I don't see why the cseq's have to be lazy, they are at the most 525
elements long.
shouldn't each sequence only be produced when it is reduced in max-key and
then discarded?
....
But it makes a difference:
(defn cseq [n]
(if (= 1 n)
[1]
(lazy-seq (cons n (cseq (if (even? n)
(/ n 2)
(+ (* 3 n) 1 )))))))
(apply max-key count (map cseq (range 1 1000000)))
gives me results.
(def a (apply max-key count (map cseq (range 1 1000000))))
gives a heap error.
Thanks for your answer.
2011/1/17 Mark Engelberg <[email protected]>
> Your cseq is not lazy, and some of the sequences can be quite long, so
> it wouldn't surprise me if that's the source of your problem.
>
> You can test if this is the problem by doing something like:
> (dorun (map cseq (range 1 1000000))) which removes the max-key from
> the computation entirely.
>
> You'll probably need to reformulate cseq with lazy lists. Even then,
> you will likely find that this program will be too slow without
> further optimizations (e.g., memoization or dynamic programming).
>
> That's the nature of project Euler problems; the "obvious" way to
> solve the problem is often too slow and further cleverness is
> required.
>
> On Mon, Jan 17, 2011 at 7:53 AM, Andreas Liljeqvist <[email protected]>
> wrote:
> > Hi.
> >
> > I am using max-key to get the longest sequence from a couple of
> sequences.
> >
> > (defn cseq [n]
> > (if (= 1 n)
> > [1]
> > (cons n (cseq (if (even? n)
> > (/ n 2)
> > (+ (* 3 n) 1 ))))))
> >
> > (apply max-key count (map cseq (range 1 1000000)))
> >
> > This gives me a heap error.
> >
> > To my understanding max-key should keep at most two sequences in memory.
> > I could certainly solve this by working around the problem, but I want to
> > understand where my logic fails.
> > Thankful for any help.
> >
> >
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