# Lua Quick Reference

*Tables, functions, metatables, coroutines, modules, patterns*

> Source: Lua Reference Manual (lua.org/manual) · MIT

## Basics

### Hello World

```
print("Hello, Lua!")
```

### Variables & Assignment

```
local name = "Lua"      -- local variable
x = 10                  -- global (avoid)
local a, b = 1, 2      -- multiple assignment
a, b = b, a            -- swap values
```

### Comments

```
-- single line comment
--[[ multi-line
     comment ]]
```

### Operators

| Command | Description |
|---------|-------------|
| `+ - * / %` | Arithmetic operators |
| `//` | Floor division (5.3+) |
| `^` | Exponentiation |
| `..` | String concatenation |
| `#` | Length operator |
| `== ~=` | Equal / not equal |
| `and or not` | Logical operators |

## Types

### Data Types

| Command | Description |
|---------|-------------|
| `nil` | Absence of value; falsy |
| `boolean` | true or false |
| `number` | Double-precision float (or integer in 5.3+) |
| `string` | Immutable byte sequence |
| `table` | Associative array (only compound type) |
| `function` | First-class closure |
| `userdata` | C data wrapped for Lua |
| `thread` | Coroutine handle |

### Type Checking

```
print(type(42))        -- "number"
print(type("hi"))      -- "string"
print(type(nil))       -- "nil"
print(type({}))        -- "table"
```

## Tables

### Array-style Tables

```
local fruits = {"apple", "banana", "cherry"}
print(fruits[1])          -- "apple" (1-indexed)
table.insert(fruits, "date")
table.remove(fruits, 2)   -- remove "banana"
print(#fruits)            -- length
```

### Dictionary-style Tables

```
local user = {name = "Alice", age = 30}
user.email = "a@b.com"   -- add field
user["name"] = "Bob"     -- bracket access
user.age = nil           -- remove field
```

### Table Functions

| Command | Description |
|---------|-------------|
| `table.insert(t, v)` | Append value to array |
| `table.insert(t, i, v)` | Insert at position i |
| `table.remove(t, i)` | Remove element at position i |
| `table.sort(t [,cmp])` | Sort array in-place |
| `table.concat(t, sep)` | Join array elements into string |
| `table.move(t,a,b,c)` | Move elements from a..b to position c |

## Functions

### Function Definition

```
local function add(a, b)
  return a + b
end
local mul = function(a, b) return a * b end
print(add(2, 3))  -- 5
```

### Variadic & Multiple Returns

```
local function sum(...)
  local s = 0
  for _, v in ipairs({...}) do s = s + v end
  return s
end
local function swap(a, b) return b, a end
local x, y = swap(1, 2)
```

### Closures

```
local function counter()
  local n = 0
  return function()
    n = n + 1; return n
  end
end
local c = counter()
print(c(), c())  -- 1  2
```

## Control Flow

### Conditionals

```
if x > 0 then
  print("positive")
elseif x == 0 then
  print("zero")
else
  print("negative")
end
```

### Loops

```
for i = 1, 10 do print(i) end
for i = 10, 1, -1 do print(i) end
for k, v in pairs(tbl) do print(k, v) end
for i, v in ipairs(arr) do print(i, v) end
```

### While & Repeat

```
while x > 0 do x = x - 1 end
repeat
  x = x + 1
until x >= 10
```

## Strings

### String Functions

| Command | Description |
|---------|-------------|
| `string.len(s) / #s` | String length in bytes |
| `string.sub(s, i, j)` | Substring from i to j |
| `string.upper(s)` | Convert to uppercase |
| `string.lower(s)` | Convert to lowercase |
| `string.rep(s, n)` | Repeat string n times |
| `string.reverse(s)` | Reverse string |
| `string.format(fmt, ...)` | Printf-style formatting |
| `string.find(s, pat)` | Find pattern, return indices |
| `string.gsub(s, pat, rep)` | Global substitution |
| `string.gmatch(s, pat)` | Iterator over pattern matches |

### Pattern Characters

| Command | Description |
|---------|-------------|
| `.` | Any character |
| `%a / %A` | Letters / non-letters |
| `%d / %D` | Digits / non-digits |
| `%w / %W` | Alphanumeric / non-alphanumeric |
| `%s / %S` | Whitespace / non-whitespace |
| `%p` | Punctuation |
| `* + - ?` | Greedy, greedy, lazy, optional |

## Metatables

### Setting Metatables

```
local mt = {}
mt.__add = function(a, b)
  return {val = a.val + b.val}
end
local a = setmetatable({val=1}, mt)
local b = setmetatable({val=2}, mt)
local c = a + b  -- c.val == 3
```

### Common Metamethods

| Command | Description |
|---------|-------------|
| `__index` | Lookup missing keys (table or function) |
| `__newindex` | Intercept new key assignment |
| `__add / __sub / __mul` | Arithmetic operators |
| `__eq / __lt / __le` | Comparison operators |
| `__tostring` | Custom string representation |
| `__len` | Custom # operator |
| `__call` | Call table as function |
| `__concat` | Custom .. operator |

### OOP with Metatables

```
local Dog = {}; Dog.__index = Dog
function Dog.new(name)
  return setmetatable({name=name}, Dog)
end
function Dog:bark() print(self.name.." says Woof") end
local d = Dog.new("Rex"); d:bark()
```

## Coroutines

### Coroutine Lifecycle

| Command | Description |
|---------|-------------|
| `coroutine.create(f)` | Create coroutine from function |
| `coroutine.resume(co, ...)` | Start or continue coroutine |
| `coroutine.yield(...)` | Suspend execution, return values |
| `coroutine.status(co)` | "running", "suspended", "dead" |
| `coroutine.wrap(f)` | Create callable coroutine wrapper |

### Coroutine Example

```
local function gen(max)
  for i = 1, max do coroutine.yield(i) end
end
local co = coroutine.wrap(gen)
print(co(5))  -- 1
print(co())   -- 2
```

## Modules

### Creating a Module

```
-- mylib.lua
local M = {}
function M.greet(name)
  return "Hello, " .. name
end
return M
```

### Using Modules

```
local mylib = require("mylib")
print(mylib.greet("World"))
```

### Standard Libraries

| Command | Description |
|---------|-------------|
| `math` | Math functions (sin, random, huge, etc.) |
| `string` | String manipulation and patterns |
| `table` | Table manipulation (insert, sort, etc.) |
| `io` | File I/O operations |
| `os` | OS facilities (time, clock, execute) |
| `debug` | Debug interface (use sparingly) |

## Common Patterns

### Ternary Idiom

```
-- Lua has no ternary; use and/or idiom
local val = condition and "yes" or "no"
-- Caution: fails if "yes" is false/nil
```

### Safe Table Access

```
local function get(t, ...)
  for _, k in ipairs({...}) do
    if type(t) ~= "table" then return nil end
    t = t[k]
  end
  return t
end
get(config, "db", "host")  -- safe nested access
```

### Iterating with ipairs vs pairs

```
-- ipairs: array part, stops at first nil
for i, v in ipairs(arr) do print(i, v) end
-- pairs: all keys (unordered)
for k, v in pairs(tbl) do print(k, v) end
```