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Scheme empty list help

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Scheme empty list help

 
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  #1
Feb 10th, 2008
I'm working on an implementation of Conway's Game of Life as a way to learn Scheme, and I'm having trouble with the updater. I keep getting the message "The object () is not applicable" when I try to use the following. I could be missing something simple, and any advice on my code is appreciated:

;;bitset and bitclear are just bit-string-set! and bit-string-clear! renamed
(define (update graph u d)
   (define (upgraph lst)
     (if (null? lst) lst
        (begin
         (bitset graph (car lst))
         (upgraph graph cdr lst)
         )   
        )   
     )     
 
   (define (downgraph lst)
     (if (null? lst) lst
       (begin
        (bitclear graph (car lst))
        (downgraph graph (cdr lst))
        )    
       )    
     )     
   (upgraph u)
   (downgraph d)
)

Thanks,
Cory
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Re: Scheme empty list help

 
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  #2
Feb 13th, 2008
That shouldn't work at all. Both upgraph and downgraph are defined to take a single argument, but you are passing multiple...

That could be the problem. Make sure also that you aren't calling bitset with '().

Sorry I can't be of more help.
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Re: Scheme empty list help

 
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  #3
Feb 13th, 2008
Ah, yes... I believe the word is "embarrassing"...
There are time when I feel rather blind and stupid. This is one of them.

I don't think I'd be calling bitset with '(), as a null list input would just return a null list. But that assumes I allowed it to take another argument....

Alright still having problems...

(define (update graph u d)
  (define (upgraph lst)
    (if (null? lst) lst
       (begin
	(bit-set graph (car lst))
	(upgraph (cdr lst))
	)
       )
    )

  (define (downgraph lst)
    (if (null? lst) lst
      (begin
       (bit-clear graph (car lst))
       (downgraph (cdr lst))
       )
      )
    )
  (upgraph u)
  (downgraph d)
  )
Last edited by cknapp; Feb 13th, 2008 at 9:36 pm.
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Re: Scheme empty list help

 
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  #4
Feb 14th, 2008
The same problem?

I see nothing wrong with your code... (I can't test it myself ATM, as I'm just recovering from a system wipe and have yet to reinstall scheme.)

Do you think it might be barfing because upgraph evaluates to '() ?
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Re: Scheme empty list help

 
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  #5
Feb 14th, 2008
Yes, same problem.

I was thinking that may be the case... but if that's so, I'd really love to know why. It wouldn't make any sense: the empty list is just like any other piece of data.

I was also thinking it may have something to do with the (begin) statement. I'd elaborate more on this, but I've spent the last 7 hours learning LaTeX and using it to writing a lab report that's due tomorrow, and my brain is fried... Sleep time; I will return to this... probably Friday, as I have a lot to get done tomorrow.
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Re: Scheme empty list help

 
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  #6
Feb 14th, 2008
A begin is implied in the body of a lambda.

The problem is that you have to be extra careful to keep data and code separate in scheme. You are mixing the two.

For example, the following is not a valid program:
(1 2 3 4)

but it is valid data:
'(1 2 3 4)

Essentially, your update function body at some point evaluates to
('() '())
...which is not a valid program.

Again, I have yet to re-install PLT so I can't be sure (it has been a few years since I've used scheme much), but you might fix it by simply changing your letrecs to use the following if inside:
(if (not (null? lst))

That way, the result is nothing, rather than an empty list.

Hope this helps.
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Re: Scheme empty list help

 
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  #7
Feb 20th, 2008
Sorry, I'm not fully following you. I understand what you're saying about ('() '()) being unintelligible for the interpreter, but how do I rectify this?

Thanks again.
Last edited by cknapp; Feb 20th, 2008 at 9:04 pm.
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Re: Scheme empty list help

 
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  #8
Feb 21st, 2008
You are getting lost on syntactic sugar.
(define (foo x y z) (+ (- x y) z))
is sugar for:
(define foo (lambda (x y z) (+ (- x y) z)))

Likewise, inside the body of a lambda, using
(define (bar q) (* q (+ q 1))) ...
is sugar for:
(letrec ((bar (lambda (q) (* q (+ q 1))))) ...)

What I suggest is replacing the lines:
(if (null? lst) lst
with:
(if (not (null? lst))

and see if that works. (I'm not sure if it will or not...)
Last edited by Duoas; Feb 21st, 2008 at 12:20 am.
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Re: Scheme empty list help

 
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  #9
Feb 21st, 2008
Rock!

Syntactic sugar causes cancer of the semi-colon.

I actually had that at one point, but my function call was embarrassing, so I had changed it, hoping it had something to do with my error.

Anyway, yeah... Now I need to remember to filter through the syntactic sugar when I'm stuck.

Thanks again,
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