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• Scheme accumulative recursion with lists

  - by John Retallack

  How can I pass a list as a parameter to a function adding elements to it recursively,and have it unmodified when it comes out of recursion?

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• Scheme redefine a list...

  - by octavio

  I have a list called hand and another one called deck, the main goal here is to take the first card (or element ) in the list deck and put it in the list hand when i call the fnction draw... (draw hand deck) (2 C) (draw hand deck) (2 C) (3 H) (draw hand deck) (2 C) (3 H) (K D) but everytime i call it the hand never changes value... I'm clueless is there a way like in O-Object to change the content of hand permenantly? and i initialy define hand empty because the player has no card to start. (define hand '())

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• Scheme homework problem, need help.

  - by poorStudent

  Okay one of my homework problems is to take a list of lists and return the car of each sublist as a list. I have it to where I can print out the values, but it's not a list. To be honest, I have no clue how to output lists. Here's what I got: (define (car-print alist) (if (null? alist) (newline) (begin (write (car (car alist))) (display " ") (car-print(cdr alist))))) This is a cheap way to do it, perhaps some help on actually solving this problem would be much appreciated. Not necessarily the full answer but steps to get there. Thanks.

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• scheme struct question

  - by qzar

  ;; definition of the structure "book" ;; author: string - the author of the book ;; title: string - the title of the book ;; genre: symbol - the genre (define-struct book (author title genre)) (define lotr1 (make-book "John R. R. Tolkien" "The Fellowship of the Ring" 'Fantasy)) (define glory (make-book "David Brin" "Glory Season" 'ScienceFiction)) (define firstFamily (make-book "David Baldacci" "First Family" 'Thriller)) (define some-books (list lotr1 glory firstFamily)) ;; count-books-for-genre: symbol (list of books) -> number ;; the procedure takes a symbol and a list of books and produces the number ;; of books from the given symbol and genre ;; example: (count-books-for-genre 'Fantasy some-books) should produce 1 (define (count-books-for-genre genre lob) (if (empty? lob) 0 (if (symbol=? (book-genre (first lob)) genre) (+ 1 (count-books-for-genre (rest lob) genre)) (count-books-for-genre (rest lob) genre) ) ) ) (count-books-for-genre 'Fantasy some-books) It produce following exception first: expected argument of type non-empty list; given 'Fantasy, I don't understand whats the problem. Can somebody give me some explanation ? Thank you very much !

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• Getting the median of 3 values using scheme's

  - by kristian Roger

  The problem this time is to get the median of three values (easy) I did this: (define (med x y z) (car(cdr(x y z))) and it was accepted but when testing it: (med 3 4 5) I get this error: Error: attempt to call a non-procedure (2 3 4) And when entering letters instead of number i get: (md x y z) Error: undefined varia y (package user) Using something besides x y z I get: (md d l m) Error: undefined variable d (package user) the question was deleted dont know how anyway write a function that return the median of 3 values Sorry for editing the question I got that I should put the values in order first not just a sill car and cdr thing so I did so 33> (define (med x y z) (if(and( (<x y) (<y z) y if(and( (<y x) (<x z) x z))))) Warning: invalid expression (if (and< (<x y) (<y z) y if (and ((<y x) (<x z) x z)))) but as u see Im getting a warning so what is wronge ?

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• Scheme sorting a list

  - by John

  Okay so I am trying to take in a list and sort it from greatest to smallest. Example: > (maxheap (list 5 6 2 1 18 7)) ;output: > (18 7 6 5 2 1) So here's what I got so far: (define (mkmaxheap heaplist) (let ((max (mymax(heaplist)))) ;mymax is a func that returns max number, it works (let (( head (car heaplist)) (tail (cdr heaplist))) (if (null? tail) newlist)))) Thats all I could get to compile, all the other code I wrote failed. Any help on solving this would be much appreciated.

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• Currying a function n times in Scheme

  - by user1724421

  I'm having trouble figuring out a way to curry a function a specified number of times. That is, I give the function a natural number n and a function fun, and it curries the function n times. For example: (curry n fun) Is the function and a possible application would be: (((((curry 4 +) 1) 2) 3) 4) Which would produce 10. I'm really not sure how to implement it properly. Could someone please give me a hand? Thanks :)

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• How to display data uri scheme into a C# WebBrowser Controler

  - by Emanuel

  How can I show an image base64 encoded using WebBrowser control in C#? I used the following code: <img src="data:image/gif;base64,/9j/4AAQSkZJRgABAgAAZABkAA7AAR R894ADkFkb2JlAGTAAAAAAfbAIQABAMDAwMDBAMDBAYEAwQGBwUEBAUHCAYGBw ... uhWkvoJfQO2z/rf4VpL6CX0Dts/63+FaS+gl9A7bP+tthWkvoJfQODCde4qfcg RiNWK3UyUeX9CXpHU43diOK915X5fG/reux5hUAUBftZ" /> but no image is displayed. One solution would be to save images locally and using absolute path, but this is not desirable. Any idea? Thanks.

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• custom URL scheme doesn't work! Navigon AppInteract

  - by rdesign

  Hey guys, It is really frustrating me. I used the doc provided by Navigon itself. Unfortunately it doesn't work as expected. Navigon launches, but stops at the main menu. All I do is this: NSString *myTestStr = [NSString stringWithFormat:@"navigon://App|Another place|FRA|75008|PARIS|rue de Turin|17|2.324621|48.881273"]; NSString *navigonStrEsc = [myTestStr stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]; NSLog(@"navigonStr: %@", navigonStrEsc); [[UIApplication sharedApplication] openURL:[NSURL URLWithString:navigonStrEsc]]; Any ideas what is wrong with my way? thanks a lot!

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• Extracting fields from a define-type object in Scheme

  - by Mike

  Hi, I am trying to extract the field 'name' or 'named-expr' from the following object: (bind 'x (num 5)) ;; note that this is not a list, but a type Binding With the Binding definition: (define-type Binding (bind (name symbol?) (named-expr WAE?))) I have tried, but received the error "reference to an identifier before its definition: Binding-name." Here is what I tried typing: (begin (Binding-name (bind 'x (num 5)))) (begin (define x (bind 'x (num 5))) (Binding-name x)) Thank you!

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• Find all paths from root to leaves of tree in Scheme

  - by grifaton

  Given a tree, I want to find the paths from the root to each leaf. So, for this tree: D / B / \ A E \ C-F-G has the following paths from root (A) to leaves (D, E, G): (A B D), (A B E), (A C F G) If I represent the tree above as (A (B D E) (C (F G))) then the function g does the trick: (define (g tree) (cond ((empty? tree) '()) ((pair? tree) (map (lambda (path) (if (pair? path) (cons (car tree) path) (cons (car tree) (list path)))) (map2 g (cdr tree)))) (else (list tree)))) (define (map2 fn lst) (if (empty? lst) '() (append (fn (car lst)) (map2 fn (cdr lst))))) But this looks all wrong. I've not had to do this kind of thinking for a while, but I feel there should be a neater way of doing it. Any ideas for a better solution (in any language) would be appreciated.

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• Getting the median of 3 values using scheme's car & cdr

  - by kristian Roger

  The problem this time is to get the median of three values (easy) I did this: (define (med x y z) (car(cdr(x y z))) and it was accepted but when testing it: (med 3 4 5) I get this error: Error: attempt to call a non-procedure (2 3 4) And when entering letters instead of number i get: (md x y z) Error: undefined varia y (package user) Using something besides x y z I get: (md d l m) Error: undefined variable d (package user) the question was deleted dont know how anyway write a function that return the median of 3 values

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• Problem with circular definition in Scheme

  - by user8472

  I am currently working through SICP using Guile as my primary language for the exercises. I have found a strange behavior while implementing the exercises in chapter 3.5. I have reproduced this behavior using Guile 1.4, Guile 1.8.6 and Guile 1.8.7 on a variety of platforms and am certain it is not specific to my setup. This code works fine (and computes e): (define y (integral (delay dy) 1 0.001)) (define dy (stream-map (lambda (x) x) y)) (stream-ref y 1000) The following code should give an identical result: (define (solve f y0 dt) (define y (integral (delay dy) y0 dt)) (define dy (stream-map f y)) y) (solve (lambda (x) x) 1 0.001) But it yields the error message: standard input:7:14: While evaluating arguments to stream-map in expression (stream-map f y): standard input:7:14: Unbound variable: y ABORT: (unbound-variable) So when embedded in a procedure definition, the (define y ...) does not work, whereas outside the procedure in the global environment at the REPL it works fine. What am I doing wrong here? I can post the auxiliary code (i.e., the definitions of integral, stream-map etc.) if necessary, too. With the exception of the system-dependent code for cons-stream, they are all in the book. My own implementation of cons-stream for Guile is as follows: (define-macro (cons-stream a b) `(cons ,a (delay ,b)))

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• make tree in scheme

  - by ???

  (define (entry tree) (car tree)) (define (left-branch tree) (cadr tree)) (define (right-branch tree) (caddr tree)) (define (make-tree entry left right) (list entry left right)) (define (mktree order items_list) (cond ((= (length items_list) 1) (make-tree (car items_list) '() '())) (else (insert2 order (car items_list) (mktree order (cdr items_list)))))) (define (insert2 order x t) (cond ((null? t) (make-tree x '() '())) ((order x (entry t)) (make-tree (entry t) (insert2 order x (left-branch t)) (right-branch t))) ((order (entry t) x ) (make-tree (entry t) (left-branch t) (insert2 order x (right-branch t)))) (else t))) The result is: (mktree (lambda (x y) (< x y)) (list 7 3 5 1 9 11)) (11 (9 (1 () (5 (3 () ()) (7 () ()))) ()) ()) But I'm trying to get: (7 (3 (1 () ()) (5 () ())) (9 () (11 () ()))) Where is the problem?

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• Scheme: Mysterious void in pattern match.

  - by Schemer

  Hi. I am writing a function called annotate that uses match-lambda -- often with recursive calls to annotate. Here is one of the pattern matches: (`(lambda (,<param1> . ,<params>) ,<stmts>) `(CLOSURE ENV (,<param1> . ,<params>) (lambda (ENV) ,(map annotate (map (lambda (x) (append `(,<param1> . ,<params>) (list x))) `(,<stmts>)))))) However, when this pattern is matched this is what returns: '(CLOSURE ENV (x) (lambda (ENV) ((CLOSURE ENV (x y) (lambda (ENV) ((+ x y)))))) #<void>) Specifically I can't figure out where "void" is coming from. In fact, if I include the line: ,(displayln (map annotate (map (lambda (x) (append `(,<param1> . ,<params>) (list x))) `(,<stmts>)))) it prints: ((CLOSURE ENV (x y) (lambda (ENV) ((+ x y))))) notably without "void". If someone could tell me what the problem is it would be greatly appreciated. Thanks.

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• Writing an Eval Procedure in Scheme?

  - by Planeman

  My problem isn't with the built-in eval procedure but how to create a simplistic version of it. Just for starters I would like to be able to take this in '(+ 1 2) and have it evaluate the expression + where the quote usually takes off the evaluation. I have been thinking about this and found a couple things that might be useful: Unquote: , (quasiquote) (apply) My main problem is regaining the value of + as a procedure and not a symbol. Once I get that I think I should just be able to use it with the other contents of the list. Any tips or guidance would be much appreciated.

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• using lambda instead of let in scheme

  - by Radagaisus

  Hey, In SICP 1.2.1 there is a function that makes a rational number, as follow: (define (make-rat n d) (let ((g (gcd n d))) (cons (/ n g) (/ d g)))) I'm just curious how you can implement the same thing using lambda instead of let, without calling GCD twice. I couldn't figure it out myself.

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• What color scheme do you use for programming?

  - by EndangeredMassa

  I get a lot of attention at work because I am the only one who bothered to change the default color settings in Visual Studio. I just modified them myself. I can provide the settings file if anyone cares to import it. Here's an example of how it looks. It reminds me of DOS/BASIC programming before I actually knew how to program. I also find it to be very readable. What color schemes do you use?! EDIT: To clarify, I only edited the text settings. The windows and panels of VS2005 are still the windows default.

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• True / false evaluation doesn't work as expected in Scheme

  - by ron

  I'm trying to compare two booleans : (if (equal? #f (string->number "123b")) "not a number" "indeed a number") When I run this in the command line of DrRacket I get "not a number" , however , when I put that piece of code in my larger code , the function doesn't return that string ("not a number") , here's the code : (define (testing x y z) (define badInput "ERROR") (if (equal? #f (string->number "123b")) "not a number" "indeed a number") (display x)) And from command line : (testing "123" 1 2) displays : 123 Why ? Furthermore , how can I return a value , whenever I choose ? Here is my "real" problem : I want to do some input check to the input of the user , but the thing is , that I want to return the error message if I need , before the code is executed , because if won't - then I would run the algorithm of my code for some incorrect input : (define (convert originalNumber s_oldBase s_newBase) (define badInput "ERROR") ; Input check - if one of the inputs is not a number then return ERROR (if (equal? #f (string->number originalNumber)) badInput) (if (equal? #f (string->number s_oldBase)) badInput) (if (equal? #f (string->number s_newBase)) badInput) (define oldBase (string->number s_oldBase)) (define newBase (string->number s_newBase)) (define outDecimal (convertIntoDecimal originalNumber oldBase)) (define result "") ; holds the new number (define remainder 0) ; remainder for each iteration (define whole 0) ; the whole number after dividing (define temp 0) (do() ((= outDecimal 0)) ; stop when the decimal value reaches 0 (set! whole (quotient outDecimal newBase)) ; calc the whole number (set! temp (* whole newBase)) (set! remainder (- outDecimal temp)) ; calc the remainder (set! result (appending result remainder)) ; append the result (set! outDecimal (+ whole 0)) ; set outDecimal = whole ) ; end of do (if (> 1 0) (string->number (list->string(reverse (string->list result))))) ) ;end of method This code won't work since it uses another method that I didn't attach to the post (but it's irrelevant to the problem . Please take a look at those three IF-s ... I want to return "ERROR" if the user put some incorrect value , for example (convert "23asb4" "b5" "9") Thanks

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• Scheme Function to reverse elements of list of 2-list

  - by sudhirc

  This is an exercise from EOPL. Procedure (invert lst) takes lst which is a list of 2-lists and returns a list with each 2-list reversed. (define invert (lambda (lst) (cond((null? lst ) '()) ((= 2 (rtn-len (car lst))) ( cons(swap-elem (car lst)) (invert (cdr lst)))) ("List is not a 2-List")))) ;; Auxiliry Procedure swap-elements of 2 element list (define swap-elem (lambda (lst) (cons (car (cdr lst)) (car lst)))) ;; returns lengh of the list by calling (define rtn-len (lambda (lst) (calc-len lst 0))) ;; calculate length of the list (define calc-len (lambda (lst n) (if (null? lst) n (calc-len (cdr lst) (+ n 1))))) This seems to work however looks very verbose. Can this be shortened or written in more elegant way ? How I can halt the processing in any of the individual element is not a 2-list? At the moment execution proceed to next member and replacing current member with "List is not a 2-List" if current member is not a 2-list.

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• If item not in the lst in scheme

  - by ms. sakura

  I'm working on context-free grammars, and I have a function that returns the (terminal values of grammar) for example: i have non-terminal function that results in (A B) , from calling say ((A cat) (B happy np) (A B sad)) so technically A and B are non terminals of the grammar. Now I want to be able to get the terminals (cat happy np sad) (define terminals (lambda (lsts) (cond ((null? lsts) lsts) ((not(member? (car(car lsts)) (n-terminals lsts))) (cons (car(car lsts)) (terminals (car (cdr lsts))))) (else (terminals (cdr lsts)))))) PS: functionality of n-terminals is described above. member? is a boolean function that returns true if an item is a member of the list, false otherwise. My function returns an empty lst. What am I missing here?

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• The Business case for Big Data

  - by jasonw

  The Business Case for Big Data Part 1 What's the Big Deal Okay, so a new buzz word is emerging. It's gone beyond just a buzzword now, and I think it is going to change the landscape of retail, financial services, healthcare....everything. Let me spend a moment to talk about what i'm going to talk about. Massive amounts of data are being collected every second, more than ever imaginable, and the size of this data is more than can be practically managed by today’s current strategies and technologies. There is a revolution at hand centering on this groundswell of data and it will change how we execute our businesses through greater efficiencies, new revenue discovery and even enable innovation. It is the revolution of Big Data. This is more than just a new buzzword is being tossed around technology circles.This blog series for Big Data will explain this new wave of technology and provide a roadmap for businesses to take advantage of this growing trend. Cases for Big Data There is a growing list of use cases for big data. We naturally think of Marketing as the low hanging fruit. Many projects look to analyze twitter feeds to find new ways to do marketing. I think of a great example from a TED speech that I recently saw on data visualization from Facebook from my masters studies at University of Virginia. We can see when the most likely time for breaks-ups occurs by looking at status changes and updates on users Walls. This is the intersection of Big Data, Analytics and traditional structured data. Ted Video Marketers can use this to sell more stuff. I really like the following piece on looking at twitter feeds to measure mood. The following company was bought by a hedge fund. They could predict how the S&P was going to do within three days at an 85% accuracy. Link to the article Here we see a convergence of predictive analytics and Big Data. So, we'll look at a lot of these business cases and start talking about what this means for the business. It's more than just finding ways to use Hadoop + NoSql and we'll talk about that too. How do I start in Big Data? That's what is coming next post.

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• Application for taking pretty screenshots (like OS X does)

  - by Oli

  I've been building a website for a guy who uses Mac OS X and occasionally he sends me screenshots of bugs. They come out looking like this: This is fairly typical of Mac screenshots. You get the window decorations, the shadow from the window and a white or transparent background (not the desktop wallpaper -- I've checked). Compare this to an Ubuntu window-shot (Alt+Print screen): It's impossible to keep a straight face and say the Ubuntu one anywhere near as elegant. My question is: Is there an application that can do this in Ubuntu? Edit: Follow up: Is there an application that can do this in one move? Shutter is pretty good but running the plugin for every screenshot is pretty tiresome as it doesn't seem to remember my preference (I want south-shadow and that requires selecting south, then clicking refresh, then save) and it's more clicks than I'd like. Is there a simple way of telling shutter I want south-shadow for all screenshots (except entire desktop and area-selection)?

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• Forbes Article on Big Data and Java Embedded Technology

  - by hinkmond

  Whoa, cool! Forbes magazine has an online article about what I've been blogging about all this time: Big Data and Java Embedded Technology, tying it all together with a big bow, connecting small devices to the data center. See: Billions of Java Embedded Devices Here's a quote: By the end of the decade we could see tens of billions of new Internet-connected devices... with billions of Internet- connected devices generating Big Data, are the next big thing. ... That’s why Oracle has put together an ecosystem of solutions for this new, Big Data-oriented device-to-data center world: secure, powerful, and adaptable embedded Java for intelligent devices, integrated middleware... This is the next big thing. Java SE Embedded Technology is something to watch for in the new year. Start developing for it now to get a head-start... Hinkmond

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• Application for taking pretty screenshots (like OS X does)

  - by Oli

  I've been building a website for a guy who uses Mac OS X and occasionally he sends me screenshots of bugs. They come out looking like this: This is fairly typical of Mac screenshots. You get the window decorations, the shadow from the window and a white or transparent background (not the desktop wallpaper -- I've checked). Compare this to an Ubuntu window-shot (Alt+Print screen): It's impossible to keep a straight face and say the Ubuntu one anywhere near as elegant. My question is: Is there an application that can do this in Ubuntu? Edit: Follow up: Is there an application that can do this in one move? Shutter is pretty good but running the plugin for every screenshot is pretty tiresome as it doesn't seem to remember my preference (I want south-shadow and that requires selecting south, then clicking refresh, then save) and it's more clicks than I'd like. Is there a simple way of telling shutter I want south-shadow for all screenshots (except entire desktop and area-selection)?

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