Lua

Lua is a lightweight, high-level programming language from 1993 designed primarily for embedded use in applications. Lua is cross-platform, since the interpreter of compiled bytecode is written in ANSI C and Lua has a relatively simple C API to embed it into applications.

Important:

In Lua, the indices of strings and tables start at 1 and not at 0! Furthermore, table is the only collection type, there are no lists or arrays.

function factorial(n)
  local x = 1
  for i = 2, n do
    x = x * i
  end
  return x
end

Data Types in Lua

Primitive Types

• strings s = 'hello'
• integer d = 42

Collections: Table

Tables are the most important data structures and, by design, the only collection type. It can contain any type.

my_table = {1, 'hi', 3.14, 1, 2, {4, 5} }

• length of the table: len = #my_table (is 6)
• access element: hi = my_table[2]
• subset of the table: hipi = {table.unpack(my_table, 2, 3)}

Warning:

The # symbol returns the value of the last item if that is an integer! If the last item is not an integer, it returns the number of items.

To iterate over a table:

for key, val in pairs(my_table) do
  print(key, ": ", val)
end

Code Examples

Working with Strings in Lua

text = "My name is %s and I am %s years old."
text:format('Alice', 42)
text = "Hi! "..text   -- .. is concatenation
print(text)

Useful Helpers

local function starts_with(text, start)
  if type(text) ~= "string" then return end
  if (type(start) == "table") then
    for key, val in pairs(start) do
      if starts_with(text, val) then return true end
    end
  elseif (type(start) == "string") then
    return text:sub(1, #start) == start
  end
end

Working with Files

function file_exists(name)
   local f=io.open(name,"r")
   if f~=nil then io.close(f) return true else return false end
end

local function write_file(fname, text)
  local f = io.open(fname, 'w')
  f:write(text)
  f:close()
end

Executing Programs

os.execute('pdf2svg tikz.pdf ' .. outfile)

Object Oriented Programming

local Vector = {}
Vector.__index = Vector

function Vector:new(x, y, z)    -- The constructor
  -- Since the function definition uses a colon,
  -- its first argument is "self" which refers
  -- to "Vector"
  return setmetatable({x = x, y = y, z = z}, self)
end

function Vector:magnitude()     -- Another method
  -- Reference the implicit object using self
  return math.sqrt(self.x^2 + self.y^2 + self.z^2)
end