[SOLVED] confused about cons (Lisp)

sycamorex · 10-06-2011, 04:10 PM

Breaking down the cddar function, I arrived at the following:

Code:

CL-USER> (cddar '((a b c) (d e f) g))
(C)
CL-USER> (cddr '(a b c))
(C)
CL-USER> (cdr '(b c))
(C)

(c) is, if I understand correctly, a list, or a chain of cons consisting of c and nil:

Code:

(cons 'c nil)

If I look at this example:

Code:

CL-USER> (cadadr '((a b c) (d e f) g))
E
CL-USER> (cadar '((d e f) g))
E
CL-USER> (cadr '(d e f))
E
CL-USER> (car '(e f))
E

Why doesn't it include parentheses? What is different here? What is E? I mean is it still a list?

tuxdev · 10-06-2011, 05:22 PM

It's easier to see why when you break the lists down into the binary trees they actually are. The list (b c) would correspond to

Code:

  /\  
 b /\
  c  nil

"car" returns the left branch, and "cdr" returns the right branch. The left branch is just the atom b, and the right branch is a cons with the atoms c and nil, which is otherwise known as a list with one element.

sycamorex · 10-06-2011, 05:25 PM

Aarrgh! Why haven't I thought of this?! Thanks a lot. It makes sense.