Lua basics for other language programmers

Data types and variables

Lua supports these data types:

• nil (to assign nil value effectively deletes the variable)
• boolean (falsy values are nil and false only; "" and 0 evaluates as true)
• number (decimals only in standard Lua, integers only in Netio Lua)
• string (same strings share same address, so equality operator measures both address and value)
• tables are associative arrays. (Tables with numeric indices are, well, just arrays.)

local numbers = {2,3,7,5} -- array (first index = 1)
local dictionary = {one=1, two=2, three=3} -- table
local numbersZeroBased = {[0]=2,3,7,5} -- array (first index = 0)

local keyword limits the variable scope to the block (ended by end) where it is declared. To access an upvalue (variable defined outside of the scope), just omit the local keyword.

Operators

Lua has its own manners:

• ~= is inequality operator
• .. is string concatenation (don't use plus)
• i++ no increment/decrement operator i = i + 1
• max = a<b ? b : a no ternary operator. Use (and get used to) and idiom instead: max = (a<b) and b or a
• or idiom works: x = x or 42 is equivalent to if not x then x = 42 end

Conditions

Lua conditions has casual syntax, see here. Just keep in mind to merge else and if to elseif or multiple ends are required.

Note that falsy values are nil and false only. Empty string and 0 evaluates as true. If you come from C, you might be surprised by following snippet:

if devices.system.output1_consumption then
  -- always happens, even if the consumption is 0
  log("Outlet 1 supplies power")
end

if not devices.system.output1_consumption then
  -- never happens, even if the consumption is 0
  log("Outlet 1 is idle")
end

Loops

Lua has break statement, but no continue (use condition inside loop instead). Additionally, Netio Lua implementation limits the loop iterations to 32k.

While

While syntax is not different to other languages:

local i = 0
while i~=3 do
  i = i + 1
  logf("%d",i)
end

Repeat until

Repeat until syntax is the same as Pascal's, other languages have similar do-while loops:

local i = 0
repeat
  i = i + 1
  logf("%d",i)
until i==3

Numeric for

Arrays in Lua are one-based (in indices are not specified, the first one is 1, not 0).

local arr = {2,3,7,5}
for i=1,#arr do logf("%d",arr[i]) end

for initVar,limit,increment do

• number assignment initVar inits loop-local variable
• number limit loops until initVar reaches this value
• optional number increment after each loop initVar increment by this value (default 1)

Generic for

Is rather sophisticated. Most common example (the order of elements in pairs() is not guaranteed):

local tab = {one=1, two=2, three=3}
for key,val in pairs(tab) do logf("%s:%d",key,val) end

Generic for syntax

for var_1, ..., var_n in explist do block end

is equivalent to (Full explanation here.)

do
  local _f, _s, _var = explist
  while true do
    local var_1, ... , var_n = _f(_s, _var)
    _var = var_1
    if _var == nil then break end
    block
  end
end

Iterator closure that holds its state

function iter(a)
  local i = 0
  return function()
    i = i+1
    return a[i]
  end
end

local arr = {2,3,7,5}
for value in iter(arr) do
  logf("%d",value)
end

Stateless iterator (in this case returns variable list: key and value)

function iter(a,i)
  i = i+1
  if a[i] then return i,a[i] end
end

local arr = {2,3,7,5}
for k,v in iter,arr,0 do
  logf("%d:%d",k,v)
end

Same effect using ipairs() Lua function (without initial state)

for k,v in ipairs(arr) do
  logf("%d:%d",k,v)
end

Now return to the most common example above using pairs() Lua function and read about pairs() and ipairs() implementation here.

Functions

Lua allows multiple results (comma separated):

function diskSpace()
  return devices.system.freeSpace, devices.system.totalSpace
end

logf("Free space: %d MB", diskSpace()) -- only first return value is used
local free,total = diskSpace() -- values are stored into LHS var-list
logf("Free space: %d %%", 100*free/total)

Variable-length arguments are also available:

function sum(...)
  local result = 0
  for _,v in ipairs(arg) do
    result = result + v
  end
  return result
end

logf("Sum: %d", sum(2,3,5))