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.]]
-- String concatenation uses the .. operator:
message = 'Winter is coming, ' .. line

Empty / Null

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

Global Variables

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

Local Variables

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

If Statement

if num < 40 then
  print('below 40')
elseif name ~= 'wayne' then  
  -- ~= is not equals.
  -- Equality check is == like Python; ok for strs.
  io.write('over 40 and Name is wayne\n')  -- Defaults to stdout.
else
  print('above 40')
end

Conditions

Relational

Symbols	Explanation
==	Equal
~=	Not equal
<=	Less than or equal
>=	Greater than or equal
<	Less than
>	Greater than

Conditional

Operator	Description	Example
and	Called Logical AND operator. If both the operands are non zero then condition becomes true.	(A and B) is false
or	Called Logical OR Operator. If any of the two operands is non zero then condition becomes true.	(A or B) is true.
not	Called Logical NOT Operator. Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false.	!(A and B) is true.

-- 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