Lua

Comments

-- Two dashes start a one-line comment.

--[[
     Adding two ['s and ]'s makes it a
     multi-line comment.
--]]

Variables

Numbers

num = 42 -- All numbers are doubles.

Strings

s = 'walternate' -- Immutable strings like Python. t = "double-quotes are also fine" u = [[ Double brackets

      start and end
      multi-line strings.]]

Empty / Null

t = nil -- Undefines t; Lua has garbage collection.

-- If clauses: if num > 40 then

 print('over 40')

elseif s ~= 'walternate' then -- ~= is not equals.

 -- Equality check is == like Python; ok for strs.
 io.write('not over 40\n')  -- Defaults to stdout.

else

 -- Variables are global by default.
 thisIsGlobal = 5  -- Camel case is common.

 -- How to make a variable local:
 local line = io.read()  -- Reads next stdin line.

 -- String concatenation uses the .. operator:
 print('Winter is coming, ' .. line)

end

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

aBoolValue = false

-- Only nil and false are falsy; 0 and are true! if not aBoolValue then print('twas false') end

-- 'or' and 'and' are short-circuited. -- This is similar to the a?b:c operator in C/js: ans = aBoolValue and 'yes' or 'no' --> 'no'

Loops

While

-- Blocks are denoted with keywords like do/end: while num < 50 do

 num = num + 1  -- No ++ or += type operators.

end

For

karlSum = 0 for i = 1, 100 do -- The range includes both ends.

 karlSum = karlSum + i

end

-- Use "100, 1, -1" as the range to count down: fredSum = 0 for j = 100, 1, -1 do fredSum = fredSum + j end

-- In general, the range is begin, end[, step].

Repeat / Do While

-- Another loop construct: repeat

 print('the way of the future')
 num = num - 1

until num == 0

Functions

function fib(n)

 if n < 2 then return 1 end
 return fib(n - 2) + fib(n - 1)

end

-- Closures and anonymous functions are ok: function adder(x)

 -- The returned function is created when adder is
 -- called, and remembers the value of x:
 return function (y) return x + y end

end a1 = adder(9) a2 = adder(36) print(a1(16)) --> 25 print(a2(64)) --> 100

-- Returns, func calls, and assignments all work -- with lists that may be mismatched in length. -- Unmatched receivers are nil; -- unmatched senders are discarded.

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

function bar(a, b, c)

 print(a, b, c)
 return 4, 8, 15, 16, 23, 42

end

x, y = bar('zaphod') --> prints "zaphod nil nil" -- Now x = 4, y = 8, values 15..42 are discarded.

-- Functions are first-class, may be local/global. -- These are the same: function f(x) return x * x end f = function (x) return x * x end

-- And so are these: local function g(x) return math.sin(x) end local g; g = function (x) return math.sin(x) end -- the 'local g' decl makes g-self-references ok.

-- Trig funcs work in radians, by the way.

-- Calls with one string param don't need parens: print 'hello' -- Works fine.