CODEWITHSUNDEEP
schemeracket
tlauer
tlauer

Reputation: 588

Scheme Pairs output

Trying to produce an output of pairs that would look like (1 1) (1 2) (1 3) (2 2) (2 3) (3 3), if taking the first 6 elements of a stream. (The first 6 have 3 columns and it should print the pairs beginning with 1 and then 2 and then 3.) The code I have is:

(define (pairs s t)
  (cons-stream (cons (stream-car s) (stream-car t))
               (cons-stream
                (stream-map (lambda (x) (cons (stream-car s) x))
                            (stream-cdr t))
                (pairs (stream-cdr t) (stream-cdr s)))))

And if I run 

(take 6 (pairs integers integers))

where take and integers are defined as follows:

(define (take n s)  
  (if (= n 0)
      '()
      (cons (stream-car s) (take (- n 1) (stream-cdr s)))))

(define integers (cons-stream 1 (add-streams ones integers)))

The result I get is: 

((1 . 1)
 ((1 . 2) . #<promise>)
 (2 . 2)
 ((2 . 3) . #<promise>)
 (3 . 3)
 ((3 . 4) . #<promise>))

Upvotes: 1

Views: 287

Answers (1)

Will Ness
Will Ness

Reputation: 71065

In Scheme,

(define (interleaved-pairs xs ys . args)
  (let ((x (stream-car xs))
        (ac (if (null? args) () (car args))))
    (stream-append 
      (stream-map stream-car (list->stream (reverse ac)))
      (stream-cons (list x (stream-car ys))
        (interleaved-pairs 
          (stream-cdr xs)
          (stream-cdr ys)
          (cons
            (stream-map (lambda(y)(list x y)) (stream-cdr ys))
            (map stream-cdr ac)))))))

This should produce results in the order you wanted: (1 1) (1 2) (2 2) (1 3) (2 3) (3 3) (1 4) ....

You tagged this as racket also. As far as I can see in the Racket documentation, it has stream-first in place of stream-car, etc. For some reason it doesn't seem to have list->stream, which can be defined quite straightforwardly with the apply and stream functions.

Here it is in a shorter notation.

ipairs xs ys = g xs ys []  where
  g (x:xs) (y:ys) ac = map head (reverse ac) ++ (x,y) : 
                         g xs ys (map ((,) x) ys : map tail ac)

Upvotes: 1

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