racket

问题 Suppose I have some function which returns a struct: (struct layer (points lines areas)) (define (build-new-layer height) ... (layer list-a list-b list-c)) I want to keep track of the last returned result something like: (define top-side (build-new-layer 0)) ; store the first result ... (set! top-side (build-new-layer 0.5)) ; throw away the first result and store the new one However, for that particular code I get the error: set!: assignment disallowed; cannot modify a constant constant: top

问题 I'm using pl in racket: https://pl.barzilay.org/ The download can be found here: http://pl.barzilay.org/pl.plt ( : f1 : -> (Pairof Symbol String)) (define (f1) (cons 'wwww "aaa")) Error: Type Checker: Polymorphic function `cons' could not be applied to arguments: Argument 1: Expected: a Given: 'wwww Argument 2: Expected: (Listof a) Given: String Result type: (Listof a) Expected result: (Pairof Symbol String) in: (cons (quote wwww) "aaa") What I did wrong and how can I fix it? 回答1: The #lang

问题 How to visualise arbitrary tree? for example: (define T1 '(and (or x1 x2)(or x3 x4 x5))) or one generated with: (define functions '(not if and or)) (define terminals '(A0 A1 A2 D0 D1)) (define functions&terminals (append terminals functions )) (define (pick-one list) (list-ref list (random (length list)))) (define arities '((if . 3)(and . 2)(or . 2)(not . 1))) (define (terminal? symbol) (find (lambda (x)(eq? x symbol)) terminals)) (define (function? symbol) (find (lambda (x)(eq? x symbol))

问题 i'm kinda of confused how i can construct a for loop in scheme. the for-loop should be implemented in Part II. where it takes a list of numbers and insert each element inside the list in Part I to find the length. I was cable to get the first element but i need a for loop or what so ever to get an output like this: '(7 10 5 16 106 37) here is my code : #lang racket ; Part I (define (sequence n) (cond [(= n 1) (list n)] [(even? n) ( cons n(sequence( / n 2)))] [(odd? n) ( cons n(sequence (+(* n

问题 I'm having some trouble debugging a case statement. I was hoping that the statement would assign numeric values to note-val , but so far it is assigning #<void> . I know it's something wrong with the case statement, because if I add an else clause, that value gets applied. Given a sample input of '(((#\3 #\A) (#\4 #\B)) ((#\4 #\C))) , what am I doing wrong here? (In regards to the case statement. I'm sure there are other errors, but I'd like to try to work those out myself if I can get this

问题 I have a file with the name “functions.rkt”, where I have some functions. And I am working in another file, let’s name it “working.rkt” I have tried the following (one by one) at “working.rkt” to use the function defined at “functions.rkt”: (require “functions.rkt”) (include “functions.rkt”) (provide “functions.rkt”) And anyone of them hasn’t worked, any help? They are in the same path. 回答1: In the file "functions.rkt: #lang racket (provide my-function) (define (my-function x) (* 2 x)) In the

问题 I am having trouble implementing a function that given something like this: '((+(d + e) + f) + (a + (a + c)) Returns this: '(d + (e + (f + (a + (a + c))))) Now the catch is that I have to use associativity to get the answer so I only want to use this property: '((a + b) + c) = '(a + (b + c)) To get the correct output. I know how to implement the function for this case: '((a + b) + c) -> '(a + (b + c)) However I cannot seem to figure out how to implement it for the first case above (or any

问题 i would like to ask you for help to complete code below with condition which is testing if lists ws and vs are not equal. If they are not equal so return text false(#f) else process code below. I stared with fulfilling variables len1 and len2 which are counting length of both lists. When i run it i am getting this error: lambda: no expression after a sequence of internal definitions in: lambda What i am doing wrong? (define (weighted-sum . ws) (define (sub . vs) (let ((len1 (length ws)) (len2

问题 I've been messing around with this program. It takes a number and adds 1 to it. I am wondering how exactly could you use stop-when here? For example, to make it stop at 5? I suppose a cond statement is necessary here. Thanks. (require 2htdp/image) (require 2htdp/universe) (define (my-tick n) (add1 n)) (define (my-render n) (text (number->string n) 36 "silver")) (big-bang 1 (on-tick my-tick 2) (to-draw my-render)) 回答1: Give stop-when a predicate that consumes a world and returns true or false.

问题 I am trying to intersect more lists in Scheme and I need a little help. The lists look like this: The first two: (((?x john) (?city new-york)) ((?x mike) (?city chicago)) ((?x mary) (?city london))) and (((?city chicago)) ((?city new-york))) I need to look in every list (say A) from the first list and see if there is a list (say B) in the second one so that A has at least one element in common with B. If there is no such element, the resulting list will not contain A. The result for the two