recursionschemelispcdrsisc

Recursive function not working '"Wrong type argument in procedure car"

I am writing a recursive function that takes an element A and a list L and returns a list equal to L, but with every occurrence of A removed. Here is what I've written:

(define (remove A L)
    (cond   ( (eq? A (car L))       (remove A (cdr L)) )
            ( (not(eq? A (car L)))  (cons (car L) (remove A (cdr L))) )
            ( (null? L)             '() )
    )
)

When compiled and ran, I get the following error:

/tmp/compile/5c6515d8-e155-11e5-9605-aa00009baa05/input/main.scheme:2:21: In procedure remove:

/tmp/compile/5c6515d8-e155-11e5-9605-aa00009baa05/input/main.scheme:2:21: In procedure car: Wrong type argument in position 1 (expecting pair): ()
Solution

  • I figured it out:

    The first check of the function needed to be (null? L) as car and cdr cannot work on any empty list.

    (define (remove A L)
    (cond   ( (null? L)             '() )
            ( (equal? A (car L))       (remove A (cdr L)) )
            ( (not(equal? A (car L)))  (cons (car L) (remove A (cdr L))) )
    )
)