Topic: [challenge] 39 : Testing for proper lists

[JohnK]

In Lisp, lists come in several different flavors; "proper lists" and "association lists (dotted pairs)".

DEMONSTRATION:

Code - Auto/Visual Lisp: [Select]

1. ;; Setup
2. (setq plst '(1 2 3))
3. (setq alst '(1 . 2))
4.  
5. ;; Checking
6. ;; -If you run the following code you will get T(rue) for each list.
7. (listp plst)
8. (listp alst)
9.  
10. ;; Execution
11. ;; -MAPCAR takes--as it's arguments--a function name and a list but
12. ;;  if you run the following code you will get an error for the `alst`.
13. (mapcar 'print plst)
14. (mapcar 'print alst)

I have been told the a simple definition for a "proper list", in the Lisp languages, is the list is considered `proper` if the list's last CDR returns nil.

Define a function which will check if a list is `proper`.

EXAMPLE:
(properp '(1 2 3 4 5 6 7))
> T
(properp '(1 . 2))
> nil
(properp nil)
> nil

[ribarm]

Who knows...
Just simple thought...

Code: [Select]

(vl-every (function atom) lst)

EDIT :
Still untested...

Code: [Select]

(defun chkproplst ( masterlst )
  (vl-every
    (function (lambda ( lst )
      (not (vl-catch-all-error-p (vl-catch-all-apply
        (function (lambda nil
          (while (and (car lst) (cons (car lst) (setq lst (cdr lst))))
            (and
              (vl-every (function atom) lst)
              (cadr lst)
            )
          )
        ))
        (list lst)
      )))
    ))
    (list masterlst)
  )
)


Code: [Select]

(defun chkproplst ( lst )
  (while (and (car lst) (cons (car lst) (setq lst (cdr lst))))
    (and
      (listp lst)
      (not (vl-catch-all-error-p (vl-catch-all-apply (function cadr) lst)))
    )
  )
)


[JohnK]

Hello ribarm,
ATOM will return T(rue) for the NIL symbol.

(vl-every (function atom) nil)
> T

[gile]

Hi,
My 2 cents.

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l)
2.   (cond
3.     ((null (cdr l)) T)
4.     ((atom (cdr l)) nil)
5.     (T (properlistp (cdr l)))
6.   )
7. )

Code: [Select]

_$ (properlistp '())
T
_$ (properlistp '(1))
T
_$ (properlistp '(1 . 2))
nil
_$ (properlistp '(1 2 3 . 4))
nil
_$ (properlistp '(1 2 (3 . 4)))
T
_$ (properlistp '(1 2 (3 4)))
T

[gile]

Could have been written:

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l)
2.   (or (null (cdr l))
3.       (and (not (atom (cdr l)))
4.            (properlistp (cdr l))
5.       )
6.   )
7. )

[Lee Mac]

Another -

Code - Auto/Visual Lisp: [Select]

1. (defun list-p ( l )
2.     (and (< 0 (vl-list-length l))
3.          (vl-every '(lambda ( x ) (or (atom x) (list-p x))) l)
4.     )
5. )

[gile]

Another -

Code - Auto/Visual Lisp: [Select]

1. (defun list-p ( l )
2.     (and (< 0 (vl-list-length l))
3.          (vl-every '(lambda ( x ) (or (atom x) (list-p x))) l)
4.     )
5. )

I forgot vl-list-length function.
May be I misundersood, my attempts do not recursively check nested lists (i.e. consider a list containing dotted pairs as a properlist).
They simply could be written:

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l) (numberp (vl-list-length l)))

[JohnK]

Maybe I went too simple on my routine (compared to the other entries).

Code - Auto/Visual Lisp: [Select]

1. (defun properp (l / my-last)
2.    (defun my-last (lst)
3.      (if (atom lst)
4.        lst
5.        (my-last (cdr lst))) )
6.    (if l (null (my-last l))) )

[bruno_vdh]

I forgot vl-list-length function.
May be I misundersood, my attempts do not recursively check nested lists (i.e. consider a list containing dotted pairs as a properlist).
They simply could be written:

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l) (numberp (vl-list-length l)))

Hello,
Well seen, it will be difficult to do better
Perhaps, we can vary like this:

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l) (< 0 (vl-list-length l)))

 To meet this condition:

(properp nil)
> nil

Although I do not understand this request, for me nil <=> ‘()

Code: [Select]

_$ '()
nilBest regards

[Lee Mac]

Another -

Code - Auto/Visual Lisp: [Select]

1. (defun list-p ( l )
2.     (and (< 0 (vl-list-length l))
3.          (vl-every '(lambda ( x ) (or (atom x) (list-p x))) l)
4.     )
5. )

I forgot vl-list-length function.
May be I misundersood, my attempts do not recursively check nested lists (i.e. consider a list containing dotted pairs as a properlist).
They simply could be written:

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l) (numberp (vl-list-length l)))

Well seen, it will be difficult to do better
Perhaps, we can vary like this:

Code - Auto/Visual Lisp: [Select]

1. (defun properlistp (l) (< 0 (vl-list-length l)))

 To meet this condition:

(properp nil)
> nil

Although I do not understand this request, for me nil <=> ‘()

Code: [Select]

_$ '()
nil

I too considered these, but wasn't sure whether this should be permitted:

Code - Auto/Visual Lisp: [Select]

1. _$ (defun properlistp (l) (numberp (vl-list-length l)))
2. PROPERLISTP
3. _$ (properlistp '(1 2 (3 . 4)))
4. T

[gile]

I too considered these, but wasn't sure whether this should be permitted:

Code - Auto/Visual Lisp: [Select]

1. _$ (defun properlistp (l) (numberp (vl-list-length l)))
2. PROPERLISTP
3. _$ (properlistp '(1 2 (3 . 4)))
4. T

JohnK said: "I have been told the a simple definition for a "proper list", in the Lisp languages, is the list is considered `proper` if the list's last CDR returns nil"
((0 . "CIRCLE") (10 0 0 0) (40 . 10)) is an association list and also a proper list. Last cdr returns: ((40 . 10)).
Instead of "association list" he should have said "dotted list" of which "dotted pair" is only a particular case (see here).

[JohnK]

Nice link.