From: Logan Capaldo <logancapaldo@...>
Date: 2006-05-16T16:57:01+09:00
Subject: Re: learn to program


On May 15, 2006, at 10:08 PM, Eric Armstrong wrote:

> That's a nice little visualization trick. Did you
> come up with that?
>
> Logan Capaldo wrote:
>> On May 15, 2006, at 6:05 PM, 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.
>>>
>>> --Posted via http://www.ruby-forum.com/.
>>>
>> It has to do with variable scoping. Try this metaphor on for size.
>> You have a stack of papers. When ever you call a function, you  
>> grab a blank piece of paper. On it, you write the names and values  
>> for all the variables in that function and you put it on top of  
>> the stack. You then execute the code inside the function. Any code  
>> in the function can only change the stuff written on the piece of  
>> paper at the top of the stack. When the function finishes, we take  
>> it's piece of paper off the top of the stack of papers and throw  
>> it away.
>> Ok so we have an initial piece of paper.
>> We write on it:
>> tough_var = ' You can\' t even touch my variable!'
>> then we call a function, little_pest. We grab a blank piece of  
>> paper for little pest, and write on it
>> tough_var = ' You can\'t even touch my variable!'
>> we put it on top of the stack.
>> Then little_pest says to set tough_var to nil.
>> We look at our piece of paper on the top of the stack of papers,  
>> scribble out tough_var and write
>> tough_var = nil
>> We then print the "HAHA!" message, and finish our function. Since  
>> we are done with our function, we take the piece paper off the top  
>> of the stack, crumple it up, and throw it away.
>> then we saee we must print tough_var. We look at the paaper on the  
>> top of the stack to see what tough var means. Oh its a string with  
>> ' You can\'t touch my variable '. Ok, we print that.
>

Well in this case yeah. But I imagine that's what the first guy who  
thought of a call stack was thinking of anyway.