04 Control

Control-Blocks Common Lisp has 3 basic operators for creating blocks of code progn block tagbody If ordinary function calls are the leaves of a Lisp program, these operators are used to build the branches

Control-Blocks > (progn (format t “a”) (format t “b”) ( ) ) ab 23 -> only the value of the last expression is returned

Control-Blocks > (block head (format t “Here we go.”) (return-from head ‘idea) (format t “We’ll never see this.”)) Here we go. IDEA Calling return-from allows your code to make a sudden but graceful exit from anywhere in a body of code Expressions after the return-from are not evaluated

Control-Blocks > (block nil (return 27)) 27 > (dolist (x ‘(a b c d e)) (format t “~A “ x) (if (eql x ‘c) (return ‘done))) A B C DONE

Control-Blocks The body of a function defined with defun is implicitly enclosed in a block with the same name as the function (defun foo ( ) (return-from foo 27) ) (defun read-integer (str) (let ((accum 0)) (dotimes (pos (length str)) (let ((i (digit-char-p (char str pos)))) (if i (setf accum (+ (* accum 10) i)) (return-from read-integer nil)))) accum))

Control-Blocks Within tagbody, you can use go > (tagbody (setf x 0) top (setf x (+ x 1)) (format t “~A “ x) (if (< x 10) (go top))) This operator is mainly something that other operators are built upon, not something you would use yourself Ugly code!!

Control-Blocks How to decide which block construct to use? Nearly all the time you’ll use progn If you want to allow for sudden exits, use block Most programmers will never use tagbody explicitly

Control-Context let Takes a body of code Allows us to establish variables for use within the body > (let ((x 7) (y 2)) (format t “Number”) (+ x y)) Number 9

Control-Context > ((lambda (x) (+ x 1)) 3) 4 ((lambda (x y) (format t “Number”) (+ x y)) 7 2)

Control-Context One let-created variable can’t depend on other variables created by the same let (let ((x 2) (y (+ x 1))) (+ x y)) ((lambda (x y) (+ x y)) 2 (+ x 1)) > (let* ((x 1) (y (+ x 1))) (+ x y)) 3 equivalent

Control-Context A let* is functionally equivalent to a series of nested lets (let ((x 1)) (let ((y (+ x 1))) (+ x y))) In both let and let*, initial values default to nil > (let (x y) (list x y)) (NIL NIL)

Control- Conditionals (if (oddp that) (progn (format t “Hmm, that’s odd.”) (+ that 1))) (when (oddp that) (format t “Hmm, that’s odd.”) (+ that 1)) equivalent

Control- Conditionals (if ) (if )  (when ) (if nil )  (unless )

Control- Conditionals (defun our-member (obj lst) (if (atom lst) nil (if (eql (car lst) obj) lst (our-member obj (cdr lst)))) (defun our-member (obj lst) (cond ((atom lst) nil) ((eql (car lst) obj) lst) (t (our-member obj (cdr lst))))) equivalent A Common Lisp implementation will probably implement cond by translating it to the “if” format

Control- Conditionals cond (cond ( …) ( …) … ( …) );cond The conditions are evaluated in order until one of them returns true When one condition returns true, the expressions associated with it are evaluated in order, and the value of the last is returns as the value of the cond > (cond (99)) ; if there are no expressions after the successful condition 99 ;, the value of the condition itself is returned.

Control- Conditionals case (case ( …) ( …)... ( …) ) ;case

Control- Conditionals (defun month-length (mon) (case mon ((jan mar may jul aug oct dec) 31) ((apr jun sept nov) 30) (feb (if (leap-year) 29 28)) (otherwise “unknown month”))) Keys Are treated as constants Will not be evaluated > (case 99 (99)) The successful clause contains only keys NIL

Control- Iteration do (do (( ) ( ) … ( )) ( ) ) ;do > (let ((x ‘a)) (do ((x 1 (+ x 1)) (y x x)) ((> x 5)) (format t “(~A ~A) “ x y))) (1 A) (2 1) (3 2) (4 3) (5 4) ;on each iteration, x gets its previous NIL ;value plus 1; y also gets the previous ;value

Control- Iteration do* Has the same relation to do as let* does to let > (do* ((x 1 (+ x 1)) (y x x)) ((> x 5)) (format t “(~A ~A) “ x y)) (1 1) (2 2) (3 3) (4 4) (5 5) NIL

Control- Iteration dolist > (dolist (x ‘(a b c d) ‘done) (format t “~A “ x)) A B C D DONE dotimes > (dotimes (x 5 x) ; for x = 0 to 5-1, return x (format t “~A “ x))

Control- Multiple values In Common Lisp, an expression can return zero or more values E.g., get-decoded-time returns the current time in nine values value Returns multiple values > (values ‘a nil (+ 2 4)) A NIL 6

Control- Multiple values > ((lambda ( ) (values 1 2) )) 1; 2 If something is expecting only one value, all but the first will be discarded > (let ((x (values 1 2))) x) 1

Control- Multiple values > (values) returns no value > (let ((x (values))) x) NIL Use multiple-value-bind to receive multiple values > (multiple-value-bind (x y z) (values 1 2 3) (list x y z)) (1 2 3) > (multiple-value-bind (x y z) (values 1 2) (list x y z)) (1 2 NIL)

Control- Multiple values > (multiple-value-bind (s m h) (get-decoded-time) (format nil “~A:~A:~A” h m s)) “4:32:13” We can pass on multiple values as the arguments to a second function using multiple-value-call > (multiple-value-call #’+ (values 1 2 3)) 6 multiple-value-list is like using multiple-value-call with #’list as the first argument > (multiple-value-list (values ‘a ‘b ‘c)) (A B C)

Control-Aborts catch and throw (defun super ( ) (catch ‘abort (sub) (format t “We’ll never see this.”))) (defun sub ( ) (throw ‘abort 99) > (super) 99

Control- Example: Date arithmetic (defconstant month #( )) (defconstant yzero 2000) (defun leap? (y) (and (zerop (mod y 4)) (or (zerop (mod y 400)) (not (zerop (mod y 100))))))

Control- Example: Date arithmetic (defun date->num (d m y) (+ (- d 1) (month-num m y) (year-num y))) (defun month-num (m y) (+ (svref month (- m 1)) (if (and (> m 2) (leap? y)) 1 0))) (defun year-num (y) (let ((d 0)) (if (>= y yzero) (dotimes (i (- y yzero) d) (incf d (year-days (+ yzero i)))) (dotimes (i (- yzero y) (- d)) (incf d (year-days (+ y i))))))) (defun year-days (y) (if (leap? y) ))

Control- Example: Date arithmetic (defun num->date (n) (multiple-value-bind (y left) (num-year n) (multiple-value-bind (m d) (num-month left y) (values d m y)))) (defun num-year (n) (if (< n 0) (do* ((y (- yzero 1) (- y 1)) (d (- (year-days y)) (- d (year-days y)))) ((<= d n) (values y (- n d)))) (do* ((y yzero (+ y 1)) (prev 0 d) (d (year-days y) (+ d (year-days y)))) ((> d n) (values y (- n prev))))))

Control- Example: Date arithmetic (defun num-month (n y) (if (leap? y) (cond ((= n 59) (values 2 29)) ((> n 59) (nmon (- n 1))) (t (nmon n))) (nmon n))) (defun nmon (n) (let ((m (position n month :test #’<))) (values m (+ 1 (- n (svref month (- m 1))))))) (defun date+ (d m y n) (num->date (+ (date->num d m y) n)))

Control- Example: Date arithmetic > (mapcar #’leap? ‘( )) (T NIL T) > (multiple-value-list (date )) ( )

Control Homework (Due April 7) Rewrite month-mon to use case instead of svref Define a single recursive function that returns, as two values, the maximum and minimum elements of a vector