CIS 400 LECTURE 12

More Lisp:

Predicate functions, continued:

>(null '()) ;empty list

t

>(null nil)

t

(null t)

nil

>(null sam)

nil

>(member 'a '(a b c)) ; find a list that has "a" as an element

(a b c)

>(member 'a '((a b) c (d e)) ; member is top level only

nil

; "not equal" in XLisp is /=

>(assoc 'c '((a b) (c d) (e f)) ; find a sub-list with "c" as its "car"

(c d)

;;;;;;; here's an interesting function!! ;;;;;;;;;;;;

>(and nil t t) ;short circuit boolean, if NO nil is found, all are evaluated and last element ;is output

nil

>(and 'sam nil 'harry)

nil

>(and 1 2 3)

3

>(defun sign (a b)

(and (oddp a) (oddp b) 'both-odd)

)

>(sign 2 3)

nil

>(sign 3 3)

t

>(or nil t t) ;short circuit boolean, first t is evaluated

t

>(or 'sam nil 'harry)

sam

>(or nil nil nil)

nil

>(defun sign (a b)

(or (oddp a) (oddp b) 'both-odd)

)

>(sign 2 3)

t

>(sign 3 3)

t

>(sign 2 2)

both-odd

>(defun compute (op x y)

(cond ((equal op 'sum) (+ x y))

((equal op 'prod) (* x y))

(t '(does not compute)) ;"t" will never evaluate as "nil", so if previous 2

) ;condition fail, the "t" results in the output "does not

) ;compute"

>(compute 'sum 2 3) ; similar to (+ 2 3)

5

>(compute 'subtract 3 2) ; all evaluations fail

(does not compute)

>(if (listp a) (car a) '(not a list)

) ; "if" is a predicate function found in Xlisp and ;common-Lisp

>(cond (( listp a) (car a))

' (t '(not list))

) ; similar function using "cond" instead of "if"

;;;;;;;;;;; creating local variables ;;;;;;;;;;;;;;;;;

>(defun augment (first second) ;"first" and "second" are passed parameters

(let ((item first) ; "let" creates local variable

(bag second)

)

(if (listp first) (setq item second bag first))

(if (member item bag) bag (cons item bag))

)

)

>(augment 2 '(1 3))

(2 1 3) ; adds 2 as new "car"

>(augment 2 '(1 2 3))

(1 2 3) ; 2 is already in list

>(defun count (L) ; this function will count number of item in a list

® (do ((count 0 (+1 count)) ; initialize count to 0, then increment by 1

(loaf L (cdr loaf))

((null loaf) count) ; test to stop

¬ ) ; ARROWS REPRESENT BEGINNING AND

) ; END OF LOCAL VARIABLE SCOPE

>(count '(1 2 3))

3

;;;;;;;;;;; input and output ;;;;;;;;;;;;;

>(setq a (read))

>1 ; enter input of "1"

>a ; ask to output the value of "a"

1

>(setq a (read))

>(defun sum (x y) (+ x y)) ; this actually makes Lisp "read" the function that is

; is defined by the input

>(eval a) ; what has been read is now evaluated

sum

>(sum 2 3)

5