For Monday Read Chapter 3 Homework: –Lisp handout 2

Atoms The simplest thing in Lisp is an atom Numbers are atoms Anything that would be a standard identifier in C++, etc., is also an atom. But alphanumeric atoms must be quoted when we refer to them (otherwise Lisp assume that they are variable names and tries to evaluate them by finding their contents--which may not exist)

Evaluating Atoms > 9 9 > > 43.2e > > abc *** - EVAL: variable ABC has no value > 'abc ABC > 'fred FRED > 'H2O H2O

Lists Lists are the crucial data structure in LISP They look like a list of atoms (or lists) enclosed in parentheses: (a b c 24 (a b)) The list above has 5 members, one of them a list with two members Like alphanumeric atoms, lists must be quoted when you type them in to the interpreter to keep Lisp from evaluating them

nil nil or () is the empty list It is the only item in Lisp which is both an atom and a list (at the same time) It is also Lisp’s value for FALSE (just like 0 is the value for FALSE in C and C++ Note that Lisp also has a value for true: it is t

More Examples > '(a b c) (A B C) > () NIL Note that Lisp is not case sensitive. clisp echoes everything in uppercase. Some Lisps use lower case.

Function Calls in Lisp (functionname parameter1 parameter2...) (+ 3 5) All functions return values

Taking Lists Apart (first ‘(lisp is fun)) lisp (rest ‘(lisp is fun)) (is fun) (first ‘((the dog) likes me)) (the dog)

Why the Quote? Quoting prevents Lisp from attempting to evaluate the list If we had (first (a b c d)) Lisp would try to find a function a and apply that function to b c d, then apply first to the result

The Old Way In the original lisp, first was spelled car and rest was spelled cdr Allowed short-cuts (cadr ‘(a b c)) ≡ (car (cdr ‘(abc)) ≡ (first (rest ‘(abc)) Not often needed, because Common Lisp provides second, third, etc. to tenth

SETF Lisp does use variables Values are given using setf (setf friends ‘(elf dwarf human)) (setf enemies ‘(orc kobold)) (setf pi 3.14) (setf ab-list ‘(a b) xy-list ‘(x y))

True and False Lisp has built-in true and false values nil, or the empty list, is the false value t is the true value (though any non-nil value will be interpreted as true) No other values can be assigned to these two atoms

Making Lists To add an item to the head of a list, use cons –(cons ‘a ‘(b c)) (a b c) To combine two lists, use append –(append ‘(a b c) ‘(d e f)) (a b c d e f) To make a list from a set of items, use list –(list ‘a ‘b ‘c) (a b c) –(list ‘(a b) ‘c) ((a b) c)

Tricky Stuff (append ‘(a b c) ‘( )) (list ‘(a b c) ‘( )) (cons ‘(a b c) ‘( ))

Shortening Lists Use nthcdr to trim multiple off the front of a list –(nthcdr 2 ‘(a b c d)) (c d) –(nthcdr 50 ‘(a b c d)) nil Use butlast to trim elements off the back of a list –(butlast ‘(a b c d)) (a b c) –(butlast ‘(a b c d) 3) (a) Use last to get a list containing just the last element of a list –(last ‘(a b c d)) (d)

Length The length primitive counts the number of top-level elements in a list –(length ‘(a b)) 2 –(length ‘((a b) (c d))) 2 –(length (append ‘(a b) ‘((a b) (c d)))) 4

Reverse The reverse primitive reverses the order of the top-level elements of a list –(reverse ‘(a b c)) (c b a) –(reverse ‘((a b) (c d))) ((c d) (a b))

Association Lists Provide hash-type capabilities Example: –(setf sarah ‘((height.54) (weight 4.4))) –(assoc ‘weight sarah) (weight 4.4) If multiple matches, only first is returned

Numbers (/ ) (/ 27 9) 3 (/ 22 7) 22/7 (float (/ 22 7)) (round (/ 22 7)) 3 1/7 (- 8) -8 (- -8) 8 (/ 2) 1/2

More Numbers (max 2 4 3) 4 (min ) 2 (expt 2 3) 8 (expt 3 2) 9 (expt ) (sqrt 9) 3 (abs 5) 5 (abs –5) 5

Defining Functions (defun both-ends (whole-list) (cons (first whole-list) (last whole-list))) (defun both-end-two-params (l m) (cons (first l) (last m)))

Let’s Try One Define palindromize, a function that takes a list as its argument and returns a “palindrome” twice as long as the original.

LET and LET* Allow you to bind values to locals (defun both-ends-with-let (whole-list) (let ((element (first whole-list)) (trailer (last whole-list))) (cons element trailer))) LET does binding “in parallel” LET* allows binding to occur in sequence– so variables can use previous variable in value computation

Equality equal: same expression eql: same symbol or number (of same type) eq: same symbol =: same number

Examples (equal (+ 2 2) 4) t (equal (+ 2 3) 3) nil (equal ‘(this is a list) (setf l ‘(this is a list))) t (eql 4 4.0) nil (eql 4 4) t (= 4 4.0) t

Member (setf sentence ‘(tell me about your mother please)) (member ‘mother sentence) (mother please) (setf pairs ‘((father son) (mother daughter))) (member ‘mother pairs) nil

Keyword Arguments (setf pairs ‘((maple shade) (apple fruit))) (member ‘(maple shade) pairs) nil; uses EQL by default We can change using a keyword (member ‘(maple shade) pairs :test #’equal) ((maple shade) (apple fruit))

Type Predicates atom –(atom ‘pi) t –(atom pi) t –(atom ‘(red black)) nil numberp –(numberp ‘pi) nil –(numberp pi) t symbolp –(symbolp ‘pi) t –(symbolp pi) nil listp –(listp ‘pi) nil –(listp pi) nil –(listp ‘(a list with pi in it)) t

Detecting Empty Lists Use NULL or ENDP (null ‘(this is not empty)) nil (endp ‘(this is not empty)) nil (null ()) t (endp ()) t (null ‘aSymbol) nil (endp ‘aSymbol) error

Number Predicates zerop plusp evenp > < may take multiple arguments

AND, OR, and NOT AND and OR work as expected –AND returns nil if any of its arguments return nil –OR return nil if all of its arguments return nil –Both are short-circuiting –AND returns value of last argument if non return nil –OR returns value of first non-nil argument NOT turn non-nil values into nil and nil into t

Selection (if ) (if (symbolp day-or-date) ‘day ‘date) (when ) (when (> temp high) (setf high temp) ‘new-record) (unless )

COND (cond ( …) ( …) … ( …)) (cond ((eq thing ‘circle) (* pi r r)) ((eq thing ‘sphere) (* 4 pi r r)))

COND cont. If no test matches, returns nil Often include a final test clause t (matching anything) (cond ((> p.75) ‘very-likely) ((> p.5) ‘likely) ((> p.25) ‘unlikely) (t ‘very-unlikely))

Recursion Standard method for doing “repetition” in functional languages is recursion (defun myexpt (m n) (if (zerop n) 1 (* m (myexpt m (- n 1)))))

Recursion Inefficiency (defun count-elements (l) (if (endp l) 0 (+ 1 (count-elements (rest l)))))

Tail Recursion If we don’t manipulate the result of a recursive call before passing it on, that is called tail recursion In a tail recursive function, you need not save any but the most recent call (because the result of the last call is the result of all calls) Good lisps (and prologs) optimize tail-recursive functions/predicates, making them just as efficient as a corresponding loop

Counting More Efficiently (defun count-elements-cleverly (lis) (count-elements-cleverly-aux lis 0)) (defun count-elements-cleverly-aux (lis result) (if (endp lis) result (count-elements-cleverly-aux (rest lis) (+ 1 result))))

Iteration Lisp has several constructs that support iteration. DOTIMES is the equivalent of a for loop. DOLIST iterates through a list. DO and LOOP are general purpose structures.

DOTIMES (defun dotimes-expt (m n) (let ((result 1)) (dotimes (count n result) (setf result (* m result)))))

DOLIST (defun count-outlyers (list-of-elements) (let ((result 0)) (dolist (element list-of-elements result) (when (or (> element boiling) (< element freezing)) (setf result (+ result 1)))))))

DO (defun do-expt (m n) (do ((result 1) (exponent n)) ((zerop exponent) result) (setf result (* m result)) (setf exponent (- exponent 1))))

LOOP LOOP is simply an infinite loop which can be exited using a return form. If no return statement is ever executed, the loop is truly infinite. Form is simply (loop )

Transforming Lists MAPCAR applies a function to each member of a list, producing a list of the results (mapcar #’oddp ‘(1 2 3)) (t nil t) Can use with your own functions. Do not have to be predicates

More List Processing All of the following take a predicate and a list and return a list based on the predicate Remove-if Remove-if-not Count-if Count-if-not

Explicitly Calling Functions funcall (funcall #’append ‘(a b) ‘(x y)) (funcall #’ ) (funcall #’first ‘(a b c)) apply (apply #’append ‘((a b) (x y))) (apply #’+ ( )) (apply #’first ‘((a b c)))

And More Common Lisp is a huge language The handouts are intended to encourage you to try out and explore the language. This is the end of lecture on Lisp, but I will answer questions at the beginning of each class.