Streams questions

Questions & Answers
21

This is what I had in mind:

(define (stream-foldr fun acc str)
  (if (stream-empty? str)
      acc
      (fun (stream-foldr fun acc (stream-rest str))
           (stream-first str))))

(define (inverterd-cons acc e) (cons e acc))
(stream-foldr inverterd-cons '() (stream 1 2 3 4 5))

=> '(1 2 3 4 5)

but it works for one valued streams only.

22

Hmm, reusing some of the code from before, perhaps something such as this?

(define (n-val-stream . ss)
  (if (ormap stream-empty? ss)
      empty-stream
      (stream-cons (apply values       (map stream-first ss))
                   (apply n-val-stream (map stream-rest  ss)))))

(define (stream-foldr fun acc str)
  (if (stream-empty? str)
      acc
      (call-with-values
       (thunk (stream-first str))
       (curry fun (stream-foldr fun acc (stream-rest str))))))

(define s1 (stream 1 2 3 4 5))
(define s2 (stream 1 2 3 4 5))

(define (inverterd-cons acc . es) (cons es acc))
(stream-foldr inverterd-cons '() (n-val-stream s1 s2))

;=> '((1 1) (2 2) (3 3) (4 4) (5 5))

Edit: ormap might be safer.

1 Like
23

This is what i found:

(define-syntax values->list
  (syntax-rules ()
    [(_ (values v ...))
     (call-with-values
      (lambda () (values v ...))
      (lambda v v))]))

(define (stream-foldr fun acc mvstr)
  (if (stream-empty? mvstr)
      acc
      (apply fun (append (values->list 
                          (stream-foldr fun acc (stream-rest mvstr)))
                         (values->list (stream-first mvstr))))))

(define (inv-cons-mul acc e1 e2) (cons (* e1 e2) acc))
(stream-foldr inv-cons-mul '() (stream (values 1 10)
                                       (values 2 100)
                                       (values 3 1000)))
;=> '(10 200 3000)

a bit more complicated.

1 Like