Lua cheatsheet

• Lua (lua.org)
• Lua Document (lua.org/docs.html)
• Learn X in Y minutes (learnxinyminutes.com)

print("Hello, World!") -- Hello, World!

-- You can omit parentheses if the argument is one string or table literal
print "Hello, World!"  -- Hello, World!

print [[multi-line
 Hello
 World]]

The famous "Hello World" program in Lua

local age = 18 -- local variable
boys, girls = 2, 3 -- global variables

-- Now x = 1, y = 2, z = 3, and 4 is thrown away.
x, y, z = 1, 2, 3, 4

-- Undefined variables return nil.
-- This is not an error:
foo = anUnknownVariable  -- Now foo = nil.

-- Variables are global by default unless declared with local.

Lua is a dynamically typed language and hence only the values will have types not the variables.

-- Writes data to the standard output or a file.
io.write("Enter your name: ")
-- Reads input from the user or a file. You can specify formats like "*l" (line), "*n" (number), or "*a" (all).
name = io.read()

io.write("Enter your age: ")
age = io.read()

-- This is a single line comments

--[[
 Comments (multi-line) could also be written like this
--]]

-- add
  result = 10 + 30  -- => 40

-- subtract
result = 40 - 10  -- => 30

-- multiply
result = 50 * 5   -- => 250

-- divide (float division)
result = 16 / 4   -- => 4.0

-- divide (integer division, Lua 5.3+)
result = 16 // 4  -- => 4

-- modulo
result = 25 % 2   -- => 1

-- power
result = 5 ^ 3    -- => 125

-- unary minus
a = 12
result = -a -- => -12

a = 10
b = 20

-- equals
print(a == b) -- false

-- not equals
print(a ~= b) -- true

-- greater than
print(a > b) -- false

-- less than
print(a < b) -- true

-- greater than or equals
print(a >= b) -- false

-- less than or equals
print(a <= b) -- true

-- and
false and nil  --> false
0 and 20       --> 20
10 and 20      --> 20

-- or
true or false  --> true
10 or 0        --> 10
12 or 4        --> 12

  -- not
not true       --> false

Only nil and false are falsy; 0 and '' are true!

i = 1
while i <= 5 do
  print(i)
  i = i + 1
end

-- Numeric for loop (start, end, step)
for i = 1, 5, 1 do
  print(i)
end

-- Generic for loop for tables
t = {10, 20, 30}
for k, v in ipairs(t) do
  print(k, v) -- prints 1 10, 2 20, 3 30
end

i = 1
repeat
  print(i)
  i = i + 1
until i > 5

while x do
  if condition then
    break
  end
end

-- prints even numbers in [|1,10|]
for i=1,10 do
   if i % 2 == 1 then
    goto continue
  end
   print(i)
   ::continue::
end

function myFunction()
  return 1
end

function myFunctionWithArgs(a, b)
  -- ...
end

-- function as arg
function operate(a, b, func)
    return func(a, b)
end



-- anonymous function
function (parameters)
    -- body of the function
end


local greet = function(name)
    return "Hello, " .. name
end


-- Not exported in the module
local function myPrivateFunction()
end


-- Splats
function doAction(action, ...)
  print("Doing '"..action.."' to", ...)
end

s = "Hello"

Concatenation

s .. " there" -- => Hello there

Commonly used methods

s:upper() -- => HELLO
s:lower() -- => hello
s:len()   -- => 5
s:find("o") -- => 5
s:reverse() -- => olleH

Sub

local s = "programming"
s:sub(3, 7) -- (extracts substring) => "ogram"

Gsub

s:gsub() --> (substitutes all matches)

Char

s = "ha"
s:rep(3) -- // repeats 3 times -> "hahaha"
local s_char = string.char(72, 101, 108, 108, 111)
print(s_char) -- "Hello"

Format

local name = "Alice"
local age = 30
local formatted = string.format("My name is %s and I am %d years old.", name, age)
print(formatted) -- "My name is Alice and I am 30 years old."

Others

s:match()
s:gmatch()
s:dump()
s:byte()

-- Array-like table (one-indexed)
local colors = {"red", "green", "blue"}
print(colors[1]) -- "red"

-- Dictionary-like table
local user = {name = "Jane", age = 25}
print(user.name) -- "Jane"
print(user["age"]) -- 25

-- Mixed table
local mixed = {1, "two", key = "value"}
print(mixed[1]) -- 1
print(mixed.key) -- "value"

-- Getting the length of an array-like table
print(#colors) -- 3

local my_table = {10, 20}

-- insert (appends 30 to the end)
table.insert(my_table, 30)

-- insertion (inserts 2 in position 1)
table.insert(my_table, 1, 2)

-- remove (remove item in position 3)
table.remove(my_table, 3)

-- Default numerical sort
local numbers = {5, 2, 8, 1}
table.sort(numbers) -- {1, 2, 5, 8}

-- Custom sort for descending order
local numbers_desc = {5, 2, 8, 1}
table.sort(numbers_desc, function(a, b)
  return a > b
end) -- {8, 5, 2, 1}

-- concat
local fruit = {"apple", "banana", "cherry"}
local fruit_string = table.concat(fruit, ", ")
print(fruit_string) -- apple, banana, cherry

A metatable is simply a table with functions in it.

mt = {}

mt.__tostring = function() return "lol" end
mt.__add      = function(b) ... end       -- a + b
mt.__mul      = function(b) ... end       -- a * b
mt.__index    = function(k) ... end       -- Lookups (a[k] or a.k)
mt.__newindex = function(k, v) ... end    -- Setters (a[k] = v)

Metatables allow you to override behavior of another table.

mytable = {}
setmetatable(mytable, mt)

print(myobject)

local file = io.open("test.txt", "w")
if file then
  file:write("Hello from Lua!")
  io.close(file)
end

local file = io.open("test.txt", "r")
if file then
  local content = file:read("*a") -- read all content
  print(content)
  io.close(file)
end