Help with lisp function
Todd Baker
having problems with the following 2 functions and was wondering if
anyone could help me understand what I'm doing wrong.
;this function works just fine
(defun atoms_help (mylist)
(cond ((null mylist) nil)
((atom mylist) (list mylist))
(t (append (atoms_help (first mylist))
(atoms_help (rest mylist))))))
;how ever for my class we are not allowed to use the cond predicate
and so
;i rewrote the function as follows but keep getting an error
(defun atoms_help(mylist)
(if (null mylist)()
(if (or (atom mylist) (list mylist))
(append (atoms_help (first mylist))
(atoms_help(rest mylist))))
)
)
if anyone could help i'd really appreciate it, thanks todd
Henrik Motakef
> Hi Group I was wondering if anyone could answer a question for me I'm
> having problems with the following 2 functions and was wondering if
> anyone could help me understand what I'm doing wrong.
You don't format your code properly and choose suboptimal identifiers ;-)
Seriously, a direct translation of the first function to use IF only
instead of COND would be
(defun atoms-help (mylist)
(if (null mylist)
nil
(if (atom mylist)
(list mylist)
(append (atoms-help (first mylist))
(atoms-help (rest mylist))))))
Your use of OR in
| (if (or (atom mylist) (list mylist))
| (append (atoms_help (first mylist))
| (atoms_help(rest mylist))))))
will return the result of APPEND if either (atom mylist) or (list
mylist) are non-nil, which is always the case because (list mylist) is
never nil (i.e. an empty list). The only way to get nil through a call
to LIST is by not passing any arguments to it, as in (list).
I guess you should reconsider your idea of what OR does, and probably
IF as well.
Additionally, even if I don't think that you wanted to do that anyway,
using IF with only a then-clause, but no else, is usually considered
bad style. Either have an else-clause that returns nil implicitly if
this nil has an interesting meaning in your program, or use WHEN
instead of IF.
Pascal Bourguignon
> Hi Group I was wondering if anyone could answer a question for me I'm
> having problems with the following 2 functions and was wondering if
> anyone could help me understand what I'm doing wrong.
>
> ;this function works just fine
> (defun atoms_help (mylist)
> (cond ((null mylist) nil)
> ((atom mylist) (list mylist))
> (t (append (atoms_help (first mylist))
> (atoms_help (rest mylist))))))
>
> ;how ever for my class we are not allowed to use the cond predicate
> and so
> ;i rewrote the function as follows but keep getting an error
Oh! If you're not allowed to use the cond predicate, then the correct
way to do it is:
(defmacro not-the-cond-predicate (&rest clauses)
(let ((ifs '()))
(dolist (clause (reverse clauses))
(setq ifs (append `(if ,(car clause) (progn ,@(cdr clause)) ,ifs))) )
ifs))
;; Note the indentation. Please use a lisp-aware editor!
(defun atoms_help (mylist)
(not-the-cond-predicate ((null mylist) nil)
((atom mylist) (list mylist))
(t (append (atoms_help (first mylist))
(atoms_help (rest mylist))))))
> (defun atoms_help(mylist)
> (if (null mylist)()
> (if (or (atom mylist) (list mylist))
> (append (atoms_help (first mylist))
> (atoms_help(rest mylist))))
> )
> )
>
> if anyone could help i'd really appreciate it, thanks todd
Then you can run:
(macroexpand (quote
(not-the-cond-predicate ((null mylist) nil)
((atom mylist) (list mylist))
(t (append (atoms_help (first mylist))
(atoms_help (rest mylist)))))
))
and compare with your version to see where the error lies...
--
__Pascal_Bourguignon__
http://www.informatimago.com/
Lowell Kirsh
expression. So even simpler than writing a bunch of code yourself for
macroexpansion, just macroexpand the original cond.
the following :
(macroexpand-1 '(cond ((null mylist) nil)
((atom mylist) (list mylist))
(t (append (atoms_help (first mylist))
(atoms_help (rest mylist))))))
evaluates (in my lisp implementation) to :
(IF (NULL MYLIST) NIL
(IF (ATOM MYLIST) (LIST MYLIST)
(APPEND (ATOMS_HELP (FIRST MYLIST)) (ATOMS_HELP (REST MYLIST)))))
Lowell