SICPゼミ第47回
練習問題4.20
- 4.18 と一緒なのでパス。
SICPゼミ第46回
練習問題4.18
(define (solve f y0 dt) (define y (integral (delay dy) y0 dt)) (define dy (stream-map f y)) y) (define (solve_ f y0 dt) (let ((y '*hoge*) (dy '*hoge*)) (let ((a (integral (delay dy) y0 dt)) (b (stream-map f y))) (set! y a) (set! dy b)) y)) (define (solve__ f y0 dt) (let ((y '*hoge*) (dy '*hoge*)) (set! y (integral (delay dy) y0 dt)) (set! dy (stream-map f y)) y))
実行結果
> (stream-ref (solve (lambda (y) y) 1 0.001) 1000) 2.716923932235896 > (stream-ref (solve_ (lambda (y) y) 1 0.001) 1000) . . car: contract violation expected: pair? given: '*hoge* > (stream-ref (solve__ (lambda (y) y) 1 0.001) 1000) 2.716923932235896
書き換えるとエラーが出る。
solve_のほうでは、(b (stream-map f y)) が評価されるときに y には '*hoge* が入っているのでエラーが出る。
solve__ のほうでは、(set! y (integral (delay dy) y0 dt)) が評価されるときに dy には '*hogeが入っているが、遅延評価なので問題なく動く。
by tube
練習問題4.19
Ben式は割といろんな言語である気がする．Eva式は逐次定義絶対殺すマンなので割と異常だと思う．ところがGoshはこの方針で実装されているのでたいへん気持ち悪い．とりあえず結論としては「ここで出てくるようなプログラムを書くのを止めろ」に尽きる．by現実
SICPゼミ第45回
練習問題4.16
(define (lookup-variable-value var env) (define (env-loop env) (define (scan vars vals) (cond (( null? vars) (env-loop (enclosing-environment env ))) ((eq? var (car vars )) (if (eq? (car vals) '*unassigned*) (error "Unassigned Variable") (car vals))) (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))
b.
(define (scan-out-defines procedure-body) (if (null? procedure-body) null (let ((exp (car procedure-body))) (if (definition? exp) (let ((var (definition-variable exp))) ((make-lambda var (begin (set! var (definition-value exp)) (scan-out-defines (cdr procedure-body)))) '*unassigned*)) (cons exp (scan-out-defines (cdr procedure-body)))))))
c. どっちでも問題はないが，make-procedureのほうが効率がいい．make-procedureに入れると，evalの一回だけscan-outするが，procedure-bodyに入れるとapplyするたびにscan-outすることになる．
練習問題4.17
SICPゼミ第44回
練習問題4.14
なんかタグとかがうまく処理されないと思う。
by tube
練習問題4.15
ネゲートして自分を突っ込む。終わり。
by tube
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))
By tube
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)))))))
by tube
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)))))
