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.

< 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!

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

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