1 = 'single quotes'
local s2 = "double quotes"
local s3 = [[multi-line
string]]

local flag = true
local active = false
-- Only nil and false are falsy

print(type(x))    -- "number", "string", etc.
print(type(nil))  -- "nil"

local sum = a + b
local diff = a - b
local prod = a * b
local quot = a / b
local mod = a % b
local pow = a ^ b  -- Exponentiation

a == b   -- Equal
a ~= b   -- Not equal (note: ~=)
a > b    -- Greater
a < b    -- Less
a >= b   -- Greater or equal
a <= b   -- Less or equal

a and b  -- Logical AND
a or b   -- Logical OR
not a    -- Logical NOT

local full = first .. " " .. last
local msg = "Count: " .. tostring(42)

#"hello"      -- 5
#mytable      -- Table length

if age >= 18 then
  print("Adult")
else
  print("Minor")
end

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

-- Only nil and false are falsy
if 0 then print("0 is truthy") end
if "" then print("Empty string is truthy") end

-- Lua doesn't have ternary, use:
local result = condition and value1 or value2

local i = 1
while i <= 10 do
  print(i)
  i = i + 1
end

local i = 1
repeat
  print(i)
  i = i + 1
until i > 10

for i = 1, 10 do
  print(i)
end

for i = 10, 1, -1 do  -- Step of -1
  print(i)
end

local arr = {10, 20, 30}
for i, v in ipairs(arr) do
  print(i, v)  -- 1 10, 2 20, 3 30
end

local t = {a=1, b=2}
for k, v in pairs(t) do
  print(k, v)  -- "a" 1, "b" 2
end

for i = 1, 100 do
  if i == 50 then break end
  print(i)
end

function greet(name)
  print("Hello, " .. name)
end

greet("Alice")

function add(a, b)
  return a + b
end

local sum = add(5, 3)

function stats(a, b)
  return a + b, a - b, a * b
end

local sum, diff, prod = stats(10, 5)

local square = function(x)
  return x * x
end

print(square(5))  -- 25

function sum(...)
  local total = 0
  for _, v in ipairs({...}) do
    total = total + v
  end
  return total
end

print(sum(1, 2, 3, 4))  -- 10

local arr = {10, 20, 30}
print(arr[1])  -- 10 (1-indexed!)

local person = {
  name = "John",
  age = 30
}
print(person.name)  -- "John"
print(person["age"])  -- 30

local t = {
  10, 20, 30,
  name = "mixed",
  [5] = "five"
}

table.insert(arr, 40)  -- Add to end
table.insert(arr, 1, 5)  -- Insert at index
table.remove(arr)  -- Remove last
table.remove(arr, 1)  -- Remove at index

local arr = {10, 20, 30}
print(#arr)  -- 3

for k, v in pairs(tbl) do
  print(k, v)
end

local t = {}
local mt = {__index = {default = 10}}
setmetatable(t, mt)
print(t.default)  -- 10

Dog = {}
Dog.__index = Dog

function Dog.new(name)
  local self = setmetatable({}, Dog)
  self.name = name
  return self
end

function Dog:bark()
  print(self.name .. " barks!")
end

local d = Dog.new("Buddy")
d:bark()

local mt = {
  __add = function(a, b)
    return a.value + b.value
  end
}
local a = setmetatable({value=5}, mt)
local b = setmetatable({value=3}, mt)
print((a + b))  -- 8