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CS 3723 Programming Languages |
Lisp Lists | |
- Stopping evaluation: Sometimes we don't want to evaluate
a list or an atom. quote is another Lisp special function
that doesn't evaluate its argument, but instead returns it unevaluated.
A single quote mark, without the parens, gives a shorthand version
of the longer form.
Quote or ', Preventing Evaluation > (quote (a b c)) (A B C) > '(a b c) (A B C) > (a b c) Error: The function A is undefined.
- car and cdr tear lists apart:
car returns the first element of a list, and
cdr returns the remainder of the list with the first element
deleted.
car and cdr: Tearing Lists Apart > (car '(a b c)) ;;; first element is a A > (cdr '(a b c)) ;;; rest of list is (b c) (B C) > (car (cdr '(a b))) ;;; cdr gives (b), and car of that is b B > (car (a b c)) ;;; lisp tries to evaluate (a b c) Error: The function A is undefined.
- cons puts two lists together:
cons assembles a list from its
car and cdr.
Cons: Assembling Lists > (cons 'a '(b c)) (A B C) > (cons (car '(a b c)) (cdr '(a b c))) (A B C)
- list and append give two more
ways to create new lists:
List: Creating Lists ; list takes any seq of S-expressions ; and makes a list out of them: > (list 'a 'b 'c) (A B C) > (list '(a) '(b c) 'd) ((A) (B C) D) > (list '(a (b c) d) '((e) f ((g)))) ((A (B C) D) ((E) F ((G)))) > (list 'a '(b c) nil 'd () ) (A (B C) NIL D NIL) ; Note: list keeps empty lists
Append: Tack Lists Together ; append takes any sequence of lists and ; combines them into a single list: > (append '(a b) '(c d)) (A B C D) > (append '(a (b c) d) '((e) f ((g)))) (A (B C) D (E) F ((G))) > (append '(a b) nil '(c) () ) (A B C) ; append drops empty lists > (append 'a '(b c)) *** - APPEND: A is not a list > (append '(a b) 'c) (A B . C) ; more on this notation later
- Evaluating lists -- Another Look
Look back at what was said under an earlier header,
"Evaluating lists." Let's see how Lisp can evaluate
using actual functions apply and eval:
Eval and Apply > (+ 3 4 5) 12 > (eval (+ 3 4 5)) 12 > (apply '+ '(3 4 5)) 12 > (apply (car '(+ 3 4 5)) (cdr '(+ 3 4 5))) 12
- List Functions:
List Functions first: same as car second third fourth ... last rest: same as cdr append list (illustrated above)
Suppose > (setq c '( (a b) c d)) caar: "car of the car", (caar x) is (car (car x)) is A cadr: "car of the cdr", (cadr x) is (car (cdr x)) is C cdar: "cdr of the car", (cdar x) is (cdr (car x)) is (B) cddr: "cdr of the cdr", (cddr x) is (cdr (cdr x)) is (D) - Other Predicates: (Boolean-valued functions,
they return t or nil)
Predicates atom: is its arg an atom? null: is its arg nil or ()? numberp: is its arg a number? listp: is its arg a list? member: two args, does the 1st appear in the 2nd (a list)? equal: do its 2 args look alike? (= is only for nums)