I am trying to run this code that supposedly removes duplicates from a sorted list of numbers.
(defun unique (x)
(cond( (null x) x )
( (null (cdr x)) x )( (equal (car x) (cdr x)) (unique (cdr x)) )( t (cons (car x) (unique (cdr x))) )))
I am trying to call this function by typing:
(print (unique '(2 2 3 4 4 5)))
My output is currently shown as follows:
(2 2 3 4 4 5)
Clearly, this doesn't seem to be removing the duplicates, if anything at all.
There is an error in the form: (equal (car x) (cdr x)). car returns the first element of a list, while cdr returns the rest of the list, so the comparison is not between the first and the second element of the list, but between the first element and the list without the first element (and this comparison will always produces false for regular lists).
To correct the error, the function should use cadr instead of cdr: (equal (car x) (cadr x)): the function cadr (or its synonym second) is equivalent to car (cdr and returns the second element of a list. So this is the correct version:
CL-USER> (defun unique (x)
(cond ( (null x) x )
( (null (cdr x)) x )
( (equal (car x) (cadr x)) (unique (cdr x)) )
( t (cons (car x) (unique (cdr x))) )))
UNIQUE
CL-USER> (unique '(1 1 2 2 3))
(1 2 3)
(let* ((tail (list 0 1 2)) (list (cons tail tail))) (assert (equal (car list) (cdr list))))
(1 . 1) :). I omitted “for regular lists”. Corrected, thanks @coredump.
(unique '(2 2 3 4 4 5)) produces (2 3 4 5). On which system did you test it?