练习3-72

原文

Exercise 3.72. In a similar way to exercise 3.71 generate a stream of all numbers that can be written as the sum of two squares in three different ways (showing how they can be so written).

代码

 (define (square x) (* x x))
 (define (sum-square x) (+ (square (car x)) (square (cadr x))))
 (define (squaresn s)
         (define (stream-cadr s) (stream-car (stream-cdr s)))
         (define (stream-caddr s) (stream-cadr (stream-cdr s)))
         (let ((scar (stream-car s))
                   (scadr (stream-cadr s))
                   (scaddr (stream-caddr s)))
                 (if (= (sum-square scar) (sum-square scadr) (sum-square scaddr))
                         (cons-stream (list (sum-square scar) scar scadr scaddr)
                                                  (squaresn (stream-cdr (stream-cdr (stream-cdr s)))))
                         (squaresn (stream-cdr s)))))
 (define square-numbers
         (squaresn (weighted-pairs integers integers sum-square)))