Admin: Admin moved page Lua basics & NETIO Lua from original Lua difference to Lua basics for other language programmers

2019-07-11T18:39:04Z

Bbakala: Created page with " == Data types and variables == Lua supports these data types: * nil (to assign nil value effectively deletes the variable) * boolean (falsy values are
2019-07-11T16:58:41Z

Created page with " == Data types and variables == Lua supports these data types: * nil (to assign nil value effectively deletes the variable) * boolean (falsy values are <i..."

New page

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

== Conditions ==
Lua conditions has casual syntax, see [https://www.lua.org/pil/4.3.1.html here]. Just keep in mind to merge <code>else</code> and <code>if</code> to <code>elseif</code> or multiple <code>end</code>s 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 <code>break</code> statement, but no <code>continue</code> (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
<code>for initVar,limit,increment do</code>
* number assignment <code>initVar</code> inits loop-local variable
* number <code>limit</code> loops until initVar reaches this value
* optional number <code>increment</code> after each loop initVar increment by this value (default 1)

=== Generic for ===
Is rather sophisticated. Most common example (the order of elements in <code>pairs()</code> 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 [https://www.lua.org/pil/7.2.html 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 <code>ipairs()</code> 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 <code>pairs()</code> Lua function and read about <code>pairs()</code> and <code>ipairs()</code> implementation [https://www.lua.org/pil/7.3.html 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))

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