I'm new on lisp and I'm finding a difficulty while working with "append". I have to reorder a list and put the first element of the input list as the last of the output list. I've trying whit "append" and "nconc", and any time I get the list I want, but with a "." before the last element. What this dot means? Is there any way to avoid this symbol to appear?
Thanks a lot!
(nconc (rest l)(first l)) >> (B C D E . A)
(append (rest l) (first l)) >> (B C D E . A)
At the most basic level, lists are created with a series of cons calls:
(cons 1 2 ) ; (1 . 2)
(cons 1 (cons 2 NIL)) ; (1 2 . NIL) = (1 2)
(cons 1 (cons 2 3 )) ; (1 2 . 3)
(cons 1 (cons 2 (cons 3 NIL))) ; (1 2 3 . NIL) = (1 2 3)
NIL is the special marker to signal the end of a list. Hence, the NIL after the . can just disappear together with the .. But with anything else after the ., it can't.
append is a higher-level function, which appends two lists:
; l = (1 2 3)
(append (rest l) (first l))
=
(cons 2 (cons 3 1))
=
( 2 3 . 1)
This is so because first l here is not a list. But we can make a list out of it:
; l = (1 2 3)
(append (rest l) (list (first l)))
=
(cons 2 (cons 3 (list 1)))
=
(cons 2 (cons 3 (cons 1 NIL)))
=
( 2 3 1 . NIL )
=
( 2 3 1 )
nconc in this respect is just like append.