Category Archives: Tutorials

Extend string.match to test and capture patterns

Pattern matching is a powerful feature of Lua’s standard string library. I use it often to automate text file conversion and reporting. For instance, I use pattern matching in a Lua script to format and email my weekly report.

I have maintained a weekly diary of projects I’ve worked on, accomplishments, meetings attended, plans, business travel, and other information since 1988. Each week is logged in a simple text file. This means that despite the evolution of text editors and file media over the decades, I can still read my original diary files. The Lua script that prepares my weekly report scans my latest diary file and uses pattern matches to extract and format an email that adheres to the current reporting standards where I work.

The string.match function is useful for both testing if a pattern exists in a string and for extracting substrings that match a pattern enclosed in parentheses. Lua calls these substrings “captures”. Often I want to do both simultaneously–test for a pattern in an if statement and capture substrings. For instance, in my weekly report generator, I have a bit of code:

if line:match("^Weekly Report.+(%d%d)/(%d%d)/(%d%d%d%d)") then
    local month = _1
    local day   = _2
    local year  = _3
    email:setSubject(("John Powers - %s-%s-%s Weekly Report"):format(year, month, day))

The first line checks to see if variable line starts with the text “Weekly Report” and contains a date. If it matches, as a side effect it also sets global variables _1, _2, and _3 to the captures, i.e. the month, day, and year captured from string line. The built-in definition of string.match does not have this side effect. But we can extend string.match to gain this new capability.

Here is the code I used to modify string.match.

do
    local smatch = string.match     -- keep the original definition of string.match
 
    -- String matching function
    -- Same results as string:match but as a side effect
    -- places the captures in global variables _1, _2, ...
    function string:match(pat)
        local matches = {smatch(self, pat)}    -- call the original match to do the work
        for i = 1, #matches do                 -- #matches == 0 if no matches
            _G["_" .. i] = matches[i]          -- assign captures to global variables
        end
        return unpack(matches)                 -- return original results
    end
end

Note the use of a do … end block. This creates a block that limits the scope of local variable smatch. Only the new function string.match can call it.

Placing captures into global variables is nothing new. Anyone familiar with the AWK, Perl, and Ruby scripting languages will recognize this feature right away.

A new, smarter way to create a Screen Manager

It hasn’t been a long time since Jim Bauwens surprised us with an elegant way to customize the gc object with your own functions. But he’s striking again.

Indeed, he came up with a new, smarter way to create a screen manager.
If you’re not familiar with this great concept, you should head over here.
For those who are, though, you should know that it’s all about thoroughly listing and linking all the events you’ll have defined later in each screen implementation.
For example :

function on.arrowKey(key) activeScreen:arrowKey(key) end
function on.timer() activeScreen:timer() end
 
-- long list of all handled events...       Then, somewhere else :
 
function myScreen:arrowKey(key)
    -- Your awesome code goes here
end
function myScreen:timer()
    print("I'm the myScreen timer, ticking....")
end
 
-- etc.

It sure works, but …. quite boring, eh ?

Well, let’s look at what Jim created.
I define (t)his new screen manager concept as “Smarter” because with this code, you won’t even have to directly (explicitly) rely on the traditional event handling you’re used to, writing things like on.arrowKey, on.enterKey… or the good old on.paint.
That’s right, with this new method : no more “function on.paint(gc) …” etc.

“What’s this sorcery about ?” , you may wonder ?
Well, once more, it’s all about intelligently using the power of Lua metatables.
In short, metatables are sets of properties defining an object (generally a table) ‘s behaviour.
There is a “__index” property that you can define, that will describe how the table will react when the script calls an undefined element of that table. Pretty useful, believe me.
Well, the thing is that when you write “function on.paint(gc)”, you’re actually defining the “paint” method of the “on” table (thus the dot).
What we want to do, is to get rid of the explicit definition and to “redirect” the paint event to whichever screen we want to.
So, we’re going to use an “eventDistributer” method that takes as arguments whatever its passed, with the use of the “…” (the event followed by its parameters, if any), and “passes” them to the screen we want (checking that the event actually exists (defined) for the screen :

local triggeredEvent = "paint"  -- first declaration, it will be overwritten anyway
local eventDistributer = function (...)
     local currentScreen = GetScreen()
     if currentScreen[triggeredEvent] then
         currentScreen[triggeredEvent](currentScreen, ...)
     end
end

This code should be rather clear to you now.

Now, what we have to do is to actually bind that function to the “on” ‘s metatable __index (notice the smart use of closures) :

local eventCatcher = {}
 
eventCatcher.__index = function (tbl, event)
    triggeredEvent = event
    return eventDistributer
end
 
setmetatable(on, eventCatcher)

That code tells the Lua script that whenever an event without an explicit “on.xxx” handler occurs, it will execute this function (which returns the function – that’s the closure I was talking about – that will take the args of the event and pass it through the eventDistributer thus actually calling the correct screen’s appropriate event handler). You might find all this confusing, but at some point you sould be able to figure it out 🙂

Anyway, here’s the full code for the screen manager and event redistributer.
(You’ll still need to create (and push) your screens as usual.)

local screens = {}
local screenLocation = {}
local currentScreen = 0
 
function RemoveScreen(screen)
    screen:removed()
    table.remove(screens, screenLocation[screen])
    screenLocation[screen] = nil
    currentScreen = #screens -- sets the current screen as the top one on the stack.
    if #screens<=0 then print("Uh oh. This shouldn't have happened ! You must have removed too many screens.") end
end
 
function PushScreen(screen)
    -- if already activated, remove it first (so that it will be on front later)
    if screenLocation[screen] then
        RemoveScreen(screen)
    end
 
    table.insert(screens, screen)
    screenLocation[screen] = #screens
 
    currentScreen = #screens
    screen:pushed()
end
 
function GetScreen()
    return screens[currentScreen] or RootScreen
end
 
Screen = class()
 
function Screen:init() end
 
function Screen:pushed() end
function Screen:removed() end
 
RootScreen = Screen() -- "fake", empty placeholder screen.
 
local eventCatcher = {}
local triggeredEvent = "paint"
 
local eventDistributer = function (...)
     local currentScreen = GetScreen()
     if currentScreen[triggeredEvent] then
         currentScreen[triggeredEvent](currentScreen, ...)
     end
end
 
eventCatcher.__index = function (tbl, event)
    triggeredEvent = event
    return eventDistributer
end
 
-- finally linking everything
setmetatable(on, eventCatcher)

I’m sure a lot of you readers love a working .tns example directly, so, I made one just for you : click here. It’s a simple example showing a few events, without a single on.xxxx event handler explicitely defined, inside 2 screens. I’ve commented the code quite well so you’d understand quickly, I’m sure 🙂

For a more “real-life” example fully demonstrating this technique (as well as the gc-customizing trick), I suggest downloading Jim’s Memory game here.

 

How to add your own functions to “gc”

Hi all,

As you know, in order to do graphical things in Lua on the Nspire platform, you have to deal with gc and its methods TI created, like drawString, drawRect, fillArc, etc.

Well, what if you wanted to make a drawRoundRect routine ?

You could certainely do something like :

function drawRoundRect(gc, x, y, wd, ht, rd)
        if rd > ht/2 then rd = ht/2 end
        gc:drawLine(x + rd, y, x + wd - (rd), y)
        gc:drawArc(x + wd - (rd*2), y + ht - (rd*2), rd*2, rd*2, 270, 90)
        gc:drawLine(x + wd, y + rd, x + wd, y + ht - (rd))
        gc:drawArc(x + wd - (rd*2), y, rd*2, rd*2,0,90)
        gc:drawLine(x + wd - (rd), y + ht, x + rd, y + ht)
        gc:drawArc(x, y, rd*2, rd*2, 90, 90)
        gc:drawLine(x, y + ht - (rd), x, y + rd)
        gc:drawArc(x, y + ht - (rd*2), rd*2, rd*2, 180, 90)
end
 
function on.paint(gc)
        drawRoundRect(gc, 100, 50, 20, 15, 5)
        ...
end

Indeed, that works.

But wouldn’t it be cool to actually have it like, “gc:drawRoundRect(100,50,20,15,5)”, so it can feel way more natural and wouldn’t need you to explicitely pass gc as an argument ? 😉
Well, here is the definitive solution to this, by Jim Bauwens 😉

function AddToGC(key, func)
        local gcMetatable = platform.withGC(getmetatable)
        gcMetatable[key] = func
end
 
local function drawRoundRect(gc, x, y, wd, ht, rd)
        -- the code above
end
 
AddToGC("drawRoundRect", drawRoundRect)
 
function on.paint(gc)
        gc:drawRoundRect(100, 50, 20, 15, 5)
        ...
end

One more thing :

You may have noticed the use platform.withGC, which is an API “2.0”+ (OS 3.2+) function. Here’s how to “recreate” it for earlier versions :

if not platform.withGC then
        platform.withGC = function(func, ...)
            local gc = platform.gc()
            gc:begin()
            func(..., gc)
            gc:finish()
        end
    end

[Update] : John Powers from TI commented that this definition of platform.withGC has some limitations, and proposed this better version (thanks !) :

if not platform.withGC then
    function platform.withGC(f)
        local gc = platform.gc()
        gc:begin()
        local result = {f(gc)}
        gc:finish()
        return unpack(result)
    end
end

A-to-Z Nspire Lua tutorials

This page links to Steve Arnold’s website where you can find excellent detailed, step-by-step tutorials.

However, they are all in English. Inspired-Lua is making its best to translate them into French too.
The french version so far is available here.

 





The lessons in this next section require OS 3.2 functionality.




How to make a .tns from a .lua file ?

< Back to Part 1

You have two options:

1) Use the Nspire computer software

Since version 3.2 (mid 2012), the Nspire software (“TINCS”) includes a script editor to directly create, edit, test, and debug your Lua code from within a TI-Nspire document 🙂

It is available from the Insert menu > Script Editor .

2) Use “Luna”

Luna is an open-source community tool, created by Olivier “ExtendeD” Armand, that can create .tns files (TI-Nspire) out of a .lua file, for instance.

You can download a Windows executable on TI-Planet.
For other platforms, you can compile the source code, available at the same link.

To use it, just write in a terminal / command prompt, the path to the Lua file followed by the path of .tns you want to create:

1
luna.exe myscript.lua mydocument.tns

Let’s go to Part 3 now!

Create the is() operator to test an object type

On TI-Nspire, Lua lets you work easily with classes with class() function, that is also used to create inherited classes, based on the mother class. Sadly, since this class concept is purely superficial, there is neither function nor syntax to work with specific kind of classes. In this tutorial, we’re going to see how to reproduce the “is” feature that we meet in C# for example.

The is() operator lets you distinguish between an object created with one class and another object created with another class. We are going to create the is() operator specific to Lua because it doesn’t support inherited classes tests.

Here is an example of what we are trying to do :

function is(obj, class)
  -- we are going to create it
end
 
A = class()
function A:init(x)
  self.x = x
end
 
B = class(A)
function B:init(x)
  self.x = x * 2
end
 
t = { A(1),  B(1) }
for k, v in pairs(t) do
  if is(v, A) then
    print("A class, value = "..v.x)
  elseif is(v, B) then
    print("B class, value = "..(v.x / 2))
  end
end

The goal is here obvious : if we store in a table objects from different classes we some common fields, we d’like to be able to distinguish objects from a particular class.

But how write such a function in that case ? Add a specific field ? No need !
Indeed, the class() function does half of the work. It stores something in the __index field of your object (in reality a table). This one contains a reference to the mother class. When you do a = A(), a.__index is equal to A.__index (by reference) !

As simple as you guessed :

function is(obj, class)
  return obj.__index == class.__index
end

NB : Since the edge between classes and object is not so far big in Lua (contrary to other OOP langages where an object is created by a class), it is possible to test whether or not two objects have the same mother class with one test less :

A = class()
function A:init() end
a = A()
b = A()
is(a, b) -- returns true

 

Call the master class constructor

You are obviously a lot that enjoyed the easy-go of Lua classes on TI-Nspire, but also noticed that it is too simple to let you work correctly on a specific case. Let’s explain.

This case is the ability to call the master class constructor in the inherited class in order to avoid repetitions. Indeed, if you change one field in the master class, you must change that field in every inherited classes.

Here is an example :

A = class()
function A:init(x)
  self.t = 0
  self.x = x
end
 
B = class(A)
function B:init(x, y)
  self.t = 0
  self.x = x
  self.y = y
end

Here, if we would like to change t = 0 from the master class constructor, we also have to change it in the B class constructor in order to be coherent. Ones could think like that :

A = class()
function A:init(x)
  self.t = 0
  self.x = x
end
 
B = class(A(0, 0)) -- construction of the object before inheriting
function B:init(x, y)
  self.x = x
  self.y = y
end

But here again, the field x is repeated.

One thing we forgot to take care here is that B is an inherited class of A, so, each field of A are obviously in B. But for the constructor, we’re just redefining it. Is there a way to call the previous constructor from A giving B as parameter ?

Actually, yes !

Something important to know in order to understand the solution, is that this :

dummy = class()
function dummy:doSomething()
  self.mark = 0
end
dummy:doSomething()

is exactly the same of that :

dummy = class()
function dummy.doSomething(self)
  self.mark = 0
end
dummy.doSomething(dummy)

or again :

dummy = class()
function dummy:doSomething()
  self.mark = 0
end
dummy.doSomething(dummy)

The colons allow to give as first parameter the object that has this method. So nothing prevents us to not use this tip and then give to the method a manual first parameter as below !

A = class()
function A:init(x)
  self.t = 0
  self.x = x
end
 
B = class(A)
function B:init(x, y)
  A.init(self, x) -- call the A class constructor in order to avoir repetitions
  self.y = y
end