Scheme Help

[nanaman]

Ok I have to write a program that creats an ADT to store a set of cards that are imput so that i can use the command (a-card rank suit) and (rank card) (suit card) to firstly construct the card and then return its rank and suit. Unfortunately I am struggling alot with this and wondering if someone could prod me in the correct direction.

(define suit (list 'clubs 'diamonds 'hearts 'spades))
(define rank (list 1 2 3 4 5 6 7 8 9 10 11 12 13))
(define (a-card rank suit)
          (list cons rank suit))

any and all prods greatly appreciated, as this is just giving me an error

[nanaman]

(define (make-card rank suit) (list rank suit))
(define (a-card rank suit) (make-card rank suit))

Using this I can get (a-card) to return the correct value, how do I then get this value to be passed out of the procedure (i.e. so I can recall it and also use it for (card) ?

Thanks

[azimuth0]

Try defining accessor functions that return particular elements of the list using car and cdr.

[Duoas]

There are no procedures in scheme, only functions. Whenever you say define, you are naming an expression that evaluates to something.

Hence, follow along:
(a-card 13 'spades) becomes (make-card 13 'spades) becomes (list 13 'spades) becomes '(13 'spades)
Themake-card and a-card functions do the same thing: take a rank and suit and return a card. Why not get rid of one of them?

I think you need to re-read your class notes about the difference between a list and a pair.

Personally, I would store the card as a pair:
(define make-card (lambda (rank suit) (cons rank suit)))
Or, using the shortcut syntax: (define (make-card rank suit) (cons rank suit))

Now, you know that a card is apair of '(rank . suit), so to get out the rank or suit you only need a new function get knows what a card looks like:
(define (rank card) (car card)) (define (suit card) (cdr card))

The purpose is to make it so that no one knows that a card is just a pair (or a list or something else). You know, because that is how you are storing it. But anyone else using your program only needs to know three functions: make-card rank suit : return a card object rank card : return the rank of a card object suit card : return the suit of a card object

Hope this helps.

[nanaman]

That is fantastic, however, the reason I was playing with a list was that I need to store 5 cards (a poker hand), would the best way be to make the pair and then store the pair in a list along with the other 5 then?

thanks again

[Duoas]

Yes. A card is a single object. If you want a list of cards, then store a list of cards, not a list of suits and ranks.

(define dead-mans-hand (list
  (make-card 13 'spades)
  (make-card 13 'clubs)
  (make-card 8 'spades)
  (make-card 8 'clubs)
  (make-card 2 'hearts)
  ))

Thereafter, you can get, say, the second card's suit with: (suit (second dead-mans-hand))

Hope this helps.

[nanaman]

(define c1 (a-card 7 'clubs))
(define c2 (a-card 8 'spades))
(define c3 (a-card 9 'hearts))
(define c4 (a-card 10 'clubs))
(define c5 (a-card 11 'diamonds))
(define (a-hand c1 c2 c3 c4 c5) (list c1 c2 c3 c4 c5))

The help so far has been fantastic, I now have the single cards working, and am working on constructing the hand, the a-hand function works fine, however the next thing is to impliment "(contents hand) - returns a list of card objects (as constructed by a-card in the hand."

Now i have no problem doing this as i could just simply put

(define (hand c1 c2 c3 c4 c5) (a-hand c1 c2 c3 c4 c5))
(define (contents hand) (list (car hand) (car (cdr hand)) (car (cdr (cdr hand))) (car (cdr (cdr (cdr hand)))) (cdr (cdr (cdr (cdr hand)))))

That the correct way to do it you think?

Thanks again for all the help

*Edit* just tried this way and it is giving me an error of "wrong type in arg1" >.<
*Edit* Just removed the (Define (hand c1 c2 .....) statement and simply type (contents (a-hand c1 c2 c3 c4 c5)) and it works fine :)

Just wondering though, how can i seperate the two things in a card in a hand? would it be

(suit (car hand)) etc?

[Duoas]

Since a "hand" object is a list of cards, and you want to get a list of cards from a hand, all you need to do is return the hand:
(define (contents hand) hand)
Now, if a "hand" object were something else, you'd need to do some work to turn it into a list...Just wondering though, how can i seperate the two things in a card in a hand? would it be
(suit (car hand))
etc?
Exactly. :)

[nanaman]

Is there anyway to sort a list of numbers (and words alphabetically) into order?

thanks

[Duoas]

Of course. You'll need to read up on recursion.

There is a basic principle: if you have a list
(a b c d)
(which, if you remember, is really: (a . (b . (c . (d . ()))))
and if you can do something to the car of the list (or a ) and the cdr of the list (that is, the rest of the list: (b c d) ),
then you can do something to the whole list.

Remember, your list is a list of 'card's. So you'll have to use the card ADT to get the rank of each card and compare its value.

Hope this gets you started. Using scheme (and other functional languages) require you to think pretty hard.

[Duoas]

Just as a random addendum. Above I said you could make your ADT as:
(define (rank card) (car card))
That's fine, but if you really want to wow, forget the fluff and just say: (define rank car)

Heh heh heh...

[Rashakil Fol]

There are no procedures in scheme, only functions.

There are nofunctions in Scheme, only procedures. That is, the word used in the R5RS specification (and probably R6RS, but I haven't read that) is "procedure". That's why there is a procedure named procedure? in Scheme, and no procedure named function? .

However, your help to the OP is stellar.

[Duoas]

Thanks. Please keep in mind that the OP (and most other readers here) haven't read (nor will they) the R5RS.

The R5RS committee needed a word for a technical thing. They chose poorly (though, considering the circumstances and the technical issues, I don't blame them).

A "procedure" is a very broad term. In most computing contexts that I am aware of, procedures return value by way of side-effect only, whereas functions explicitly return value.

In Scheme, a combination (a lambda), is a function call: local values are bound to an expression which is evaluated for value. That value then replaces the local context. Please keep in mind that this is a functional operation --hence the term functional languages.

Please understand that I'm not being rude. You are very correct. However, you must be careful not to confuse those reading here with those who care about all the technicalities and terminologies specific to functional languages and Scheme in particular.

To be short: in Scheme it is meaningless to talk about the difference between a procedure and a function. But in the minds of programmers everywhere, unfamiliar with Scheme, the difference is in "how do I then get this value to be passed out of the procedure". The answer is, there are no procedures in scheme (or any other functional language). Only functions (or lambdas) that evaluate to (or "return") a new value. You pass a value out by evaluating to that value.

I hope I've explained my choice in language to everyone's satisfaction. If you are a Scheme junkie, know that I am too (it is my all-time favorite functional language, and I've used it in professional contexts). I know as well as you that in Scheme combinations are called "procedures". I don't take this as an excuse to use terms that confuse rather than help the poor saps struggling to figure this stuff out. By the time they get to the point where they read the R5RS about procedure?, they will already know enough to bridge the gap.

Hope this helps.

[nanaman]

Thanks so much for all the help Duoas, rep points coming your way, got the assignment all working and handed in, again thanks :D

[Rashakil Fol]

In Scheme, a combination (a lambda), is a function call: local values are bound to an expression which is evaluated for value. That value then replaces the local context. Please keep in mind that this is a functional operation --hence the term functional languages.

Well, yeah. I agree with calling functions (when used functionally in Scheme) functions. And calling procedures (when used procedurally, with side effects) procedures.

Of course, Scheme isn't really a functional language. :-) troll>

I don't understand why you have "(a lambda)" there, saying that a 'lambda (expression?)' is a synonym for the term 'combination', though.

[Duoas]

I think that's actually part of the reason they called them procedures, because Scheme allows side effects...

The use of the word "combination" is another one of those Scheme goodies... But in an evaluative/transformational way of thinking it makes sense... (as Scheme is all about list transformation).

Are you in to program transformation too? (All the Haskell junkies I know are into that.)

They all worry about me because I like my side effects being transparent. I don't want to have to dink with monads for every function that uses a function that may have side effects... That's what compilers are for IMO. :S :)