# [plt-scheme] match question

 From: praimon (praimon at gmail.com) Date: Mon May 12 23:34:45 EDT 2008 Previous message: [plt-scheme] match question Next message: [plt-scheme] match question Messages sorted by: [date] [thread] [subject] [author]

```thanks for your response. Would you say that the greediness of
Here's a very simple example:

(match '(3 2 4 3 2 1)
((list x y ... x z ...) (append y z)))
=> (2 4 3 2)

Surely the result should be (2 4 2 1)? At least that's the intuitively
correct answer (and the one returned by Mathematica, e.g.).

Changing ... to ..1 produces a different counter-intuitive answer: (2 4 3 1).
For people like me, who are merely using your language rather than trying
to understand its inner workings, this is worrisome.

regards,
praimon

On Sun, May 11, 2008 at 10:37 PM, Sam TH <samth at ccs.neu.edu> wrote:
> The general answer is the all ... patterns are greedy, but in order,
>  from left to right.
>
>  So we match as many elements to `x' as possible, then one to `y', then
>  as many to `z' as possible, then another to the second `y'.
>
>  In the particular case, we match as many elements to `x' as possible,
>  while allowing the whole pattern to match.  That's the first 3
>  elements, and the fifth element gets matched to `z'.  In your second
>  example, we have to match at least two elements to `z', so we only
>  match 2 to `x'.
>
>  Does that help explain things?
>
>  sam th
>
>
>
>  On Sat, May 10, 2008 at 6:52 PM, praimon <praimon at gmail.com> wrote:
>  > hello,
>  >  in 3.99.0.24, using scheme/match:
>  >
>  >  (map (lambda (x)
>  >        (match x ((list x ... y z ..1 y) (append x z))))
>  >      '((1 2 3 4 3)
>  >        (1 2 3 4 3 4)
>  >        (1 2 3 4 3 3)))
>  >
>  >  => ((1 2 4) (1 2 3 3) (1 2 3 3))
>  >
>  >  I understand the first two matches, but
>  >  shouldn't the last result be (1 2 4 3)?
>  >
>  >  This produces my expected answer:
>  >
>  >  (match '(1 2 3 4 3 3)
>  >        ((list x ... y z ..2 y) (append x z)))
>  >
>  >  => (1 2 4 3)
>  >
>  >  but in this context, aren't ..1 and ..2 equivalent?
>  >
>  >  thanks,
>  >  praimon
>  >  _________________________________________________