Topic: Evaluate a list and return the nil

[JohnK]

I was thinking this morning and i have been trying to come up with a slick way to test a list and return the nil member of a list. I cant think of anything. (Am i missing anything?) What is the opposite of "cond"? Is there one?

The only thing i could come up with is: "(mapcar 'cond '(a b c d))" to evaluate each member of the list but all i get is something like "(T T T nil)" as a return. I was wondering if anyone knew of a way to return the item that is nil in a list?

Am i nuts?! ...I must be nuts.

[Mark]

do what..............
I think you've been taking to much cold medicine.

[hendie]

funnily enough I was trying to do exactly the same thing two days ago.


I gave up    

[JohnK]

Smartass! Funny, cause I need to go get more. (I'm out)

Ok, lets say I have a list of stuff.
(setq a 1 b 2 c 3 d nil)
and I want to run thru the list to see what member dose not have a value.

I can use a "(mapcar 'cond "mylist")" to evaluate each member of my list but that dosent tell me which one in the list is nil.

[JohnK]

Well it can be done. Here is "a way" but it aint smoooth (like me)

Run each of these lines in your vlide and watch the return values.

Code: [Select]

(setq a 1 b 2 c 3 d nil)
  (setq x (list a b c d))
  (setq y '(a b c d))
  (setq n (- (length x) 1))
  (setq nu
        (1+ (while
              ;; test each
              (equal (nth n x) nil)
              ;; drop the n variable down one increment
              (setq n (1- n))
              ))
        )
 ;; return the nil variable.
  (nth nu y)

[SomeCallMeDave]

If you are sure that you only will have one nil, you could use vl-position

Command: (setq aa (list "A" "b" nil "c"))
("A" "b" nil "c")
Command: (vl-position nil aa)
2

[JohnK]

If you are sure that you only will have one nil, you could use vl-position

Command: (setq aa (list "A" "b" nil "c"))
("A" "b" nil "c")
Command: (vl-position nil aa)
2

Oh crap! im sorry, i forgot to mention that i wanted a way without VL. But nice.

Edit: Im actualy just playing around with the lisp language; looking for cool stuff that i overlooked.  Sorry, i prolly should have mentioned that huh?!

[Mark]

Ok, lets say I have a list of stuff.
(setq a 1 b 2 c 3 d nil)
and I want to run thru the list to see what member dose not have a value.

OK..........now you're startin to make sense.

Check out he eval function.

Code: [Select]

(if (not (eval x))(prompt "is nil.."))

[CAB]

Se7en,
Putting the nil in the middle of the list resulted in error.
I had to move the 1+ for the index var and don't see why eval is needed.
Still just a variation on your routine..

Code: [Select]

 (setq a 1 b 2 c nil d 3)
  (setq x (list a b c d))
  (setq y '(a b c d))
  (setq n (- (length x) 1))
  (setq nu (while
    (nth n x)
    (setq n (1- n))
  )
  )
  ;; return the nil variable.
  (nth (1+ nu) y)

[CAB]

This appears to work as well.

Code: [Select]

 (setq a 1 b nil c 3 d 4)
  (setq x (list a b c d))
  (setq y '(a b c d))
  (setq nu (-(length x)(length (member nil x))))
  (nth nu y);; return the nil variable. If no nil returns nil

[Mark]

Code: [Select]

(setq a 1 b 2 c nil d 3)
 (setq syms '(a b c d))
   
  (foreach item syms
    (if (not (eval item))
      (princ item)
      )
    )

[Kerry]

am I missing something ??

 (setq xxxx (list 1 2 3 nil 5 6))
==>> (1 2 3 nil 5 6)

(member nil xxxx)
==>> (nil 5 6)

(length xxxx) ==>> 6
(length nil) ==>> 0
(length (member nil xxxx)) ==>> 3

[Mark]

am I missing something ??

 (setq xxxx (list 1 2 3 nil 5 6))
==>> (1 2 3 nil 5 6)

(member nil xxxx)
==>> (nil 5 6)

member function

Searches a list for an occurrence of an expression and returns the remainder of the list, starting with the first occurrence of the expression

[Kerry]

yep

.. if the return from (member nil xxxx) is not nil

(- (length xxxx) (length (member nil xxxx))) will be the position of the nil.

it's early here, so I may be incorrect need coffee

[JohnK]

Mark, That is what i was attmepting to do. Damn! I cant find my "foreach" test. (I must have destroyed it.) But it was craaaazy.  

Look at this one:
"(mapcar 'if (not (cond (eval x))) (setq x (princ "this one")) '(a b c d))"
Dahell is that?!!!

Kids, do you see what cold med's do to your mind. They make you go nuts!

[Mark]

looks correct to me, but se7en while have to make the final say-so since he started this mess........

[Kerry]

if you are after the index, this may suit :

Code: [Select]


(defun test (theList / tmp)
  (if (setq tmp (member nil theList))
    (1+ (- (length theList) (length tmp)))
    nil
  )
)



[JohnK]

Oh i dont care. I was origionaly looking for the opposite of "cond" like it was the fountian of youth or something but anyof those will do.  Thanx alot guys.

[JohnK]

If i had to pick one i would have to say marks because its cleaner.  But their all cool.

[Kerry]

< snip > and I want to run thru the list to see what member dose not have a value.

I can use a "(mapcar 'cond "mylist")" to evaluate each member of my list but that dosent tell me which one in the list is nil.

ahh ok ..