# [racket] Racket style question

Thanks Rodolfo and Eli for the education, very elegant solutions.
I really like the clever use of the "(and (right-triangle? a b c) (list a b
c))))" idiom.
I had to look up in-value... unfortunately the manual is a bit sparse
there, but I got the gift by running some examples... thanks.
After going "D'oh" about the infinite loop, here is the code I ended up
with:
(define (pythagorean-triple n)
(let loop-ab ([a 1] [b 2])
(define c (- n a b))
(cond [(>= a n) '()]
[(<= c b) (loop-ab (add1 a) (+ a 2))]
[(right-triangle? a b c) (list a b c)]
[else (loop-ab a (add1 b))])))
I noticed that the sequence-based solutions are quite a bit slower than the
code above probably because they don't short-cut on (<= c b), is there an
elegant way to speed them up?
Thanks again!
-joe
On Sun, Mar 18, 2012 at 11:25 PM, Eli Barzilay <eli at barzilay.org> wrote:
>* 20 minutes ago, Rodolfo Carvalho wrote:
*>* >
*>* > (define (pythagorean-triple/alt n)
*>* > (for*/first ([a (in-range 1 n)]
*>* > [b (in-range a n)]
*>* > [c (in-value (- n a b))]
*>* > #:when (right-triangle? a b c))
*>* > (list a b c)))
*>*
*>* You can also use `for*/or':
*>*
*>* (define (pythagorean-triple/alt n)
*>* (for*/or ([a (in-range 1 n)]
*>* [b (in-range a n)])
*>* (define c (- n a b))
*>* (and (right-triangle? a b c) (list a b c))))
*>*
*>* --
*>* ((lambda (x) (x x)) (lambda (x) (x x))) Eli Barzilay:
*>* http://barzilay.org/ Maze is Life!
*>*
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