用球拍實現擴展功能

Question

我似乎無法弄清楚如何編寫此功能。我試圖寫一個函數expand是獲得一個列表lst的形式'(a (2 b) (3 c))的參數，並進行評估，以'(a b b c c c)

Answer 1

這看起來像功課，所以我不會給你一個直接的答案。相反，我會給你一些正確的方向。最有用的提示是，你應該將問題分成兩個程序，一個用於處理「外部」列表，另一個用於生成在內部子列表中編碼的重複。

注意這兩個過程是相互遞歸的（例如，它們互相呼叫）。 expand程序遍歷列表，而repeat程序重複遍歷重複次數。這是建議的解決方案的總體結構，填充了空白：

; input: lst - list to be processed 
; output: list in the format requested 
(define (expand lst) 
    (cond ((null? lst)    ; if the list is null 
     '())     ; then return null 
     ((not (pair? (car lst))) ; if the first element of the list is an atom 
     (cons <???> <???>))  ; cons the atom and advance the recursion 
     (else     ; if the first element of the list is a list 
     <???>)))    ; call `repeat` with the right params 


; input: n - number of repetitions for the first element in the list 
;  lst - list, its first element is of the form (number atom) 
; output: n repetitions of the atom in the first element of lst 
(define (repeat n lst) 
    (if (zero? n)   ; if the number of repetitions is zero 
     (expand (cdr lst)) ; continue with expand's recursion 
     (cons <???>  ; else cons the atom in the first element and 
      <???>)))  ; advance the recursion with one less repetition 

Answer 2

由於這三年前得到的回答是，我不認爲我的家庭作業幫助。只想指出這兩個函數真的不是need是相互遞歸的。作爲replicate是一個相當普遍的功能，我建議：

(define (replicate what n) 
    (if (zero? n) 
     (list) 
     (cons what (replicate what (- n 1))))) 

(define (my-expand xs) 
    (if (empty? xs) 
     (list) 
     (let ((x (first xs))) 
     (if (list? x) 
      (let ((the-number (first x)) 
        (the-symbol (cadr x))) 
       (flatten (cons (replicate the-symbol the-number) 
        (my-expand (rest xs))))) 
      (cons x (my-expand (rest xs))))))) 

當然最好是使用兩個列表並進行壓平在最後，是這樣的：

(define (my-expand xs) 
    (define (inner-expander xs ys) 
    (if (empty? xs) (flatten (reverse ys)) 
     (let ((x (first xs))) 
     (if (list? x) 
      (let ((the-number (first x)) 
        (the-symbol (cadr x))) 
       (inner-expander (rest xs) (cons (replicate the-symbol the-number) ys)))      
      (inner-expander (rest xs) (cons x ys)))))) 
    (inner-expander xs (list)))