From: Xiong Chiamiov <xiong.chiamiov+ruby_forum@...>
Date: 2008-09-12T06:12:56+09:00
Subject: Re: pass by reference in each loop

Pedro Silva wrote:
> Ruby doesn't pass arguments by reference it passes them by value. What 
> is passed to functions/blocks is the reference value.
Although that sounds exactly like passing by reference (doesn't it?), 
I've come to understand today the nuance of how Ruby acts differently 
(at least, I think).

In, say, C (ugh!), if I have some variable x and pass it to a function 
by reference, it modifies the original x.  If I pass by copy instead, it 
copies it and modifies the copy, but not the original.
Ruby, however, (correct me if I'm wrong!) passes (essentially) by 
reference.  However, the difference is in how it treats the value 
passed.  When you modify it, instead of modifying the object that the 
reference points to, you modify the reference itself to point to 
something else.

This brings me back to my original question.  While map! looked exactly 
right, it modifies where the references in the array point to, rather 
than modifying the values they pointed to, which means that my original 
variables are not modified.

Robert Klemme wrote:
>@question, @answer_explanation =
>   [@question, @answer_explanation].map {|x| f(x)}
which *does* work, as it assigns the values back to the variables, but 
it defeats the purpose of the thing.

Perhaps I should explain a little bit better why I'm going about this:

I'm building a lightweight parsing language.  The function wml() parses 
this.  I have a lot of user input, some of which I want to parse, but 
some I don't.

So then, I wanted to avoid writing:
@a = wml(@a)
@b = wml(@b)
@c = wml(@c)
...
because the list could potentially be quite long, and I'm the good kind 
of lazy (the one referred to in the Camel book).

Robert's solution would allow me to only write the function name once, 
but the variables I'd still have to type twice, and I really like DRY.

My first thought was to store a list of all those variables in an array 
and loop through them, as it's then very easy to add an additional 
variable.  Perhaps I'm approaching this the complete wrong way, though?
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