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.
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