From: "Ara.T.Howard" <ahoward@...>
Date: 2004-04-14T07:54:14+09:00
Subject: Re: New Ruby questions...

On Tue, 13 Apr 2004, Jeff Massung wrote:

> I've just started Ruby a couple days ago (man this is cool). Coming from the
> embedded world of Forth and C, being able to do some string parsing easily
> is what I'm looking forward to. So, onto a couple questions that I can't
> seem to find the answers to:

welcome aboard.


> Quickly, the "teach myself" project is a simple ARM assembler. I've done it
> numerous times, and it is something I'm familiar and comfortable with (let
> alone something I need ATM).
> 
> 1. How can a regexp get the longer of two possibilities that are ambiguous?
> For example, I need to be able to strip out register names:
> 
> registers = '(r0|r1|r2|r3...|r10|r11|r12...)'
> 
> In the regular expression, how can I get it to find r10 or r11 instead of
> stopping at r1? I get the same problem with opcode mnemonics (like B instead
> of BX or BL).

you need to make a pattern that says "a register string is an 'r' followed by
one, or more, digits".  pattern matching is greedy by default, so that alone
will do it


  irb(main):001:0> register_pat = /r\d+/o
  => /r\d+/

  irb(main):002:0> "r1"[register_pat]
  => "r1"

  irb(main):003:0> "r11"[register_pat]
  => "r11"

  irb(main):004:0> "r111"[register_pat]
  => "r111"

however, pattern matching is trickier than it looks:

  irb(main):005:0> "car_number1"[register_pat]
  => "r1"

you probably wan't to say something like: "a register is the string 'r'
followed by one, or more, digits, but when the whole thing is bounded by
non-word chars"

  irb(main):007:0> "r1"[pat]
  => "r1"

  irb(main):008:0> "carr1"[pat]
  => nil

in general, it's best to always anchor patterns somehow.



> 2. I don't seem to understand the @ convensions for variable names. Is this
> just a naming convension that people stick to (like *name* system names in
> Lisp) but aren't required?
> 
> I noticed that if I have a global variable:
> 
> x = 10
> 
> In some function:
> 
> def show_x print x end
> 
> This fails (unknown local x). But if I use @x in both cases it works just
> fine. Can someone explain this to me? Also, what about @@ variables?
> Likewise, is there @@@ or @@@@ ?


'@' means instance var - it's OO speak for __my__ var.  eg:

  ~ > cat foo.rb
  class Car
    attr :model
    def initialize model
      @model = model
    end
  end

  audi = Car.new 'audi'
  p audi.model

  jag = Car.new 'jaguar'
  p jag.model


  ~ > ruby foo.rb
  "audi"
  "jaguar"


'@@' means class var.  they could be thought of as 'static storage on a class
hierachy basis'.  eg:

  ~ > cat foo.rb
  class Widget 
    @@total_widgets = 0

    def Widget.n_widgets
      @@total_widgets
    end


    attr :name
    def initialize name 
      @name = name
      @@total_widgets += 1
    end
  end


  widgets = []
  42.times{|i| widgets << Widget.new(i) }

  p widgets.size
  p Widget.n_widgets

  ~ > ruby foo.rb
  42
  42


there are also, 'class instance vars', but that's a longer story...

there is no @@@+

> Thanks in advance.

no problem.  you'll find this (the pickaxe) most useful:

  http://www.pragmaticprogrammer.com/ruby/downloads/book.html

i use it, literally, every day.  i always use the electronic copy, but bought
three and gave them away to support the authors - good to support them too, i
hear they are working on a new release...

cheers.


-a
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