With zero tolerance, the O(n^2) problem does not arise.
Henry Rich On 8/19/2014 10:14 PM, bill lam wrote:
2. for numeric matrix and zero tolerance, will ~. perform better than O(n^2) if it computes hash for each row? (or it has already done so) On Aug 20, 2014 7:43 AM, "Roger Hui" <[email protected]> wrote:0. For a floating point vector, ~.!.0 is faster than ~., but not to the extent that Bill Lam implied. timer=: 6!:2 x=: 1e6 ?@$ 0 10 timer '~.x' 0.136236 10 timer '~.!.0 x' 0.0814054 Same comments apply to other functions in the index-of family. 1. For non-numeric arguments (and boxed arrays not involving numbers), ~.!.0 should run at the same speed as ~. x=: a.{~ 1e6 12 ?@$ 256 10 timer '~.x' 0.13204 10 timer '~.!.0 x' 0.130772 2. Floating point matrices can have O(n^2) performance. It's best to use ~.!.0 if you can get away with it. 3 : '10 timer ''~.t'' [ t=. 1.5,.y ?@$ 0'"0 ]10^0 1 2 3 4 3.03225e_6 5.45805e_6 0.000412806 0.012665 1.01016 3 : '10 timer ''~.!.0 t'' [ t=. 1.5,.y ?@$ 0'"0 ]10^0 1 2 3 4 4.05855e_6 4.3851e_6 3.11155e_5 0.000177643 0.0015026 3. Boxed arrays involving numerics (even all integers) can have O(n^2) performance. It's best to use ~.!.0 if you can get away with it. I don't remember the exact details of how this comes about. Best to check the source, or at least run a few (small) benchmarks. 3. The interpreter checks for interrupts predominantly during memory allocation. If there is a computation that runs for a long time and appears to be uninteruptible, this is the most likely culprit. Note: most anything in J can not proceed without allocating memory. ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
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