# Chapter 3

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Chapter 3 discussions go here.

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I've worked through almost all of the problems in 3.1. I skipped the problems in 3.1.2 for some reason, all the others are here roughly in order. I'd like to get feedback on them:

(define (make-accumulator init)    (let ((total init))      (lambda (a)             (begin (set! total (+ total a))                    total))))(define (make-monitored func)  (let ((times-called 0))    (lambda (arg)      (if (symbol? arg)          (cond ((eq? 'how-many-calls? arg)                 times-called)                ((eq? 'reset-count arg)                 (set! times-called 0)))          (begin (set! times-called (+ times-called 1))                 (func arg))))))(define (make-account balance password)  (define (withdraw amount)    (if (>= balance amount)        (begin (set! balance (- balance amount))               balance)        "Insufficient funds"))  (define (deposit amount)    (set! balance (+ balance amount))    balance)  (define (dispatch m pass)    (if (eq? pass password)        (cond ((eq? m 'withdraw) withdraw)              ((eq? m 'deposit) deposit)              (else (error "Unknown request -- MAKE-ACCOUNT"                           m)))        (begin (print "Incorrect password")               () )))  dispatch)(define (make-shared-account account original-pass new-pass)  (define (dispatch m pass)    (if (eq? pass new-pass)        (account m original-pass)        "Incorrect password"))  dispatch)(define f  ;f returns the previous number given to it  ;or 0 if it's the first time called  (let ((last-x 0))    (lambda (x)      (let ((a last-x))        (begin (set! last-x x)               a)))))

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Your make-accumulator works correctly. Two small details: the extra 'total' variable is not really needed, you could directly use the argument to keep track of the accumulator's value. Also, you don't need to use 'begin': 'lambda'-forms will let you put multiple statements one after the other without trouble.

(define (make-accumulator value)  (lambda (step)    (set! value (+ value step))    value))

make-monitored: again it works perfectly. You don't really need the separate 'symbol?' check, and you could try to make it work with functions taking 0 or 2+ arguments (although you need 'apply' for that, and I don't think SICP has already mentioned that function).

If you're interested in how to work with variable-length argument lists, here's how:
(define (make-monitored func)  (let ((counter 0))    (lambda args      (define (call-function)        (set! counter (+ 1 counter))        (apply func args))      (if (= (length args) 1)          (let ((arg (car args)))            (cond ((eq? arg 'how-many-calls?)                   counter)                  ((eq? arg 'reset-count)                   (set! counter 0))                  (else                   (call-function))))          (call-function)))))

make-account: and here I was beginning to think there were no comments to be made, but ha! you print "incorrect password" instead of returning the error-string! That's no good.

shared-account: which exercise is this? If I assume that both passwords need to be accepted (otherwise I wouldn't know what would be so shared about the account), then your code doesn't work, as it only accepts the new one.

The 'f'-function: 'let' will let you put multiple statements in its body, without needing an explicit 'begin'.

So, except for a few unneeded 'begin'-statements I don't really have anything wrong to say about your code.

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Thanks for the feedback. I swear that I tried putting multiple lines into functions, but perhaps I only tried it using the (define (func args) ...) style, not with lambda.

shared-account was for Exercise 3.7. I guess I changed the function name. It was supposed to be make-joint (insert bad joke about function name here). Basically, it's supposed to let you access an account via a separate password.

Oh, yes. I have one quick question. I find returning a string when the caller expects a function awkward. The error message produced is unnecessarily long and confusing (complains not just that the password is incorrect but that it's a string, not a function, etc.). Is that just life, or are there some other ways to deal with these types of problems?

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AFAIK, it's possible to put multiple lines in defines, lambda's, lets, conds etc. wherever there's no ambiguity possible (so, not in the if's).

Regarding the string-as-an-error... well, that's why exceptions were invented, so you don't need to check the return value each time. But I don't think Scheme offers exceptions. You could also use the 'error'-function (however, it looks like R5RS doesn't guarantee the existence of this function, but it works in DrScheme).
Another way of handling it is what kind of reminds me of the way monads would solve it (approximately): instead of returning a string, you return a dummy object with an interface compatible with that of a valid result, and that dummy error-object would just "swallow" all messages. After multiple operations, you could check if the end result is valid or not. However, this approach only works when programming in functional style. To illustrate:

;; (lambda args <body>) accepts any number of arguments;; The arguments are placed in a list bound to the symbol 'args'(define undefined-result  (lambda args undefined-result))(define (is-undefined-result? obj)  (eq? undefined-result obj))(define (make-vector2d x y)  (lambda (arg)    (case arg      ((x) x)      ((y) y))))(define (vector+ u v)  (if (or (is-undefined-result? u)          (is-undefined-result? v))      undefined-result      (make-vector2d (+ (u 'x) (v 'x))                     (+ (u 'y) (v 'y))))) (define (vector/ v f)  (if (or (is-undefined-result? v)          (= f 0))      undefined-result      (make-vector2d (/ (v 'x) f)                     (/ (v 'y) f))))

I guess you could make this a bit more elegant, e.g. by using macros, but that's the idea behind it.

[Edited by - SamLowry on April 2, 2007 5:50:03 PM]

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Quote:
 Original post by EzbezOh, yes. I have one quick question. I find returning a string when the caller expects a function awkward. The error message produced is unnecessarily long and confusing (complains not just that the password is incorrect but that it's a string, not a function, etc.). Is that just life, or are there some other ways to deal with these types of problems?

In this case, returning a function that returns a string is sufficient.

error isn't in the standard, but it is specified in SRFI-23. Using it is a safe option.

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Here's my 3.1 answers. Moderate testing done, but I make no guarantees. [wink]

>>> Scroll down <<<;; A bit of syntax for testing an expression;; against the expected result.(define-syntax test  (syntax-rules ()    ((test expr result)     (let ((actual expr))       (unless (equal? actual result)         (display "Failed test: ")         (print 'expr)         (display "   Expected: ")         (print 'result)         (display "        Got: ")         (print actual))))));; 3.1(define (make-accumulator value)  (define (accumulate addend)    (set! value (+ value addend))    value)  accumulate)(define A (make-accumulator 5))(test (A 10) 15)(test (A 10) 25);; 3.2(define (make-monitored fn)  (let ((calls 0))    (define (monitor . arguments)      (case (car arguments)        ((how-many-calls?)         calls)        ((reset!)         (set! calls 0))        (else         (set! calls (+ calls 1))         (apply fn arguments))))      monitor))(define s (make-monitored sqrt))(test (s 100) 10.0)(test (s 'how-many-calls?) 1);; 3.3(define (make-account balance actual-password)  (define (withdraw amount)    (if (>= balance amount)        (begin (set! balance (- balance amount))               balance)        "Insufficient funds"))  (define (deposit amount)    (set! balance (+ balance amount))    balance)  (define (mutter . ignore)    "Incorrect password")  (define (dispatch password m)    (cond ((not (eq? password actual-password))           mutter)          ((eq? m 'withdraw)           withdraw)          ((eq? m 'deposit)           deposit)          (else (error "Unknown request -- MAKE-ACCOUNT"                       m))))  dispatch)(define acc (make-account 100 'secret-password))(test ((acc 'secret-password 'withdraw) 40) 60)(test ((acc 'some-other-password 'deposit) 50) "Incorrect password");; 3.4(define (call-the-cops)  )(define (make-account balance actual-password)  (let ((failures 0))    (define (withdraw amount)      (if (>= balance amount)          (begin (set! balance (- balance amount))                 balance)          "Insufficient funds"))    (define (deposit amount)      (set! balance (+ balance amount))      balance)    (define (mutter . ignore)      (set! failures (+ failures 1))      (if (< 7 failures)          (begin            (call-the-cops)            "Cops called")          "Incorrect password"))    (define (dispatch password m)      (cond ((or (< 7 failures)                 (not (eq? password actual-password)))             mutter)            ((eq? m 'withdraw)             withdraw)            ((eq? m 'deposit)             deposit)            (else             (error "Unknown request -- MAKE-ACCOUNT"                    m))))    dispatch))(define acc (make-account 100 'secret-password))(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Incorrect password")(test ((acc 'some-other-password 'withdraw) 40) "Cops called")(test ((acc 'secret-password 'withdraw) 40) "Cops called");; 3.5;; SRFI-27 provides a random number generator.;; It boggles the mind that a PRNG doesn't come as standard.(use srfi-27)(define (random range)  (* (random-real) range))(random-source-randomize! default-random-source)(define (monte-carlo trials experiment)  (define (iter trials-remaining trials-passed)    (cond ((= trials-remaining 0)           (/ trials-passed trials))          ((experiment)           (iter (- trials-remaining 1) (+ trials-passed 1)))          (else           (iter (- trials-remaining 1) trials-passed))))  (iter trials 0))(define (random-in-range low high)  (let ((range (- high low)))    (+ low (random range))))(define (estimate-integral p x1 x2 y1 y2 trials)  (define (experiment)    (let ((x (random-in-range x1 x2))          (y (random-in-range y1 y2)))      (p x y)))  (monte-carlo trials experiment))(define (point-in-circle x y)  (>= 1.0 (sqrt (+ (* x x) (* y y)))));; We can't (test) this, since the result is random.(print (* 4.0 (estimate-integral point-in-circle -1 1 -1 1 1000)));; 3.6(define rand  (let ((rs (make-random-source)))    (lambda (action)      (case action        ((generate)         ((random-source-make-reals rs)))        ((reset)         (lambda (new-value)           (random-source-pseudo-randomize! rs 0 new-value)))))));; 3.7(define (make-joint account old-password new-password)  (let ((failures 0))    (define (mutter . ignore)      (set! failures (+ failures 1))      (if (< 7 failures)          (begin            (call-the-cops)            "Cops called")          "Incorrect password"))    (define (dispatch password m)      (if (or (< 7 failures)              (not (eq? password new-password)))          mutter          (account old-password m)))    dispatch))(define peter-acc (make-account 100 'open-sesame))(define paul-acc  (make-joint peter-acc 'open-sesame 'rosebud))(test ((paul-acc 'rosebud 'withdraw) 10) 90);; 3.8(define f  (let ((v 1))    (define (f n)      (set! v (* v n))      v)    f))(print (+ (f 0) (f 1)))

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Ok, I managed to get to section 3.3 (yay!)
I'm currently on 3.16

Exercise 3.16.  Ben Bitdiddle decides to write a procedure to count the number of pairs in any list structure. It's easy,'' he reasons. The number of pairs in any structure is the number in the car plus the number in the cdr plus one more to count the current pair.'' So Ben writes the following procedure:(define (count-pairs x)  (if (not (pair? x))      0      (+ (count-pairs (car x))         (count-pairs (cdr x))         1)))Show that this procedure is not correct. In particular, draw box-and-pointer diagrams representing list structures made up of exactly three pairs for which Ben's procedure would return 3; return 4; return 7; never return at all.

This one was somewhat confusing, so I'd just like to confirm my answers to see if its what I was supposed to do.

[source lang='lisp']For 3...(define a (list 'a 'b 'c))simpleFor never returning(define x (list 'a 'b 'c))(set-cdr! (cddr x) x)For 4(define y (list 'a 'b 'c))(set-car! y (cddr y))and for seven...(define v (list 'a 'b 'c))(set-car! (cdr v) (cddr v))(set-car! v (cdr v))

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Quote:
 Original post by GameDev DoctorOk, I managed to get to section 3.3 (yay!)I'm currently on 3.16*** Source Snippet Removed ***This one was somewhat confusing, so I'd just like to confirm my answers to see if its what I was supposed to do.*** Source Snippet Removed ***

I haven't checked thoroughly, but that's how I would've proceeded too.

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Time to get feedback on 3.17 to 3.19, and confirm that my answer to 3.19 is indeed in constant space.
;initems? is used for both 3.17 and 3.18(define (initems? x listpart);tests if x is eq? to any pair in a       (cond ((null? listpart);list of pairs             #f)            ((eq? (car listpart) x)             #t)            (else             (initems? x (cdr listpart)))));3.17(define count-pairs  (let ((items '()))    (define (count x);1 check if x eq? anything in items 2 add stuff      (if (or (not (pair? x)) (initems? x items))          0          (begin (set! items (cons x items)) (+ 1 (count (car x)) (count (cdr x))))))    (lambda (x) (begin (set! items '()) (count x)))));3.18(define (is-cycle? x)  (define (test-cycle x sofar)    (cond ((initems? x sofar)            #t)           ((not (pair? x))            #f)           (else (test-cycle (cdr x) (cons x sofar)))    ))  (test-cycle x '())  );3.19(define (cycle? x)  (define (cycletest items end distance)    (define (cycletest-iter start end distance)      (cond        ((= distance 0)         #f)        ((eq? start end)         #t)        (else (cycletest-iter (cdr start) end (- distance 1)))      ))    (cond ((not (pair? end))          #f)          ((cycletest-iter items end distance)           #t) ;one is found in range          (else           (cycletest items (cdr end) (+ distance 1)))               ))  (if (not (pair? x))      #f      (cycletest x (cdr x) 1))   )

The implementation of count-pairs is a bit wierd, with the lambda at the end, but I had trouble making items reset in subsequent calls.

For 3.19, the algorithm takes increasing chunks of the list, checking if each part of that chunk is eq? to the last member of that chunk until it can verify that the list is or is not a cycle. (it does not, of course, check car, just cdr)

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