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• Writing lambda functions in Scala

  - by user2433237

  I'm aware that you can write anonymous functions in Scala but I'm having trouble trying to convert a piece of code from Scheme. Could anyone help me convert this to Scala? (define apply-env (lambda (env search-sym) (cases environment env (empty-env () (eopl:error 'apply-env "No binding for ~s" search-sym)) (extend-env (var val saved-env) (if (eqv? search-sym var) val (apply-env saved-env search-sym))) (extend-env-rec (p-name b-var p-body saved-env) (if (eqv? search-sym p-name) (proc-val (procedure b-var p-body env)) (apply-env saved-env search-sym)))))) Thanks in advance

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• Help with dynamic-wind and call/cc

  - by josh

  I am having some trouble understanding the behavior of the following Scheme program: (define c (dynamic-wind (lambda () (display 'IN)(newline)) (lambda () (call/cc (lambda (k) (display 'X)(newline) k))) (lambda () (display 'OUT)(newline)))) As I understand, c will be bound to the continution created right before "(display 'X)". But using c seems to modify itself! The define above prints (as I expected) IN, X and OUT: IN X OUT And it is a procedure: #;2> c #<procedure (a9869 . results1678)> Now, I would expect that when it is called again, X would be printed, and it is not! #;3> (c) IN OUT And now c is not a procedure anymore, and a second invokation of c won't work! #;4> c ;; the REPL doesn't answer this, so there are no values returned #;5> (c) Error: call of non-procedure: #<unspecified> Call history: <syntax> (c) <eval> (c) <-- I was expecting that each invokation to (c) would do the same thing -- print IN, X, and OUT. What am I missing?

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• Please explain some of Paul Graham's points on LISP

  - by kunjaan

  I need some help understanding some of the points from Paul Graham's article http://www.paulgraham.com/diff.html A new concept of variables. In Lisp, all variables are effectively pointers. Values are what have types, not variables, and assigning or binding variables means copying pointers, not what they point to. A symbol type. Symbols differ from strings in that you can test equality by comparing a pointer. A notation for code using trees of symbols. The whole language always available. There is no real distinction between read-time, compile-time, and runtime. You can compile or run code while reading, read or run code while compiling, and read or compile code at runtime. What do these points mean How are they different in languages like C or Java? Do any other languages other than LISP family languages have any of these constructs now?

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• (1 2 3 . #<void>)- heapsort

  - by superguay

  Hello everybody: I tried to implement a "pairing heap" with all the regular operations (merge, delete-min etc.), then I've been requested to write a function that would sort a list using my newly constructed heap implementation. Unfortunately it seems that someting goes wrong... Here's the relevant code: (define (heap-merge h1 h2) (cond ((heap-empty? h1) h2) ((heap-empty? h2) h1) (else (let ((min1 (heap-get-min h1)) (min2 (heap-get-min h2))) (if ((heap-get-less h1) min1 min2) (make-pairing-heap (heap-get-less h1) min1 (cons h2 (heap-get-subheaps h1))) (make-pairing-heap (heap-get-less h1) min2 (cons h1 (heap-get-subheaps h2)))))))) (define (heap-insert element h) (heap-merge (make-pairing-heap (heap-get-less h) element '()) h)) (define (heap-delete-min h) (define (merge-in-pairs less? subheaps) (cond ((null? subheaps) (make-heap less?)) ((null? (cdr subheaps)) (car subheaps)) (else (heap-merge (heap-merge (car subheaps) (cadr subheaps)) (merge-in-pairs less? (cddr subheaps)))))) (if (heap-empty? h) (error "expected pairing-heap for first argument, got an empty heap ") (merge-in-pairs (heap-get-less h) (heap-get-subheaps h)))) (define (heapsort l less?) (let aux ((h (accumulate heap-insert (make-heap less?) l))) (if (not (heap-empty? h)) (cons (heap-get-min h) (aux (heap-delete-min h)))))) And these are some selectors that may help you to understand the code: (define (make-pairing-heap less? min subheaps) (cons less? (cons min subheaps))) (define (make-heap less?) (cons less? '())) (define (heap-get-less h) (car h)) (define (heap-empty? h) (if (null? (cdr h)) #t #f)) Now lets get to the problem: When i run 'heapsort' it returns the sorted list with "void", as you can see: (heapsort (list 1 2 3) <) (1 2 3 . #)..HOW CAN I FIX IT? Regards, Superguay

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• async handler deleted by the wrong thread in django

  - by user3480706

  I'm run this algorithm in my django application.when i run several time from my GUI django local server will stopped and i got this error Exception RuntimeError: RuntimeError('main thread is not in main loop',) in ignored Tcl_AsyncDelete: async handler deleted by the wrong thread Aborted (core dumped) code print "Learning the sin function" network =MLP.MLP(2,10,1) samples = np.zeros(2000, dtype=[('x', float, 1), ('y', float, 1)]) samples['x'] = np.linspace(-5,5,2000) samples['y'] = np.sin(samples['x']) #samples['y'] = np.linspace(-4,4,2500) for i in range(100000): n = np.random.randint(samples.size) network.propagate_forward(samples['x'][n]) network.propagate_backward(samples['y'][n]) plt.figure(figsize=(10,5)) # Draw real function x = samples['x'] y = samples['y'] #x=np.linspace(-6.0,7.0,50) plt.plot(x,y,color='b',lw=1) samples1 = np.zeros(2000, dtype=[('x1', float, 1), ('y1', float, 1)]) samples1['x1'] = np.linspace(-4,4,2000) samples1['y1'] = np.sin(samples1['x1']) # Draw network approximated function for i in range(samples1.size): samples1['y1'][i] = network.propagate_forward(samples1['x1'][i]) plt.plot(samples1['x1'],samples1['y1'],color='r',lw=3) plt.axis([-2,2,-2,2]) plt.show() plt.close() return HttpResponseRedirect('/charts/charts') how can i fix this error ?need a quick help

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• How to convert RGB to BGR?

  - by Adam

  This is probably easy, but I'm trying to convert from a source which provides colors in RGB strings to an output in BGR strings in Java. I've been busting my brain and time on shifting and Long.decode and Long.toHexString. Feel free to also throw alpha values in there (RGBA - ABGR), though I think I can extend the principles.

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• Override colorscheme

  - by RymdPung

  I often find myself wanting to change just something little in a colorscheme, but i don't want to edit the original file. I tried putting my change in '~/.vim/after/colors/blah.vim', but that doesn't work for me. Example, I want to change the CursorLine highlight in BusyBee.vim.. ~/.vim/colors/BusyBee.vim I create the file '~/.vim/after/colors/BusyBee.vim' and add this: hi CursorLine guibg=#000000 ctermbg=Black cterm=none However, i don't see the change. Of course it works if i change the line in the originial BusyBee.vim, but like i said i'd prefer not to do that. Doing... :colo Busy<TAB> Shows me... BusyBee BusyBee

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• DrScheme versus mzscheme: treatment of definitions

  - by speciousfool

  One long term project I have is working through all the exercises of SICP. I noticed something a bit odd with the most recent exercise. I am testing a Huffman encoding tree. When I execute the following code in DrScheme I get the expected result: (a d a b b c a) However, if I execute this same code in mzscheme by calling (load "2.67.scm") or by running mzscheme -f 2.67.scm, it reports: symbols: expected symbols as arguments, given: (leaf D 1) My question is: why? Is it because mzscheme and drscheme use different rules for loading program definitions? The program code is below. ;; Define an encoding tree and a sample message ;; Use the decode procedure to decode the message, and give the result. (define (make-leaf symbol weight) (list 'leaf symbol weight)) (define (leaf? object) (eq? (car object) 'leaf)) (define (symbol-leaf x) (cadr x)) (define (weight-leaf x) (caddr x)) (define (make-code-tree left right) (list left right (append (symbols left) (symbols right)) (+ (weight left) (weight right)))) (define (left-branch tree) (car tree)) (define (right-branch tree) (cadr tree)) (define (symbols tree) (if (leaf? tree) (list (symbol-leaf tree)) (caddr tree))) (define (weight tree) (if (leaf? tree) (weight-leaf tree) (cadddr tree))) (define (decode bits tree) (define (decode-1 bits current-branch) (if (null? bits) '() (let ((next-branch (choose-branch (car bits) current-branch))) (if (leaf? next-branch) (cons (symbol-leaf next-branch) (decode-1 (cdr bits) tree)) (decode-1 (cdr bits) next-branch))))) (decode-1 bits tree)) (define (choose-branch bit branch) (cond ((= bit 0) (left-branch branch)) ((= bit 1) (right-branch branch)) (else (error "bad bit -- CHOOSE-BRANCH" bit)))) (define (test s-exp) (display s-exp) (newline)) (define sample-tree (make-code-tree (make-leaf 'A 4) (make-code-tree (make-leaf 'B 2) (make-code-tree (make-leaf 'D 1) (make-leaf 'C 1))))) (define sample-message '(0 1 1 0 0 1 0 1 0 1 1 1 0)) (test (decode sample-message sample-tree))

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• handleOpenURL not called using a custom url schema in iPhone OS

  - by favo

  Hi, I have successfuly added my own url schemes to my App. The App correctly launches using the schemes. Now I want to handle the incoming data but the delegate is not called. It is an universal app and I have added the following function to both AppDelegates: - (BOOL)application:(UIApplication *)application handleOpenURL:(NSURL *)url { if (!url) { return NO; } NSString *URLString = [url absoluteString]; UIAlertView *alert = [[UIAlertView alloc] initWithTitle:NSLocalizedString(@"test message", nil) message:URLString delegate:self cancelButtonTitle:nil otherButtonTitles:@"OK", nil]; [alert show]; [alert release]; return YES; } I am testing with a schema like: myapp://appalarm.com …and would expect to be appalarm.com in the URLString What is wrong with it? Thanks for your responses!

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• Looking for examples of "real" uses of continuations

  - by Sébastien RoccaSerra

  I'm trying to grasp the concept of continuations and I found several small teaching examples like this one from the Wikipedia article: (define the-continuation #f) (define (test) (let ((i 0)) ; call/cc calls its first function argument, passing ; a continuation variable representing this point in ; the program as the argument to that function. ; ; In this case, the function argument assigns that ; continuation to the variable the-continuation. ; (call/cc (lambda (k) (set! the-continuation k))) ; ; The next time the-continuation is called, we start here. (set! i (+ i 1)) i)) I understand what this little function does, but I can't see any obvious application of it. While I don't expect to use continuations all over my code anytime soon, I wish I knew a few cases where they can be appropriate. So I'm looking for more explicitely usefull code samples of what continuations can offer me as a programmer. Cheers!

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• Racket list in struct

  - by Tim

  I just started programming with Racket and now I have the following problem. I have a struct with a list and I have to add up all prices in the list. (define-struct item (name category price)) (define some-items (list (make-item "Book1" 'Book 40.97) (make-item "Book2" 'Book 5.99) (make-item "Book3" 'Book 20.60) (make-item "Item" 'KitchenAccessory 2669.90))) I know that I can return the price with: (item-price (first some-items)) or (item-price (car some-items)). The problem is, that I dont know how I can add up all Items prices with this. Answer to Óscar López: May i filled the blanks not correctly, but Racket mark the code black when I press start and don't return anything. (define (add-prices items) (if (null? items) (+ 0 items) ; Here I don't really know what to write for a 0. ; I tried differnt thnigs like null and this version. (+ (item-price (first some-items)) (add-prices (item-price (rest some-items))))))

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• Inserting mutable pairs into a mutable list

  - by Romelus

  How can I push a mutable pair onto a stack such that i'm only creating one stack. I have some code that works but creates lists within lists within lists.... Here is what I believe should work but throws an error. (define func (arg1 arg2 arg3) // Where arg3 is an empty list (mappend (mcons arg1 arg2) arg3)) The above code complains and says: "mcar: expects argument of type ; given ... Can anyone show me how I can get a result that looks like so,: (list (arg1 arg2) (arg# arg#) ...)

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• SICP making change

  - by RyanD

  So; I'm a hobbiest who's trying to work through SICP (it's free!) and there is an example procedure in the first chapter that is meant to count the possible ways to make change with american coins; (change-maker 100) = 292. It's implemented something like: (define (change-maker amount) (define (coin-value n) (cond ((= n 1) 1) ((= n 2) 5) ((= n 3) 10) ((= n 4) 25) ((= n 5) 50))) (define (iter amount coin-type) (cond ((= amount 0) 1) ((or (= coin-type 0) (< amount 0)) 0) (else (+ (iter amount (- coin-type 1)) (iter (- amount (coin-value coin-type)) coin-type))))) (iter amount 5)) Anyway; this is a tree-recursive procedure, and the author "leaves as a challenge" finding an iterative procedure to solve the same problem (ie fixed space). I have not had luck figuring this out or finding an answer after getting frustrated. I'm wondering if it's a brain fart on my part, or if the author's screwing with me.

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• Vector addition of lists

  - by ntimes

  If I had a N lists each of length M, how could I write a nice clean function to return a single list of length M, where each element is the sum of the corresponding elements in the N lists? (starting to learn lisp - go easy!)

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• Choosing colour schemes

  - by DanDan

  How do you choose your colour schemes for your applications and/or web designs? Is it a gut instinct thing or can logic be applied here too? I have looked at some colour theory but my combinations seemed wrong. I am looking at a monochrome webpage. Rather than pluck colours out of the air as usual I would like to see if there is a science behind this. Links and opinions welcome.

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• What is the best color combination for readability, easy of use, and reduced eye strain?

  - by Nick Berardi

  I am trying to pick out the optimal set of colors for a new website project. I want to do a traditional black on white look and feel for the main content. However my partner on the project wants to do a color combination that more looks like the traditional Windows Forms look and feel. Is there any research available on the best color combination's to use for readability, ease of use, and reduced eye strain?

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• Lisp Web Frameworks?

  - by jsight

  What are the popular (ok, popular is relative) web frameworks for the various flavours of LISP?

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• How to parse out base file name using Script-Fu

  - by ongle

  Using Gimp 2.6.6 for MAC OS X (under X11) as downloaded from gimp.org. I'm trying to automate a boring manual process with Script-Fu. I needed to parse the image file name to save off various layers as new files using a suffix on the original file name. My original attempts went like this but failed because (string-search ...) doesn't seem to be available under 2.6 (a change to the scripting engine?). (set! basefilename (substring filename 0 (string-search "." filename))) Then I tried to use this information to parse out the base file name using regex but (re-match-nth ...) is not recognized either. (if (re-match "^(.*)[.]([^.]+)$" filename buffer) (set! basefilename (re-match-nth orig-name buffer 1)) ) And while pulling the value out of the vector ran without error, the resulting value is not considered a string when it is passed into (string-append ...). (if (re-match "^(.*)[.]([^.]+)$" filename buffer) (set! basefilename (vector-ref buffer 1)) ) So I guess my question is, how would I parse out the base file name?

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• How Can I Improve This Algorithm (LCS)

  - by superguay

  (define (lcs lst1 lst2) (define (except-last-pair list) (if (pair? (cdr list)) (cons (car list) (except-last-pair (cdr list))) '())) (define (car-last-pair list) (if (pair? (cdr list)) (car-last-pair (cdr list)) (car list))) (if (or (null? lst1) (null? lst2)) null (if (= (car-last-pair lst1) (car-last-pair lst2)) (append (lcs (except-last-pair lst1) (except-last-pair lst2)) (cons (car-last-pair lst1) '())) **(if (> (length (lcs lst1 (except-last-pair lst2))) (length (lcs lst2 (except-last-pair lst1)))) (lcs lst1 (except-last-pair lst2)) (lcs lst2 (except-last-pair lst1)))))) I dont want it to run over and over.. Regards, Superguay

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• Day dreaming about automatic RGB color function in PHP

  - by Sam

  Hello fellow earthlings. A quesion about RGB color and its usefulness in a simple tiny php code: Imagine I have variable $colorA containning a valid six char color. say B1B100, a greenish natural color. Now If I would like to make a new color from that, which is, say, ten steps lighter thatn that original color, roughly. $colorA = B1B100 // original color php code with little color engine lightening stuff up goes here $colorB = ?????? // original color lightened up Is there a php ready function that KNOWS rgb colors something like php function RGB ( input color, what to do, output color) Where what to do could be +/- 255 values of brightness etc etc. Is something like this already possible or am I day dreaming? rgb($colorA, +10, $colorB) If this does not exist, what would be the shortest code for doing this? Link suffices. Suggestions and ideas are answers to me. Thanks.

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• Equivalence Classes

  - by orcik

  I need to write a program for equivalence classes and get this outputs... (equiv '((a b) (a c) (d e) (e f) (c g) (g h))) => ((a b c g h) (d e f)) (equiv '((a b) (c d) (e f) (f g) (a e))) => ((a b e f g) (c d)) Basically, A set is a list in which the order doesn't matter, but elements don't appear more than once. The function should accept a list of pairs (elements which are related according to some equivalence relation), and return a set of equivalence classes without using iteration or assignment statements (e.g. do, set!, etc.). However, set utilities such as set-intersection, set-union and a function which eliminates duplicates in a list and built-in functions union, intersection, and remove-duplicates are allowed. Thanks a lot! By the way, It's not a homework question. A friend of mine need this piece of code to solve smilar questions.

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• How to decide which colors to use that look most similar across most screens/monitors?

  - by Lyon

  Hi, I'm baffled. I'm trying to find suitable colors for a logo that would look similar across most monitors/screens. I know it's near impossible, but how does one end up with the color palette that new Google logo and Facebook "blue" uses for example? I've a monitor that has been calibrated, and a few laptop screens that have default settings. Yet both the Google logo and facebook's look similar (although they aren't using colors restricted to the web safe 216 palette. Any ideas? thoughts? Thanks

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• Java style FOR loop in a clojure interpeter ?

  - by Kevin

  I have a basic interpreter in clojure. Now i need to implement for (initialisation; finish-test; loop-update) { statements } inside my interpreter. I will attach my interpreter code I got so far. Any help is appreciated. Interpreter (declare interpret make-env) ;; (def do-trace false) ;; ;; simple utilities (def third ; return third item in a list (fn [a-list] (second (rest a-list)))) (def fourth ; return fourth item in a list (fn [a-list] (third (rest a-list)))) (def run ; make it easy to test the interpreter (fn [e] (println "Processing: " e) (println "=> " (interpret e (make-env))))) ;; for the environment (def make-env (fn [] '())) (def add-var (fn [env var val] (cons (list var val) env))) (def lookup-var (fn [env var] (cond (empty? env) 'error (= (first (first env)) var) (second (first env)) :else (lookup-var (rest env) var)))) ;; -- define numbers (def is-number? (fn [expn] (number? expn))) (def interpret-number (fn [expn env] expn)) ;; -- define symbols (def is-symbol? (fn [expn] (symbol? expn))) (def interpret-symbol (fn [expn env] (lookup-var env expn))) ;; -- define boolean (def is-boolean? (fn [expn] (or (= expn 'true) (= expn 'false)))) (def interpret-boolean (fn [expn env] expn)) ;; -- define functions (def is-function? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'lambda (first expn))))) (def interpret-function (fn [expn env] expn)) ;; -- define addition (def is-plus? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '+ (first expn))))) (def interpret-plus (fn [expn env] (+ (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define subtraction (def is-minus? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '- (first expn))))) (def interpret-minus (fn [expn env] (- (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define multiplication (def is-times? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '* (first expn))))) (def interpret-times (fn [expn env] (* (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define division (def is-divides? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '/ (first expn))))) (def interpret-divides (fn [expn env] (/ (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define equals test (def is-equals? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '= (first expn))))) (def interpret-equals (fn [expn env] (= (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define greater-than test (def is-greater-than? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '> (first expn))))) (def interpret-greater-than (fn [expn env] (> (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define not (def is-not? (fn [expn] (and (list? expn) (= 2 (count expn)) (= 'not (first expn))))) (def interpret-not (fn [expn env] (not (interpret (second expn) env)))) ;; -- define or (def is-or? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'or (first expn))))) (def interpret-or (fn [expn env] (or (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define and (def is-and? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'and (first expn))))) (def interpret-and (fn [expn env] (and (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define with (def is-with? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'with (first expn))))) (def interpret-with (fn [expn env] (interpret (third expn) (add-var env (first (second expn)) (interpret (second (second expn)) env))))) ;; -- define if (def is-if? (fn [expn] (and (list? expn) (= 4 (count expn)) (= 'if (first expn))))) (def interpret-if (fn [expn env] (cond (interpret (second expn) env) (interpret (third expn) env) :else (interpret (fourth expn) env)))) ;; -- define function-application (def is-function-application? (fn [expn env] (and (list? expn) (= 2 (count expn)) (is-function? (interpret (first expn) env))))) (def interpret-function-application (fn [expn env] (let [function (interpret (first expn) env)] (interpret (third function) (add-var env (first (second function)) (interpret (second expn) env)))))) ;; the interpreter itself (def interpret (fn [expn env] (cond do-trace (println "Interpret is processing: " expn)) (cond ; basic values (is-number? expn) (interpret-number expn env) (is-symbol? expn) (interpret-symbol expn env) (is-boolean? expn) (interpret-boolean expn env) (is-function? expn) (interpret-function expn env) ; built-in functions (is-plus? expn) (interpret-plus expn env) (is-minus? expn) (interpret-minus expn env) (is-times? expn) (interpret-times expn env) (is-divides? expn) (interpret-divides expn env) (is-equals? expn) (interpret-equals expn env) (is-greater-than? expn) (interpret-greater-than expn env) (is-not? expn) (interpret-not expn env) (is-or? expn) (interpret-or expn env) (is-and? expn) (interpret-and expn env) ; special syntax (is-with? expn) (interpret-with expn env) (is-if? expn) (interpret-if expn env) ; functions (is-function-application? expn env) (interpret-function-application expn env) :else 'error)))

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• Fermat factorization method limit

  - by Fakrudeen

  I am trying to implement Fermat's factorization [Algorithm C in Art of computer programming Vol. 2]. Unfortunately in my edition [ISBN 81-7758-335-2], this algorithm is printed incorrectly. what should be the condition on factor-inner loop below? I am running the loop till y <= n [passed in as limit]. (if (< limit y) 0 (factor-inner x (+ y 2) (- r y) limit)) Is there anyway to avoid this condition altogether, as it will double the speed of loop? (define (factor n) (let ( ( square-root (inexact->exact (floor (sqrt n))) ) ) (factor-inner (+ (* 2 square-root) 1) 1 (- (* square-root square-root) n) n) ) ) (define (factor-inner x y r limit) (if (= r 0) (/ (- x y) 2) (begin (display x)(display " ")(display y)(display " ")(display r)(newline) ;(sleep-current-thread 1) (if (< r 0) (factor-inner (+ x 2) y (+ r x) limit) (if (< limit y) 0 (factor-inner x (+ y 2) (- r y) limit)) ) ) ) )

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• Help me write my LISP :) LISP environments, Ruby Hashes...

  - by MikeC8

  I'm implementing a rudimentary version of LISP in Ruby just in order to familiarize myself with some concepts. I'm basing my implementation off of Peter Norvig's Lispy (http://norvig.com/lispy.html). There's something I'm missing here though, and I'd appreciate some help... He subclasses Python's dict as follows: class Env(dict): "An environment: a dict of {'var':val} pairs, with an outer Env." def __init__(self, parms=(), args=(), outer=None): self.update(zip(parms,args)) self.outer = outer def find(self, var): "Find the innermost Env where var appears." return self if var in self else self.outer.find(var) He then goes on to explain why he does this rather than just using a dict. However, for some reason, his explanation keeps passing in through my eyes and out through the back of my head. Why not use a dict, and then inside the eval function, when a new "sub-environment" needs to be created, just take the existing dict and update the key/value pairs that need to be updated, and pass that new dict into the next eval? Won't the Python interpreter keep track of the previous "outer" envs? And won't the nature of the recursion ensure that the values are pulled out from "inner" to "outer"? I'm using Ruby, and I tried to implement things this way. Something's not working though, and it might be because of this, or perhaps not. Here's my eval function, env being a regular Hash: def eval(x, env = $global_env) ........ elsif x[0] == "lambda" then ->(*args) { eval(x[2], env.merge(Hash[*x[1].zip(args).flatten(1)])) } ........ end The line that matters of course is the "lambda" one. If there is a difference, what's importantly different between what I'm doing here and what Norvig did with his Env class? If there's no difference, then perhaps someone can enlighten me as to why Norvig uses the Env class. Thanks :)

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