Functional Programming04 Control

Control-BlocksCommon Lisp has 3 basic operators for creating blocks of

codeprognblocktagbody

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”) (+ 11 12) )ab23 -> 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 CDONE

Control-BlocksThe 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-BlocksWithin tagbody, you can use go > (tagbody

(setf x 0) top (setf x (+ x 1)) (format t “~A “ x) (if (< x 10) (go top)))1 2 3 4 5 6 7 8 9 10

This operator is mainly something that other operators are built upon, not something you would use yourself

Ugly code!!

Control-BlocksHow to decide which block construct to use?

Nearly all the time you’ll use prognIf you want to allow for sudden exits, use blockMost programmers will never use tagbody explicitly

Control-Contextlet

Takes a body of codeAllows us to establish variables for use within the body> (let ((x 7)

(y 2)) (format t “Number”) (+ x y))Number9

Control-Context> ((lambda (x) (+ x 1)) 3)

4((lambda (x y)

(format t “Number”) (+ x y))72)

Control-ContextOne 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-ContextA 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 <test> <then form> <else form>)(if <test> <then form>)

(when <test> <then form>)

(if <test> nil <else form>) (unless <test> <else form>)

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- Conditionalscond

(cond (<test 1> <consequent 1-1> …) (<test 2> <consequent 2-1> …) … (<test m> <consequent m-1> …));cond

The conditions are evaluated in order until one of them returns trueWhen 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- Conditionalscase

(case <key form> (<key 1> <consequent 1-1> …) (<key 2> <consequent 2-1> …) ... (<key m> <consequent m-1> …)) ;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 keysNIL

Control- Iterationdo

(do ((<parameter 1> <initial value 1> <update form 1>) (<parameter 2> <initial value 2> <update form 2>) … (<parameter n> <initial value n> <update form n>)) (<termination test> <intermediate forms, if any> <result form>) <body> ) ;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 previousNIL ;value plus 1; y also gets the previous ;value

Control- Iterationdo*

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- Iterationdolist

> (dolist (x ‘(a b c d) ‘done) (format t “~A “ x))A B C DDONE

dotimes> (dotimes (x 5 x) ; for x = 0 to 5-1, return x

(format t “~A “ x))0 1 2 3 4 5

Control- Multiple valuesIn Common Lisp, an expression can return zero or more

valuesE.g., get-decoded-time returns the current time in nine

values value

Returns multiple values> (values ‘a nil (+ 2 4))

ANIL6

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))

6multiple-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-Abortscatch 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

#(0 31 59 90 120 151 181 212 243 273 304 334 365))

(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) 366 365))

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? ‘(1904 1900 1600))

(T NIL T)> (multiple-value-list (date+ 17 12 1997 60))

(15 2 1998)

ControlHomework (Due April 7)

Rewrite month-mon to use case instead of svrefDefine a single recursive function that returns, as two

values, the maximum and minimum elements of a vector