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[REBOL] Re: On ordinal and cardinal numbers...

From: larry:ecotope at: 9-Jul-2001 12:59

Hi all, Oops, my bad. There was a stupid (and critical) typo in the recursive version, in the function body it called the iterative version RING-SKIP instead of itself, RING-SKIP2.
> The second uses recursion: > > ring-skip2: func [s [series!] n [integer!]] [ > if n = 0 [return s] > either positive? n [ > if tail? s: next s [s: head s] > ring-skip s n - 1 > ][ > s: back either head? s [tail s][s] > ring-skip s n + 1 > ] > ]
When corrected, the recursive version fails with a stack overflow at a skip value of 1515. The timings where b is a block of 10000 integers and the skip value is 1514 are:
>> t: now/time/precise loop 100 [ring-skip b 1514] now/time/precise - t
== 0:00:00.44
>> t: now/time/precise loop 100 [ring-skip2 b 1514] now/time/precise - t
== 0:00:02.14 So the iterative version is about 5 times faster for the maximum skip value allowed by the recursive version. BTW I get the same results as Mark with his recursion test, a maximum stack depth of 3147. Clearly, as Joel indicated, we should prefer the iterative version, if we anticipate using it on very large series. Sorry for the confusion. -Larry