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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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• racket scheme get-argb-pixels

  - by Giles Roberts

  I have a 32 by 32 pixel png file. I'm trying to read the values within it using get-argb-pixels. My code is as follows: #lang racket/gui (require racket/gui/base) (define floor (make-object bitmap% "C:\\floortile.png")) (define pixels (make-bytes (* 32 32 4))) (send floor get-argb-pixels 0 0 32 32 pixels) After executing the code I was expecting a series of 8 bit values to be contained within pixels. Examining pixels gives me the following output: > pixels #"\377\2148\30\377\214<\30\377\234E\30\377\245I\30\377\245I\30\377\234E\30\377\234A\30\377\224A\30\377\224A\30\377\234A\30\377\245E\30\377\255M\30\377\224A\30\377\234E\30\377\245I\30\377\224A\30\377\234E\30\377\234E\30\377\255I\30\377\245I\30\377\245E\30\377\234A\30\377\234E\30\377\234A\30\377\245I\30\377\265Q\30\377\306]\30\377\306Y\30\377\275U\30\377\265Q\30\377\306a!\377\306]!\377\214<\30\377\224A\30\377\224A\30\377\245I\30\377\255M\30\377\255M\30\377\245I\30\377\234E\30\377\234E\30\377\245I\30\377\245I\30\377\255M\30\377\234A\30\377\245E\30\377\265M\30\377\234A\30\377\245I\30\377\245E\30\377\245E\30\377\255M\30\377\245I\30\377\245E\30\377\234A\30\377\234E\30\377\255I\30\377\275U\30\377\306]\30\377\326e!\377\336e!\377\265Q\30\377\326e!\377\326a!\377{8\30\377\224E\30\377\224A\30\377\245I\30\377\245I\30\377\255M\30\377\255M\30\377\255I\30\377\245I\30\377\245I\30\377\245I\30\377\265Q\30\377\214<\30\377\245I\30\377\255M\30\377\265M\30\377\255M\30\377\265M\30\377\265M\30\377\245I\30\377\234E\30\377\234E\30\377\234E\30\377\245E\30\377\275Q\30\377\306]!\377\316a!\377\336i!\377\357q!\377\275Y\30\377\316e!\377\347i!\377k0\20\377\2048\30\377\224A\30\377\265U!\377\234E\30\377\234E\30\377\245M\30\377\255M\30\377\255M\30\377\255I\30\377\255M\30\377\265M\30\377\2048\20\377\245I\30\377\245I\30\377\306Y!\377\265Q\30\377\255M\30\377\265Q\30\377\265M\30\377\265M\30\377\255I\30\377\265Q\30\377\265M\30\377\275U\30\377\316a!\377\326i!\377\347m!\377\367\216)\377\347y)\377\336m)\377\326]!\377s0\20\377{8\30\377{4\20\377\2048\30\377s4\20\377\2048\30\377{4\20\377\214<\30\377\224A\30\377\234A\30\377\224A\30\377\224A\30\377\214<\30\377\2148\30\377\224<\30\377\234A\30\377\214<\30\377\234E\30\377\245I\30\377\265Q\30\377\255M\30\377\265U\30\377\306Y\30\377\265Q\30\377\275U\30\377\326a!\377\336i!\377\347u!\377\357\206)\377\326q)\377\326i)\377\347i!\377s0\20\377{4\20\377{4\30\377\214<\30\377s4\20\377\2048\30\377s4\20\377\224A\30\377\224A\30\377\224<\30\377\234A\30\377\245E\30\377\234E\30\377\234I\30\377\234E\30\377\245I\30\377\245I\30\377\245I\30\377\265Q\30\377\306]\30\377\255M\30\377\255M\30\377\255M\30\377\255M\30\377\275U\30\377\255M\30\377\275U\30\377\316a!\377\306]!\377\326i!\377\367\206)\377\377})\377s4\30\377k,\20\377k,\20\377s4\30\377{4\20\377\2048\30\377c(\20\377\2048\20\377\234A\30\377\245E\30\377\245E\30\377\255I\30\377\245I\30\377\245I\30\377\245I\30\377\265Q\30\377\275U!\377\255M\30\377\255M\30\377\255M\30\377\255M\30\377\275U\30\377\306]!\377\275U\30\377\255M\30\377\306Y!\377\306]!\377\316e!\377\316a!\377\326i!\377\347y)\377\367y)\377c(\20\377c,\20\377k,\20\377s0\20\377s4\20\377\2048\30\377\214<\30\377{4\20\377\214<\30\377\234E\30\377\234A\30\377\245I\30\377\255M\30\377\255M\30\377\255M\30\377\265Q\30\377\265M\30\377\265Q\30\377\265Q\30\377\255M\30\377\265U\30\377\255M\30\377\265Q\30\377\265U\30\377\265U\30\377\265Q\30\377\306Y\30\377\316a!\377\306a!\377\326i!\377\347y)\377\367\202)\377c,\20\377k,\20\377s0\20\377s0\20\377{8\30\377\214<\30\377s,\20\377s0\20\377\214<\30\377\234A\30\377\245E\30\377\255M\30\377\255M\30\377\245I\30\377\255M\30\377\275U\30\377\265U\30\377\265Q\30\377\265Q\30\377\275Y\30\377\255M\30\377\255I\30\377\275U\30\377\275Y!\377\275Y\30\377\265U\30\377\306Y!\377\326e!\377\336m!\377\336q!\377\347})\377\357\202)\377s0\20\377s4\30\377s4\30\377\2048\30\377\234E\30\377\2048\20\377\2148\30\377c(\20\377\2048\30\377\214<\30\377\234E\30\377\265Q\30\377\265Q\30\377\255M\30\377\265M\30\377\316a!\377\275U\30\377\275Y\30\377\265Q\30\377\265Q\30\377\265Q\30\377\265Q\30\377\255M\30\377\275Y\30\377\275Y!\377\275Y\30\377\275U\30\377\316a!\377\347q)\377\367\202)\377\357})\377\347y)\377k0\20\377\2048\30\377{4\30\377\2048\30\377\214A\30\377\2048\30\377\2044\30\377s4\20\377k,\20\377\2048\30\377\224A\30\377\245I\30\377\234A\30\377\234E\30\377\255Q\30\377\275U\30\377\306Y!\377\265Q\30\377\255Q\30\377\265U\30\377\265U\30\377\265Q\30\377\265Q\30\377\316e!\377\316a!\377\306]!\377\275Y!\377\306]!\377\336i!\377\357})\377\367\202)\377\336u)\377k0\20\377s0\30\377\2048\30\377\204<\30\377\204<\30\377\2048\30\377\2048\30\377\214<\30\377s4\20\377\2048\30\377\214A\30\377\224A\30\377\224E\30\377\234E\30\377\265Q\30\377\275U\30\377\306Y\30\377\255M\30\377\265Q\30\377\265Q\30\377\316]!\377\326e!\377\316]!\377\336m)\377\336i)\377\316e!\377\306a!\377\326e!\377\367y)\377\367})\377\367\2061\377\367\2061\377s4\30\377{8\30\377{8\30\377\214<\30\377\214<\30\377\204<\30\377{4\20\377\214<\30\377\214<\30\377\214<\30\377\234E\30\377\234E\30\377\224E\30\377\234E\30\377\245M\30\377\275U\30\377\275U!\377\275U\30\377\306]!\377\306]!\377\316a!\377\326i)\377\347u)\377\326e!\377\347q)\377\336q)\377\326i)\377\326i)\377\347q)\377\367\202)\377\367})\377\367\2061\377{8\30\377s4\30\377{8\30\377\214<\30\377\224A\30\377\234E\30\377\214<\30\377\214<\30\377\224A\30\377\224<\30\377\234E\30\377\255M\30\377\245I\30\377\245I\30\377\255M\30\377\265U\30\377\306]!\377\306]!\377\316a!\377\316e!\377\326e!\377\347m)\377\316e!\377\306]!\377\347u)\377\347u)\377\336m)\377\316e)\377\336q)\377\357y)\377\367\202)\377\367\2021\377{4\30\377s4\30\377\2048\30\377\214A\30\377\224A\30\377\224A\30\377\224E\30\377\214<\30\377\214<\30\377\245I\30\377\234A\30\377\255M\30\377\255M\30\377\245I\30\377\255M\30\377\255Q\30\377\306]!\377\306]!\377\316a!\377\316a!\377\316a!\377\347u)\377\326e!\377\275Y!\377\265Y!\377\336m)\377\306a)\377\336m)\377\336m)\377\326e)\377\347q)\377\336q)\377s0\30\377s0\30\377\204<\30\377\224A\30\377\224E\30\377\234I\30\377\234E\30\377\234E\30\377\255M\30\377\234E\30\377{4\30\377\224<\30\377\245M\30\377\255I\30\377\234A\30\377\255M\30\377\245E\30\377\255M\30\377\265Q\30\377\245I\30\377\275U\30\377\255I\30\377\234A\30\377\224E\30\377\265U\30\377\234I\30\377\224E\30\377\245M\30\377\234M!\377\224A\30\377\234I\30\377\224E\30\377k0\20\377s0\20\377{4\30\377\214<\30\377\224E\30\377\234E\30\377\245I\30\377\234I\30\377\245I\30\377\214<\30\377\214<\30\377\255M\30\377\265Q!\377\265Q!\377\255M\30\377\306]!\377\316a)\377\326e)\377\326a!\377\316]!\377\265Q\30\377\326a!\377\255M\30\377\204<\20\377\245M\30\377\234I\30\377\245M\30\377\275]!\377\234I\30\377\255U!\377\265Y!\377\245M!\377c(\20\377k,\20\377k0\20\377\2048\30\377\224A\30\377\224E\30\377\234I\30\377\245I\30\377\275Y!\377\234E\30\377\245I\30\377\245I\30\377\265U!\377\265Q!\377\265Q!\377\306]!\377\306]!\377\316a!\377\316a!\377\336e)\377\326a!\377\316]!\377\265Q\30\377\224A\30\377\234I\30\377\275]!\377\265Y!\377\275Y!\377\275Y!\377\265U!\377\265])\377\275])\377s4\30\377c(\20\377k,\20\377s0\20\377\214A\30\377\224E\30\377\234E\30\377\306]!\377\234I\30\377\224A\30\377\265U!\377\245I\30\377\265Q!\377\265Q!\377\265M!\377\255M\30\377\306]!\377\306Y!\377\306]!\377\306]!\377\326a)\377\336i)\377\275U!\377\224A\30\377\224A\30\377\306a!\377\275]!\377\275Y!\377\275]!\377\275])\377\275Y!\377\275Y!\377s0\20\377k,\20\377s0\20\377\2048\30\377\214<\30\377\224E\30\377\234E\30\377\245M\30\377\224A\30\377\214A\30\377\245M\30\377\265Q!\377\275U!\377\255I\30\377\245I\30\377\245I\30\377\275U!\377\306Y!\377\275Y!\377\306]!\377\306]!\377\265Q\30\377\255M\30\377\224A\30\377\204<\30\377\255Q\30\377\245M\30\377\255Q!\377\265Y!\377\275]!\377\265Y!\377\275Y!\377s0\20\377k0\20\377{0\20\377\2048\30\377\204<\30\377\204<\30\377\214<\30\377\224A\30\377\316a!\377\245I\30\377\234E\30\377\245I\30\377\265U!\377\265Q!\377\265Q!\377\255M!\377\275U!\377\275U!\377\275U!\377\275Q!\377\255I\30\377\255M\30\377\255Q\30\377\234E\30\377{4\30\377\224A\30\377\214<\30\377\275Y!\377\275Y!\377\275]!\377\255Q!\377\255Q!\377k,\20\377k,\20\377s0\20\377\2044\30\377{4\20\377{4\30\377\2048\30\377\245M\30\377\265U!\377\245I\30\377\214A\30\377\275Y!\377\234A\30\377\255M\30\377\265Q!\377\245M!\377\265U!\377\275Q!\377\245I\30\377\245I\30\377\245M\30\377\265Q!\377\255Q!\377\255M\30\377\214<\30\377\245I\30\377\255Q!\377\306]!\377\306]!\377\275U!\377\265U!\377\255Q!\377c(\20\377Z(\20\377c(\20\377k,\20\377{8\30\377{4\30\377\214<\30\377\214A\30\377\234E\30\377\245I\30\377\245M\30\377\214<\30\377s0\30\377\255M!\377\255I!\377\255M!\377\255M!\377\265Q!\377\265Q!\377\255M!\377\265Q\30\377\245I\30\377\255I\30\377\306Y!\377\275Y!\377\214A\30\377\255Q\30\377\275]!\377\306])\377\306]!\377\255U!\377\255Q!\377k0\30\377k0\20\377k,\20\377s0\20\377s0\20\377s4\30\377{4\30\377\214<\30\377\245I\30\377\224A\30\377\306Y!\377\234A\30\377s0\20\377\245I\30\377\255M!\377\255M!\377\255M!\377\255M!\377\255Q!\377\265M!\377\316]!\377\245E\30\377\316]!\377\306Y!\377\275U!\377{8\30\377\255Q!\377\265Y!\377\275]!\377\275Y!\377\275Y!\377\255Q!\377k0\20\377c,\20\377k,\20\377s0\20\377{4\30\377s0\20\377{4\30\377{4\30\377\2048\30\377\245I\30\377\234E\30\377\234E\30\377k,\20\377\214<\30\377\224A\30\377\245M\30\377\214<\30\377\224E\30\377\214<\30\377\214<\30\377\214<\30\377\245I\30\377\234E\30\377\224A\30\377\224A\30\377k,\20\377\224A\30\377\265Q!\377\265Y!\377\265U!\377\265Y!\377\275Y!\377c(\20\377c,\20\377k,\20\377s0\20\377k,\20\377k0\20\377\2048\30\377\234A\30\377\214<\30\377\224A\30\377\245I\30\377\245I\30\377k0\20\377{4\20\377\214<\30\377\214A\30\377\214<\30\377\204<\30\377\2048\30\377\234I\30\377\224A\30\377\214<\30\377\234E\30\377\234E\30\377\255M\30\377\224A\30\377{8\30\377\255Q!\377\265U!\377\255U!\377\255U!\377\265U!\377c(\20\377k,\20\377s0\20\377s0\20\377{4\20\377\2048\30\377\2048\30\377\2048\30\377\224A\30\377\245M\30\377\245E\30\377\234E\30\377{4\20\377Z(\20\377k0\30\377{8\30\377\2048\30\377\214A\30\377\224A\30\377\234I\30\377\214<\30\377\224A\30\377\224A\30\377\234E\30\377\245I\30\377\255M\30\377{8\30\377\214A\30\377\255U!\377\255U!\377\245Q!\377\265U!\377c,\20\377c(\20\377k,\20\377Z$\20\377Z$\20\377Z(\20\377c(\20\377k,\20\377k,\20\377c(\20\377c(\20\377c,\20\377k0\20\377R \20\377c,\20\377s4\30\377{4\30\377\204<\30\377\2048\30\377\214A\30\377\224A\30\377\224A\30\377\234I\30\377\234I\30\377\224A\30\377\245I\30\377\224E\30\377\2048\30\377\255U!\377\255Q!\377\265U!\377\265Y)\377J \20\377J\34\20\377Z(\20\377k,\20\377k(\20\377c(\20\377R \20\377c(\20\377k,\20\377s0\20\377k0\20\377k0\20\377c,\20\377Z(\20\377c,\20\377s4\20\377s4\20\377\2048\30\377\2048\20\377\2048\30\377\224A\30\377\214A\30\377\234I\30\377\245I\30\377\234I\30\377\245M!\377\245M\30\377\204<\30\377\234M!\377\245Q!\377\265U!\377\255U!\377c,\20\377c,\20\377c(\20\377c(\20\377c(\20\377c(\20\377c(\20\377k,\20\377k,\20\377k,\20\377k0\20\377s4\20\377s0\30\377\204<\30\377c(\20\377s4\20\377s4\30\377{4\20\377{4\30\377\204<\30\377\224A\30\377\234E\30\377\234E\30\377\234E\30\377\234E\30\377\245I\30\377\245I\30\377\224E\30\377\204<\30\377\245Q!\377\255Q!\377\347\327\326\377R$\20\377k,\20\377c(\20\377c(\20\377c(\20\377c(\20\377Z$\20\377c(\20\377c(\20\377k,\20\377k,\20\377k,\20\377s0\20\377{4\30\377\204<\30\377k,\20\377s4\20\377s4\20\377{8\30\377\2048\20\377\214<\30\377\214<\30\377\214A\30\377\234E\30\377\234E\30\377\224A\30\377\224E\30\377\224E\30\377s4\30\377\214A\30\377\265\206s\377\377\377\377\377c,\20\377k,\30\377k0\20\377k,\20\377c(\20\377Z$\20\377Z(\20\377Z$\20\377c(\20\377c(\20\377c(\20\377k,\20\377k0\20\377{4\30\377{8\30\377k,\20\377c,\20\377k0\20\377s0\20\377{8\30\377\2048\30\377\2048\30\377\214<\30\377\224A\30\377\224E\30\377\214<\30\377\214A\30\377\234I\30\377{8!\377\214M1\377\377\373\377\377\377\377\377" This doesn't look like a series of 8 bit values to me. Have I done something wrong or am I misinterpreting the results?

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• Racket: change dotted pair to list

  - by user2963128

  I have a program that recursively calls a hashtable and prints out data from it. Unfortunately my hashtable seems to be saving data as dotted pairs so when I call the hashtable I get an error saying that there is no data for it because its tryign to search the hashtable for a dotted pair instead of a list. Is there an easy way to make the dotted pair into a regular list? IE im getting '("was" . "beginning") instead of '("was" "beginning") Is there a way to change this without re-writing how my hashtable store stuff? im using the let function to set a variable to this and then calling another function based on this variable (let ((data ( list-ref(hash-ref Ngram-table key) (random (length (hash-ref Ngram-table key)))))) is there a way to make the value stored in data just a list like this '("var1" "var2") instead of a dotted pair? edit: im getting dotted pairs because im using let to set data to the part of the hashtable's key and one of the elements in that hash.

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• Using hash tables in racket

  - by user2963128

  Im working on an Ngrams program and im having trouble filling out my hash table. i want to write out a recursive function that will take the words and add them to the hash table. The way its supposed to work is given the data set 1 2 3 4 5 6 7 the first entry in the hash table should have a key of [1 2] and the data should be 3. The second entry should be: [2 3] and its data should be 4 and continuing on until the end of the text file. we are given a predefined function called readword that will simply return 1 word from the text. But im not sure how to make these calls overlap each other. The calls would look something like this if the data was hard coded in. (hash-set! (list "1" "2") 3 (hash-set! (list "2" "3") 4 2 calls that ive tried look like this (hash-set! Ngram-table(list((word1) (word2)) readword in))) (hash-set! Ngram-table(append((cdr data) word1)) readword in) apparently the in after readword is supposed to tell the computer that this is and input instead of an output or something like that. How would I call this to make the data in the key of the hashtable overlap like this? And what would the recursive call look like? edit: also we are not allowed to use assigment statements in this program.

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• How do I step through and debug a Scheme program using Dr. Racket?

  - by Thomas Owens

  I'm using the Dr. Racket development environment and the language definition #lang scheme to do work for a course. However, I'm not sure how to best use this tool for debugging. I would like to be able to execute a function and step through it, observing the values of different functions at various points in execution. Is this possible? If not, what is the typical method of stepping through the execution of a Scheme program and debugging it?

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• Idiomatic usage of filter, map, build-list and local functions in Racket/Scheme?

  - by Greenhorn

  I'm working through Exercise 21.2.3 of HtDP on my own and was wondering if this is idiomatic usage of the various functions. This is what I have so far: (define-struct ir (name price)) (define list-of-toys (list (make-ir 'doll 10) (make-ir 'robot 15) (make-ir 'ty 21) (make-ir 'cube 9))) ;; helper function (define (price< p toy) (cond [(< (ir-price toy) p) toy] [else empty])) (define (eliminate-exp ua lot) (cond [(empty? lot) empty] [else (filter ir? (map price< (build-list (length lot) (local ((define (f x) ua)) f)) lot))])) To my novice eyes, that seems pretty ugly because I have to define a local function to get build-list to work, since map requires two lists of equal length. Can this be improved for readability? Thank you.

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• How do I change the list using mutable lists in Racket or Scheme?

  - by exOne

  I need your help Can anyone help me? first question This procedure should modify each element which’s index in the list is even. The value of the element should be equal to the new-element. For example: (define number-list (1 56 8 1 43 9 78)) (modify-even-indices number-list 10) The new list: 1 10 8 10 43 10 78 second question This problem is the extended version of Question 1. (modify-even-odd-indices my-list new-element modification-type) This procedure should modify each element according to the modification-type. The modification-type can be even or odd. For example: (define number-list (1 56 8 1 43 9 78)) (modify-even-odd-indices number-list 10 ‘even’) The new list: 1 10 8 10 43 10 78 (modify-even-odd-indices number-list 10 ‘odd’) The new list: 10 56 10 1 10 9 10

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• If Scheme is untyped, how can it have numbers and lists?

  - by Dokkat

  Scheme is said to be just an extension of the Untyped Lambda Calculus (correct me if I am wrong). If that is the case, how can it have Lists and Numbers? Those, to me, look like 2 base types. So I'd say Racket is actually an extension of the Simply Typed Lambda Calculus. No? Question: Is Scheme's type system actually based or more similar to Simply Typed or Untyped Lambda Calculus? In what ways does it differ from Untyped and or Simply Typed Lambda Calculus? (The same question is valid for "untyped" languages such as Python and JavaScript - all of which look like they have base types to me.)

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• Racket regular-expression matching

  - by Inaimathi

  I'm trying to create a regex that matches the inverse of a certain string type (so, strings not ending in ".js", for example). According to the documentation, that should be the expression #rx"(?!\\.js$)", but it doesn't seem to work. To test it out, I have this function: (define (match-test regex) (map (lambda (text) (regexp-match? regex text)) '("foo.js" "bar.css" "baz.html" "mumble.gif" "foobar"))) (match-test #rx"\\.js$") returns (#t #f #f #f #f) as expected, but (match-test #rx"(?!\\.js$)") returns (#t #t #t #t #t), where I would expect (#f #t #t #t #t). What am I doing wrong, and how do I actually get a regex in Racket to express the idea "match anything which does not contain [x]"?

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• how to save and load the state of a game in scheme

  - by user3667664

  I'm creating the game of chess in scheme, but do not know how to save and load game state is a part I have this code (define-struct ficha(color se-movio? tipo-ficha )) ;;tablero lista de listas de fichas (define-struct estado (tablero turno fichaSel)) (define bpawn (bitmap "b-peon.png")) (define brook (bitmap "b-torre.png")) (define bcaballo (bitmap "b-caballo.png")) (define bbish (bitmap "b-arfil.png")) (define bquee (bitmap "b-reina.png")) (define bking (bitmap "b-rey.png")) (define wpawn (bitmap "w-peon.png")) (define wrook (bitmap "w-torre.png")) (define wcaballo (bitmap "w-caballo.png")) (define wbish (bitmap "w-arfil.png")) (define wquee (bitmap "w-reina.png")) (define wking (bitmap "w-rey.png")) (define board (bitmap "board.jpg")) This is the board that is a list of lists (define tableroini (list (list torreb caballob arfilb reinab reyb arfilb caballob torreb) (list peonb peonb peonb peonb peonb peonb peonb peonb) (list empty empty empty empty empty empty empty empty) (list empty empty empty empty empty empty empty empty) (list empty empty empty empty empty empty empty empty) (list empty empty empty empty empty empty empty empty) (list peonw peonw peonw peonw peonw peonw peonw peonw) (list torrew caballow arfilw reinaw reyw arfilw caballow torrew))) I did this to save the state of the game: (define (Guardar-en-archivo archivo) (write-file (string-append Subcarpeta archivo ".txt") "game state" )) But not as you insert the game state on "game state" for me to save the game How I can do this ?

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• Binary Trees in Scheme

  - by Javier

  Consider the following BNF defining trees of numbers. Notice that a tree can either be a leaf, a node-1 with one subtrees, or a node-2 with two subtrees. tree ::= (’leaf number) | (’node-1 tree) | (’node-2 tree tree) a. Write a template for recursive procedures on these trees. b. Define the procedure (leaf-count t) that returns the number of leaves in t > (leaf-count ’(leaf 5)) 1 > (leaf-count ’(node-2 (leaf 25) (leaf 17))) 2 > (leaf-count ’(node-1 (node-2 (leaf 4) (node-2 (leaf 2) (leaf 3))))) 3 Here's what I have so far: ;define what a leaf, node-1, and node-2 is (define leaf list) (define node-1 list) (define node-2 list) ;procedure to decide if a list is a leaf or a node (define (leaf? tree) (number? (car tree))) (define (node? tree) (pair? (car tree))) (define (leaf-count tree) (cond ((null? tree) 0) ((number? tree) 0) ((leaf? tree) 1) (else (+ (leaf-count (car tree)) (leaf-count (cdr tree)))))) It looks like it should run just fine, but when I try to run it using a simple test case like (leaf-count '(leaf 5)) I get the following error message: car: expects argument of type pair; given leaf What does this error message mean? I am defining a leaf as a list. But for some reason, it's not seeing that and gives me that error message.

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• About "If.." in Scheme (plt-scheme)

  - by Amit

  I had a pretty simple requirement in my Scheme program to execute more than one statement, in the true condition of a 'if'. . So I write my code, something like this: (if (= 1 1) ((expression1) (expression2)) ; these 2 expressions are to be executed when the condition is true (expression3) ) Obviously, the above doesn't work, since I have unintentionally created a # procedure with # arguments. So, to get my work done, I simply put the above expressions in a new function and call it from there, in place of the expression1, expression2. It works. So, my point here is: is there any other conditional construct which may support my requirement here?

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• Matrix addition in Scheme

  - by user285012

  I am trying to add a matrix and it is not working... (define (matrix-matrix-add a b) (map (lambda (row) (row-matrix-add row b)) a)) (define (row-matrix-add row matrix) (if (null? (car matrix)) '() (cons (add-m row (map car matrix)) (row-matrix-add row (map cdr matrix))))) (define (add-m row col) (if (null? col) 0 (+ (car row) (car col) (add-m (cdr row) (cdr col)))))

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• How to 'destroy/dispose' frame% in plt-scheme?

  - by Thura

  I want to destory my previously shown frame when a certain event is triggered. I can't find anything regarding this in the reference manual.

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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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• PLT Scheme sort function

  - by Danny

  PLT Scheme guide says that it's implemented sort function is able to sort a list according to an extarcted value using a lambda function. link text The guide provides an unworking code example of this- (sort '(("aardvark") ("dingo") ("cow") ("bear")) #:key car string<?) Which returns an error. How is this function is supposed to be calles so that it will actually sort a list according to values calculated by a given function?

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• How do I make a module in PLT Scheme?

  - by kunjaan

  I tried doing this: #lang scheme (module duck scheme/base (provide num-eggs quack) (define num-eggs 2) (define (quack n) (unless (zero? n) (printf "quack\n") (quack (sub1 n))))) But I get this error: module: illegal use (not at top-level) in: (module duck scheme/base (provide num-eggs quack) (define num-eggs 2) (define (quack n) (unless (zero? n) (printf "quack\n") (quack (sub1 n))))) what is the correct way?

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• plt-scheme : catching mouse click event on canvas

  - by Thura

  I am writing a tic-tac-toe game in plt-scheme as my AI course project. The idea for gui is a grid with 9 boxes, each with a canvas, using panes ... When the user click on a canvas, 'X' or 'O' will be drawn accordingly ... The question is how can I catch mouse click event on canvas? I found out I need to use on-event, but still don't know how? Any clues?

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• Is there an equivalent to Lisp's "runtime" primitive in Scheme?

  - by Bill the Lizard

  According to SICP section 1.2.6, exercise 1.22: Most Lisp implementations include a primitive called runtime that returns an integer that specifies the amount of time the system has been running (measured, for example, in microseconds). I'm using DrScheme, where runtime doesn't seem to be available, so I'm looking for a good substitute. I found in the PLT-Scheme Reference that there is a current-milliseconds primitive. Does anyone know if there's a timer in Scheme with better resolution?

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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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• How do I profile in DrScheme?

  - by kunjaan

  How Do I profile my functions using DrScheme? (require profile) (define (factorial n) (cond ((= n 1) 1) (else (* n (factorial (- n 1)))))) (profile factorial) The above code returns Profiling results ----------------- Total cpu time observed: 0ms (out of 0ms) Number of samples taken: 0 (once every 0ms) ==================================== Caller Idx Total Self Name+srcLocal% ms(pct) ms(pct) Callee ==================================== > I tried: - (profile (factorial 100)) - (profile factorial) (factorial 100) But it gives me the same result. What am I doing wrong?

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• Scheme Beginner question

  - by Robert

  Hello.I am trying to put the following statement in Dr.Scheme: {with {x {+ 5 5}} {+ x x}} but I got an error: expand: unbound identifier in module in: with anyone could help me?Thanks.

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• Scheme - Memory System

  - by Eric

  I am trying to make a memory system where you input something in a slot of memory. So what I am doing is making an Alist and the car of the pairs is the memory location and the cdr is the val. I need the program to understand two messages, Read and Write. Read just displaying the memory location selected and the val that is assigned to that location and write changes the val of the location or address. How do I make my code so it reads the location you want it to and write to the location you want it to? Feel free to test this yourself. Any help would be much appreciated. This is what I have: (define make-memory (lambda (n) (letrec ((mem '()) (dump (display mem))) (lambda () (if (= n 0) (cons (cons n 0) mem) mem) (cons (cons (- n 1) 0) mem)) (lambda (msg loc val) (cond ((equal? msg 'read) (display (cons n val))(set! n (- n 1))) ((equal? msg 'write) (set! mem (cons val loc)) (set! n (- n 1)) (display mem))))))) (define mymem (make-memory 100))

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• Pattern Matching in Scheme

  - by kunjaan

  How do I accept the following input? (list of 0 or more charcters and ends with 3) or (list of 1 or more characters 4 and 0 or more characters after 4) something like (match ( list 3)) -> #t (match ( list 1 2 3)) -> #t (match (list 1 2 3 4)) -> #t (match (list 1 2 3 4 5)) -> #t (match (list 4)) -> #f EDIT: THIS IS NOT MY HOMEWORK. I trying to write something like ELIZA from PAIP but I know only how to write a pattern that begins with a word.

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• What Scheme Does Ghuloum Use?

  - by Don Wakefield

  I'm trying to work my way through Compilers: Backend to Frontend (and Back to Front Again) by Abdulaziz Ghuloum. It seems abbreviated from what one would expect in a full course/seminar, so I'm trying to fill in the pieces myself. For instance, I have tried to use his testing framework in the R5RS flavor of DrScheme, but it doesn't seem to like the macro stuff: src/ghuloum/tests/tests-driver.scm:6:4: read: illegal use of open square bracket I've read his intro paper on the course, An Incremental Approach to Compiler Construction, which gives a great overview of the techniques used, and mentions a couple of Schemes with features one might want to implement for 'extra credit', but he doesn't mention the Scheme he uses in the course. Update I'm still digging into the original question (investigating options such as Petit Scheme suggested by Eli below), but found an interesting link relating to Gholoum's work, so I am including it here. [Ikarus Scheme](http://en.wikipedia.org/wiki/Ikarus_(Scheme_implementation)) is the actual implementation of Ghuloum's ideas, and appears to have been part of his Ph.D. work. It's supposed to be one of the first implementations of R6RS. I'm trying to install Ikarus now, but the configure script doesn't want to recognize my system's install of libgmp.so, so my problems are still unresolved. Example The following example seems to work in PLT 2.4.2 running in DrEd using the Pretty Big (require lang/plt-pretty-big) (load "/Users/donaldwakefield/ghuloum/tests/tests-driver.scm") (load "/Users/donaldwakefield/ghuloum/tests/tests-1.1-req.scm") (define (emit-program x) (unless (integer? x) (error "---")) (emit " .text") (emit " .globl scheme_entry") (emit " .type scheme_entry, @function") (emit "scheme_entry:") (emit " movl $~s, %eax" x) (emit " ret") ) Attempting to replace the require directive with #lang scheme results in the error message foo.scm:7:3: expand: unbound identifier in module in: emit which appears to be due to a failure to load tests-driver.scm. Attempting to use #lang r6rs disables the REPL, which I'd really like to use, so I'm going to try to continue with Pretty Big. My thanks to Eli Barzilay for his patient help.

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