From: Natevw <natevw@...>
Date: 2006-05-16T08:45:10+09:00
Subject: Re: learn to program

corey konrad wrote:
> I realy dont understand what Chris is talking about here in his book 
> "learning to program" Can someoe shed some light on this
> 
> def little_pest tough_var
> tough_var = nil
> puts ' HAHA! I ruined your variable!'
> end
> 
> tough_var = ' You can\' t even touch my variable!'
> little_pest tough_var
> puts tough_var
> 
> output:
> HAHA! I ruined your variable!
> You can' t even touch my variable!
> 
> excerpt from his book:
> In fact, two variables in that little program are named tough_var:
> one inside little_pest and one outside of it. They don’t communicate.
> They aren’t related. They aren’t even friends. When we called
> little_pest tough_var, we really just passed the string from one
> tough_var to the other (via the method call, the only way they can even
> sort of communicate) so that both were pointing to the same string.
> Then little_pest pointed its own local tough_var to nil, but that
> did nothing to the tough_var variable outside the method.
> 

Looks like he's trying to get at the "scope" of variables.

I think that his example might be more clearly written with parentheses:

[code]
1: def little_pest(tough_var)
2:  tough_var = nil
3:  puts ' HAHA! I ruined your variable!'
4: end
5:
6: tough_var = ' You can\' t even touch my variable!'
7: little_pest(tough_var)
8: puts tough_var
[/code]

Pay attention to 'tough_var' through the code, it actually refers to two 
different variables:
1) in line 1, 'tough_var' is the name given to the one argument (~=input 
value) that the little_pest function takes. It is a local variable.
   in line 2, this local variable is set to nil.

2) in line 6, a global variable named 'tough_var' is declared and set to 
the "You can't touch..." line
    in line 7, little_pest is called with the value of the global tough_var


So when tough_var is set to nil in line 2, all that is being set to nil 
is the local variable named 'tough_var', not the variable from lines 6-8.


To the Ruby interpretter, the code might as well be written as:

1: def little_pest(var1)
2:  var1 = nil
3:  puts ' HAHA! I ruined your variable!'
4: end
5:
6: var2 = ' You can\' t even touch my variable!'
7: little_pest(var2)
8: puts var2



HTH...I'm just learning Ruby, so maybe somebody could correct the 
details (passing by value,etc.)....but that's what the author is getting at.



-natevw