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From: Chuck Brotman on 21 Jun 2010 19:15
Robert,

Thanks again for your (second) response. I will probably print it out
and study it. I appreciate you taking your time to explain and provide
examples

And yes, it occurred to me (last night) that I should work to hide the
implementation... but first I want to have an implementation to hide ;)


Also a big thank you to Gary Wright Whose reply reached me by email, but
I haven't seen in the forum. He also gave me a lengthy reply, replete
with sound advice and examples!

Thanks, I'll be reviewing both responses until I get a handle on this

Best regards,

Chuck
--
Posted via http://www.ruby-forum.com/.

From: Chuck Brotman on 25 Jun 2010 15:55
Robert,

In looking over the code samples you provided:
------------
def add_edge(from, to)
(@nodes[from] ||= []) << to
self
end

def edges(node_id)
@nodes[node_id] || []
end

def node(node_id)
Node.new(self, node_id)
end
end
------------
I find I am a puzzled:
Even given that to, from are fixnums:
1) what does "@nodes[node_id] || []" return?
2) same question for: "@nodes[from] ||= []) << to"
I could find no reference to this " ||= " syntax (in Pickax, or
other ruby texts, nor online Nor do I understand it's semantics...

3) how about Node.new(self, node_id) I have trouble with the use of
"self" here, the whole usage of "self" has caused me confusion, ever
since I started playing with OO (back when java was IT!)

Thanks,
Chuck

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From: Robert Klemme on 26 Jun 2010 15:40
On 06/25/2010 09:55 PM, Chuck Brotman wrote:
> Robert,
>
> In looking over the code samples you provided:
> ------------
> def add_edge(from, to)
> (@nodes[from] ||= []) << to
> self
> end
>
> def edges(node_id)
> @nodes[node_id] || []
> end
>
> def node(node_id)
> Node.new(self, node_id)
> end
> end
> ------------
> I find I am a puzzled:
> Even given that to, from are fixnums:
> 1) what does "@nodes[node_id] || []" return?

It returns @nodes[node_id] if that is not nil and not false - otherwise
you get a new empty Array (from [] which is just special syntax for
Array.new). This is done so the caller can rely on an Array being
returned from this method under all circumstances so you can do

gr.edges(17).each do |node|
puts "17 -> #{node}"
end

and do not have to verify that the result of #edges is non nil.

> 2) same question for: "@nodes[from] ||= []) << to"
> I could find no reference to this " ||= " syntax (in Pickax, or
> other ruby texts, nor online Nor do I understand it's semantics...

I don't have my Pickaxe here right now but I would be surprised to find
||= not explained. There also were some threads about operator ||=
recently. It boils down to

a ||= b

being short for

a || (a = b)

In this case

@nodes[from] || (@nodes[from] = [])

In prose: set @nodes[from] to an empty Array if unset.

The "<< to" at the end simply concatenates to the Array returned from
the expression in brackets.

> 3) how about Node.new(self, node_id) I have trouble with the use of
> "self" here, the whole usage of "self" has caused me confusion, ever
> since I started playing with OO (back when java was IT!)

In a Ruby program at any point in time "self" points to an object. You
cannot assign to "self". "self" is implicitly set when a method call
with an explicit receiver is executed. If you execute "a.foo" then
"self" is set to "a" while inside the method.

Maybe you have a look at Chris Pine's learning to program. A general
advice: if you are unsure about expressions you find in programs or
postings here you can pretty easily try them out in IRB and see what
they do.

Kind regards

robert


--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/
From: Chuck Brotman on 27 Jun 2010 10:00
Robert

The more I learn about Ruby, the more impressed I am! (though it takes a
fair amount of work to try and get a handle on it all)

I wrote you a line by line response to your latest reply, but it appears
that it got swallowed by "the system" and hasn't shown up here yet! In
any case, I'll be away for a a week or two and will reestablish activity
in this thread when I return -- or can find a pc to correspond from. :)

TTY then...

Thanks again for you help!

Chuck

--
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