SICPゼミ第43回

練習問題4.11

(define (make-frame variables values)
  (if (= (length variables values))
      (if (null? variables)
          nil
         (cons (cons (car variables) (car values))) (make-frame (cdr variables) (cdr values)))
      (if (< (length variables) (length values))
          (error "Too many arguments supplied" variables values)
          (error "Too few arguments supplied" variables values ))))

(define (frame-variables frame)
  (if (null? frame)
      nil
      (cons (caar frame) (frame-variables (cdr frame)))))
(define (frame-values frame)
  (if (null? frame)
      nil
      (cons (cdar frame) (frame-variables (cdr frame)))))

(define (add-binding-to-frame! var val frame)
  (set! frame (cons (cons var val) frame)))

by pine

練習問題4.12
(define (enviroment-searching-alg var find-operation error-operation env)
  (define (env-loop env)
    (define (scan vars vals)
      (cond ((null? vars)
        (env-loop (enclosing-environment env)))
        ((eq? var (car vars)) (find-operation (car vals) (car vars)))
        (else (scan (cdr vars) (cdr vals)))))
    (if (eq? env the-empty-environment)
      (error-operation)
      (let ((frame (first-frame env)))
        (scan (frame-variables frame)
        (frame-values frame)))))
  (env-loop env))

(define (lookup-variable-value var env)
  (enviroment-searching-alg var (lambda (x y) x) (error 'hoge) env))

(define (set-variable-value! var val env)
  (enviroment-searching-alg var (lambda (x y) (set! x val)) (error 'hoge) env))

(define (define-variable! var val env)
  (enviroment-searching-alg var (lambda (x y) (set! x val)) 
    (add-binding-to-frame! var val (car env)) (list (car env))))

by dolicas

練習問題4.13
(define (make-unbound! var env) 
    (define (env-loop env) 
        (define (scan vars vals) 
            (cond ((null? vars) (env-loop (enclosing-environment env))) 
                    ((eq? var (car vars))
                     (begin (set! vals (cdr vals))
                              (set! vars (cdr vars)))) 
                    (else (scan (cdr vars) (cdr vals))))) 
        (if (eq? env the-empty-environment) 
             (error "Unbound variable" var) 
             (let ((frame (first-frame env))) 
                    (scan (frame-variables frame) 
                            (frame-values frame))))) 
    (env-loop env))

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SICPゼミ第42回

練習問題4.7
(define (let*->nested-lets exp)
  (let ((defs (car exp)) (body (cdr exp)))
    (if (null? (cdr defs))
      (list 'let (car defs) body)
      (list 'let (car defs)
        (let*->nested-lets (pair (cdr defs) body))))))

by dolicas

問題文で言ってる派生式でうまくいくはず．

練習問題4.8
(define (let->combination exp)
    (if (pair? (cadr exp))
        (let->combination-prototype exp)
        (let* ((defs (let-defs exp)) 
            (body (cadddr exp)) 
            (func (make-lambda defs body)))
          (let->combination-prototype
            (list 'let (cons ((cadr exp) func) defs) body))))

(define (let->combination-prototype exp)
    (let ((defs (let-defs exp)))
    ((make-lambda (let-variable-from-defs defs) (let-body exp))
     (let-defbody-from-defs defs))))
(define (let-defs exp)
    (cadr exp))
(define (let-body exp)
    (caddr exp))
(define (let-variable-from-defs defs)
        (if (null? defs)
            '()
            (cons (caar defs) (let-variable-from-defs (cdr defs)))))
(define (let-defbody-from-defs defs)
        (if (null? defs)
            '()
            (cons (cdar defs) (let-variable-from-defs (cdr defs)))))

by dolicas

練習問題4.9
(define (for? exp) (tagged-list? exp 'for))
(define (for-initial exp) cadr)
(define (for-terminate exp) caddr)
(define (for-body exp) cadddr)

(define (for->rec exp)
        (let->combination
          ('let for_iter
                ((i (for-initial exp)))
                (make-if
                  (> i (for-terminate exp))
                  'done
                  (begin (for-body exp)
                         (for_iter (+ i 1)))))))

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練習問題4.10
(define (eval-if exp env)
(if (true? (eval (if-predicate exp) env))
(eval (if-alternative exp) env)
(eval (if-consequent exp) env)))

と変えるとifが逆になるという新しい文法のschemeになる．

SICPゼミ第41回

練習問題4.4
(define (eval-and exp env)
  (cond
    ((null? exp) #t)
    ((null? (cdr exp)) (eval (cdr exp) env))
    ((not (eval (car exp) env)) #f)
    (else (eval-and (cdr exp) env))))

(define (and? exp) (tagged-list? exp 'and))

by dolicas

派生式の方もできた

(define (vars exp))
(define (and->if exp)
  (make-if (null? (vars exp))
                 #t
                 (make-if (car (vars exp))
                                (and-if ('and (cdr (vars exp)))
                                false))

by ぷに

練習問題4.5
(define (expand-clauses clauses)
    (if (null? clauses)
'false ;
(let ((first (car clauses))
    (rest (cdr clauses)))
(if (cond-else-clause? first)
    (if (null? rest)
        (sequence->exp (cond-actions first))
        (error "ELSE clause isn't last: COND->IF"
            clauses))
    (make-if (cond-predicate first)
        (if (cond-test? (cond-actions first))
            ((eval (cdr cond-actions) env) (eval cond-predicate env))
            (sequence->exp (cond-actions first)))
        (expand-clauses rest))))))

(define (cond-test? clause) 
    (= (cond-predicate clase) '=>))

と変更する

by dolicas

練習問題4.6
(define (let->combination exp)
    (let ((defs (let-defs exp)))
    ((make-lambda (let-variable-from-defs defs) (let-body exp))
     (let-defbody-from-defs defs))))
(define (let-defs exp)
    (cadr exp))
(define (let-body exp)
    (caddr exp))
(define (let-variable-from-defs defs)
        (if (null? defs)
            '()
            (cons (caar defs) (let-variable-from-defs (cdr defs)))))
(define (let-defbody-from-defs defs)
        (if (null? defs)
            '()
            (cons (cdar defs) (let-variable-from-defs (cdr defs)))))

SICPゼミ第40回

2章はやばいので4章に帰ってきました。

練習問題4.2

applicaionかどうかの判定 (application?) はタグを気にせずペアかどうかしか見ていない．これが最後でないと (define …) みたいなのも適用として扱ってしまうのでおかしくなる． call を使うことにするのであれば，ほかのやつと同じように，callというタグが付いてると思って扱えばよい．

by dolicas

練習問題4.3

(define (eval exp env)
  (if (self-evaluating? exp)
    exp
    (if (get 'eval (get-tag exp))
      ((get 'eval (get-tag exp)) (get-content exp) env)
      (apply (eval (operator exp) env) (list-of-values (operands exp) env)) 
    )
  ))
(define (get-tag exp)
  (car exp))
(define (get-content exp) (cdr exp))

by dolicas

SICPゼミ第38回

練習問題2.91

(define (install-polynomial-package)
  ;; 内部手続き
  ;; poly の表現
  (define (make-poly variable term-list) (cons variable term-list ))
  (define (variable p) (car p))
  (define (term-list p) (cdr p))
  (define same-variable? eq?)
  (define variable? symbol?)
  ;; 項と項リストの表現
  (define (add-poly p1 p2)
    (if (same-variable? (variable p1) (variable p2))
        (make-poly (variable p1)
                   (add-terms (term-list p1) (term-list p2)))
        (error "Polys not in same var: ADD-POLY" (list p1 p2))))
  (define (add-terms L1 L2)
    (cond (( empty-termlist? L1) L2)
          (( empty-termlist? L2) L1)
          (else
           (let ((t1 (first-term L1))
                 (t2 (first-term L2)))
             (cond ((> (order t1) (order t2))
                    (adjoin-term
                     t1 (add-terms (rest-terms L1) L2)))
                   ((< (order t1) (order t2))
                    (adjoin-term
                     t2 (add-terms L1 (rest-terms L2))))
                   (else
                      (adjoin-term
                     (make-term (order t1)
                                (add (coeff t1) (coeff t2)))
                     (add-terms (rest-terms L1)
                                (rest-terms L2 )))))))))
  (define (inversion termList)
    (if (empty-termlist? termList)
        termList
        (let ((term (first-term termList)))
          (if (eq? (type-tag term) 'polynomial)
              (adjoin-term
               (make-term (order term)
                          (inversion (coeff term)))
               (inversion (rest-terms termList)))
              (adjoin-term
               (make-term (order term)
                          (mul (make-scheme-number -1) (coeff term)))
               (inversion (rest-terms termList)))))))
  (define (mul-poly p1 p2)
    (if (same-variable? (variable p1) (variable p2))
        (make-poly (variable p1)
                   (mul-terms (term-list p1) (term-list p2)))
        (error "Polys not in same var: MUL-POLY" (list p1 p2))))
  (define (mul-terms L1 L2)
    (if (empty-termlist? L1)
        (the-empty-termlist)
        (add-terms (mul-term-by-all-terms (first-term L1) L2)
                   (mul-terms (rest-terms L1) L2))))
  (define (mul-term-by-all-terms t1 L)
    (if (empty-termlist? L)
        (the-empty-termlist)
        (let ((t2 (first-term L)))
          (adjoin-term
           (make-term (+ (order t1) (order t2))
                      (mul (coeff t1) (coeff t2)))
           (mul-term-by-all-terms t1 (rest-terms L))))))
  (define (adjoin-term term term-list)
    (if (=zero? (coeff term ))
        term-list
        (cons term term-list )))
  (define (div-terms L1 L2)
    (if (empty-termlist? L1)
        (list (the-empty-termlist) (the-empty-termlist ))
        (let ((t1 (first-term L1))
              (t2 (first-term L2)))
          (if (> (order t2) (order t1))
              (list (the-empty-termlist) L1)
              (let ((new-c (div (coeff t1) (coeff t2)))
                    (new-o (- (order t1) (order t2))))
                (let ((rest-of-result
                       (let ((rest (add-terms L1
                                              (inversion
                                               (mul-terms L2 (list (make-term new-o new-c)))))))
                         (div-terms rest L2))))
                  (cons (adjoin-term (make-term new-o new-c)
                                     (car rest-of-result))
                        (cdr rest-of-result))
                  ))))))
  (define (div-poly p1 p2)
     (if (same-variable? (variable p1) (variable p2))
        (make-poly (variable p1)
                   (div-terms (term-list p1) (term-list p2)))
        (error "Polys not in same var: ADD-POLY" (list p1 p2))))
(define (the-empty-termlist) '())
(define (first-term term-list) (car term-list ))
(define (rest-terms term-list) (cdr term-list ))
(define (empty-termlist? term-list) (null? term-list ))
(define (make-term order coeff) (list order coeff ))
(define (order term) (car term ))
(define (coeff term) (cadr term ))
  ;; システムのほかの部分とのインターフェイス
  (define (tag p) (attach-tag 'polynomial p))
  (put 'add '(polynomial polynomial)
       (lambda (p1 p2) (tag (add-poly p1 p2))))
  (put 'mul '(polynomial polynomial)
       (lambda (p1 p2) (tag (mul-poly p1 p2))))
  (put 'make 'polynomial
       (lambda (var terms) (tag (make-poly var terms ))))
  (put 'div '(polynomial polynomial)
       (lambda (p1 p2) (tag (div-poly p1 p2))))
  'done)

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