Question

In Chapter 9 of the Little Schemer, the Author presents the following two functions

(define Q 
  (lambda (str n) 
    (cond 
      ((zero? (remainder (first$ str ) n)) 
        (Q (second$ str ) n)) 
      (t (build (first$ str ) 
        (lambda ( ) 
          (Q (second$ str ) n))))))) 

(define P
  (lambda (str)
    (build (first$ str)(lambda () (P (Q str (first$ str)))))))

and proposes that they are evaluated with the following execution:

(frontier (P (second$ (second$ int)))  10)

How would you write the P and Q functions in Common Lisp?

(I have translated the Y-Combinator myself - but I m finding this one challenging)

--Helper Functions--

(define frontier
  (lambda (str n)
    (cond
      ((zero? n) (quote ()))
        (t (cons (first$ str) (frontier (second$ str) (sub1 n)))))))

(define str-maker
  (lambda (next n)
    (build n (lambda () (str-maker next (next n))))))

(define int (str-maker add1 0))

(define second$
  (lambda (str)
    ((second str))))

(define first$ first)

(define build
  (lambda (a1 a2)
    (cond
      (t (cons a1
        (cons a2 (quote ())))))))))

(define first
  (lambda (p)
    (cond
       (t (car p)))))

(define second
  (lambda (p)
    (cond
      (t (car (cdr p))))))

(define add1 
  (lambda (n)
    (+ 1 n)))

(define remainder 
  (lambda  (n m)
    (cond
      (t (- n (* m (/ n m ))))))

(Disclaimer - This Is Not A Homework Question - it is for my understanding and learning)

Answer 1

I assumed:

In P definition, with "(Q (str (first$ str)))" you meant: "(Q str (first$ str))", as Q is a two-argument function.
build is a helper which does creates something on which first$ and second$ work: list

With this in mind, the direct translation of Scheme into Common Lisp gives:

(defun first$ (list) (first list))
(defun second$ (list) (funcall (second list)))
(defun build (a b) (list a b))

(defun frontier (str n)
  (if (zerop N)
    ()
    (cons (first$ str) (frontier (second$ str) (1- n)))))

(defun str-maker (next n)
  (list n (lambda () (str-maker next (funcall next n)))))

(setq int-maker (str-maker # 1+ 0))

(defun Q (str n)
  (if (zerop (rem (first$ str) n))
    (Q (second$ str) n)
    (list (first$ str) (lambda () (Q (second$ str) n)))))

(defun P (str)
  (list (first$ str) (lambda () (P (Q str (first$ str))))))

(frontier (P (second$ (second$ int-maker))) 10)

Whose last line returns:

(2 3 5 7 11 13 17 19 23 29)

which is a well known series, so I assume the translation is successful :-)

友情链接