Solution to 4.33. This had been difficult to get right, though conceptually it was

[sicp.git] / src / sicp / ex3_70.rkt

#lang racket

(define (merge-weighted p1 p2 weightfn)
  (cond ((stream-null? p1) p2)
        ((stream-null? p2) p1)
        (else
         (let ((p1car (stream-car p1))
               (p2car (stream-car p2)))
           (let ((w1 (weightfn (car p1car) (car (cdr p1car))))
                 (w2 (weightfn (car p2car) (car (cdr p2car)))))
             (cond ((< w1 w2)
                    (cons-stream p1car 
                                 (merge-weighted (stream-cdr p1) p2 weightfn)))
                   ((> w1 w2)
                    (cons-stream p2car
                                 (merge-weighted p1 (stream-cdr p2) weightfn)))
                   (else
                    (cons-stream p1car
                                 (cons-stream p2car
                                              (merge-weighted (stream-cdr p1)
                                                              (stream-cdr p2)
                                                              weightfn))))))))))


(define (weighted-pairs s t weightfn)
  (cons-stream
   (list (stream-car s) (stream-car t))
   (merge-weighted
    (stream-map (lambda (x) (list (stream-car s) x))
                (stream-cdr t))
    (weighted-pairs (stream-cdr s) (stream-cdr t) weightfn)
    weightfn)))

#|

> (display-stream (weighted-pairs integers integers (lambda (i j) (+ i j))))

(1 1)
(1 2)
(1 3)
(2 2)
(1 4)
(2 3)
(1 5)
(2 4)
(3 3)
(1 6)
(2 5)
(3 4)
(1 7)
(2 6)
(3 5)
(4 4)
(1 8)
(2 7)
(3 6)
(4 5)
(1 9)
(2 8)
(3 7)
(4 6)
(5 5)
(1 10)
(2 9)
(3 8)
(4 7)
(5 6)
(1 11)
(2 10)
(3 9)
(4 8)
(5 7)
(6 6)
(1 12)
(2 11)
(3 10)
(4 9)
(5 8)
(6 7)
(1 13)
(2 12)
(3 11)
(4 10)
(5 9)
(6 8)
(7 7)
(1 14)
(2 13)
(3 12)
(4 11)
(5 10)
(6 9)
(7 8)
(1 15)
(2 14)
(3 13)
(4 12)
(5 11)
(6 10)
(7 9)
(8 8)
(1 16)
(2 15)
(3 14)

|#

;; part 2.


(display-stream (stream-filter (lambda (p) 
                                 (let ((i (car p)) 
                                       (j (car (cdr p)))) 
                                   (not (or (divides? 2 i)
                                            (divides? 2 j)
                                            (divides? 3 i)
                                            (divides? 3 j)
                                            (divides? 5 i)
                                            (divides? 5 j)))))
                               (weighted-pairs integers integers (lambda (i j) (+ (* 2 i) (* 3 j) (* 5 i j))))))

#|

(1 1)
(1 7)
(1 11)
(1 13)
(1 17)
(1 19)
(1 23)
(1 29)
(1 31)
(7 7)
(1 37)
(1 41)
(1 43)
(1 47)
(1 49)
(1 53)
(7 11)
(1 59)
(1 61)
(7 13)
(1 67)
(1 71)
(1 73)
(1 77)
(1 79)
(7 17)
(11 11)
(1 83)
(1 89)
(1 91)
(7 19)
(11 13)
(1 97)
(1 101)
(1 103)
(1 107)
(1 109)
(7 23)
(1 113)
(13 13)
(1 119)
(1 121)
(11 17)
(1 127)
(1 131)
(1 133)
(1 137)
(1 139)
(7 29)
(11 19)
(1 143)
(13 17)
(7 31)
(1 149)
(1 151)
(1 157)
(1 161)
(1 163)
(13 19)
(1 167)
(1 169)
(11 23)
(1 173)
(7 37)
(1 179)
(1 181)
(1 187)
(1 191)
(17 17)
(1 193)
(7 41)
(1 197)
(13 23)
(1 199)
(1 203)
(7 43)
(1 209)
(1 211)
(11 29)
(17 19)
(1 217)
(1 221)
(1 223)
(7 47)
(1 227)
(11 31)
(1 229)
(1 233)
(7 49)
(19 19)
(1 239)
(1 241)
(1 247)
(13 29)
(1 251)
(1 253)
(7 53)
(1 257)
(17 23)
(1 259)
(1 263)
(13 31)
(1 269)
(11 37)
(1 271)
(1 277)
(1 281)
(7 59)
(1 283)
(19 23)
(1 287)
(1 289)
(7 61)
(1 293)
(1 299)
(11 41)
(1 301)
(1 307)
(1 311)
(1 313)
(11 43)
(1 317)
(13 37)
(1 319)
(7 67)

|#