From: James Edward Gray II <james@...>
Date: 2005-12-09T00:16:22+09:00
Subject: Re: increasing counter whithin loop?

On Dec 8, 2005, at 6:41 AM, Steve Litt wrote:

> I just ran your code, and got the exact output you got, but I don't  
> understand
> a bit of it.

One interesting technique when you are struggling, is to ask Ruby  
your questions directly.  Let's try it!

> First, I'm unclear as to the use of variables starting with colon.  
> What are
> they, when are they used?

 >> :what_am_i.class
=> Symbol

Ruby says those are Symbols.  We can even ask what they can do:

 >> :what_do_i_do.methods - Object.new.methods
=> ["id2name", "to_sym", "to_i", "to_int"]

Wow, not much, huh?  We could then look up the class and methods in  
the documentation for more details.

Symbols are just immutable Strings in Ruby.  They generally have good  
performance (because they can't change) and can be easy on memory  
(because every time you ask for :obj, you get the exact same  
object).  They are never garbage collected though, so don't generate  
a huge number of them.

> Also, are prv, this and nxt just any old variables, or are they  
> somehow
> supplied by enumerator?

 >> (1..10).each_cons(4) do |one, two, three, four|
?>   p [one, two, three, four]
 >> end
[1, 2, 3, 4]
[2, 3, 4, 5]
[3, 4, 5, 6]
[4, 5, 6, 7]
[5, 6, 7, 8]
[6, 7, 8, 9]
[7, 8, 9, 10]
=> nil

Looks like just variables, eh?

each_cons() yields ranges of "cons"ecutive members.  In the example  
you mentioned, this is used to simulate next and previous objects.

> Is variable a a string, an array, or something else?

 >> a=[:a, :b, :b, :c, :b, :d, :e, :e]
=> [:a, :b, :b, :c, :b, :d, :e, :e]
 >> a.class
=> Array
 >> a.map { |e| e.class }
=> [Symbol, Symbol, Symbol, Symbol, Symbol, Symbol, Symbol, Symbol]

Ruby says it is an Array of Symbols.

> Why did you place an array containing only nil before and after a  
> in the
> parentheses?

 >> [nil] + a + [nil]
=> [nil, :a, :b, :b, :c, :b, :d, :e, :e, nil]

That just puts two new items in the Array.  Hmm, so if we combine  
that with the iterator:

 >> ([nil] + a + [nil]).each_cons(3) do |prv, this, nxt|
?>   p [prv, this, nxt]
 >> end
[nil, :a, :b]
[:a, :b, :b]
[:b, :b, :c]
[:b, :c, :b]
[:c, :b, :d]
[:b, :d, :e]
[:d, :e, :e]
[:e, :e, nil]
=> nil

Ah, now I see it.  It's used to add a nil object to prv and nxt at  
the ends of iteration.

> Thank you for this code. When I asked my question about how to stop  
> writing
> Ruby with a Perl accent, I was hoping to understand constructs like  
> this one.
> I have about 8 days experience with Ruby and so have some catching  
> up to do.

Welcome to Ruby!

James Edward Gray II