CODEWITHSUNDEEP
lispappend
Rob L
Rob L

Reputation: 3303

Issue using cons function in Lisp?

I am learning Lisp and I had to write a function whose return value was a list containing the odd integers (if any) from the given input. In code I have this:

(defun f3 (a)
   (cond
    ((null a) nil )
    ((and (numberp (car a)) (oddp (car a))) (cons (car a) (f3 (cdr a))))
    (T (f3 (cdr a)))
   ) ; end cond
)

I originally wanted to use the append function, but I kept getting errors.

It was recommended to me to use cons function. When I did this my function started working (code is above). I originally had this:

(defun f3 (a)
    (cond
       ((null a) ())
       ((and (numberp (car a)) (oddp (car a))) (append (f3 (cdr a)) (car a))))
       (T (append () (f3 (cdr a))))
    )
)

but kept getting errors. For example, if I called (f3 '(1 2 3)) it would say "error 3 is not type LIST". So, my questions are why does cons work here and why did append not work? How does cons work? Thanks in advance.

Upvotes: 0

Views: 147

Answers (3)

Linuxios
Linuxios

Reputation: 35803

While this isn't a fix to your actual problem, which @mbratch provided, here's the way I would implement something like this using the LOOP macro (another part of the standard library):

(defun keep-odd-numbers (list)
  (loop for x in list collecting x when (and (numberp x) (oddp x))))

Upvotes: 1

lurker
lurker

Reputation: 58244

append wants list arguments, and (car a) is not a list. Instead of (car a) you'd need (list (car a)). In other words, (append (f3 (cdr a)) (list (car a))).

That will basically work, but you'll get the result in reverse order. So that should be (append (list (car a)) (f3 (cdr a))).

Also note that your (append () (f3 (cdr a))) is equivalent to just (f3 (cdr a)).

The resulting changes in your original would be:

(defun f3 (a)
    (cond
       ((null a) ())
       ((and (numberp (car a)) (oddp (car a))) 
          (append (list (car a)) (f3 (cdr a)))))
       (T (f3 (cdr a)))))

But, you wouldn't normally use append to prepend a single element to a list. It would more naturally be done using cons. So

  (append (list (car a)) (f3 (cdr a)))

Is more appropriately done by:

  (cons (car a) (f3 (cdr a)))

Which finally takes you right to the working version you showed.

Upvotes: 6

Joshua Taylor
Joshua Taylor

Reputation: 85843

While something like mbratch's answer will help you in learning about list manipulation (and so is probably a more useful answer for you at this point in your study), it's also important to learn about the standard library of the language that you're using. In this case, you're trying to filter out everything except odd numbers. Using remove-if-not, that's just:

(defun keep-odd-numbers (list)
  (remove-if-not (lambda (x)
                   (and (numberp x) (oddp x)))
                 list))

CL-USER> (keep-odd-numbers '(1 a 2 b 3 c 4 d 5 e))
;=> (1 3 5)

Upvotes: 5

Related Questions