Oops! should have been
timespacex '100 findxth 1;A'
0.0925588 1.35533e8
On Tue, Aug 30, 2022 at 8:02 AM Don Guinn <dongu...@gmail.com> wrote:
> NB. Find the index of some instance of a value
>
> findxth=:4 : 0
>
> 'value list'=.y
>
> try. x{,4$.$.value=list
>
> catch. _1
>
> end.
>
> )
>
> _20,\z=:?.100#10
>
> 4 6 8 6 5 8 6 6 6 9 3 2 3 1 9 2 7 0 9 5
>
> 7 7 9 7 4 8 7 4 2 1 1 0 4 3 9 3 2 7 4 4
>
> 0 3 7 5 9 6 3 2 8 2 0 3 5 4 0 0 3 1 5 0
>
> 4 0 2 9 2 4 0 0 7 7 5 7 3 1 6 3 1 5 8 9
>
> 8 5 3 9 9 8 9 5 1 1 0 4 1 7 3 2 3 4 0 4
>
> NB. Find index of the second occurence of 2
>
> 1 findxth 2;z
>
> 15
>
> NB. Find index of the third from last of 7
>
> _3 findxth 7;z
>
> 69
>
> NB. Try to find index of the tenth of 5 but does not exist
>
> 9 findxth 5;z
>
> _1
>
> NB. find the index of the middle or next to middle 8
>
> ({~<.@-:@#),4$.$.8=z
>
> 48
>
> A=. ?. 1e8#1e3
>
> 100 (2 i.~ +/\)@:E. A
>
> 1599
>
> timespacex '100 (2 i.~ +/\)@:E. A'
>
> 0.213069 1.20796e9
>
> timespacex '100 findxth 2;A'
>
> 0.0890775 1.35533e8
>
>
> On Mon, Aug 29, 2022 at 6:32 PM 'Pascal Jasmin' via General <
> gene...@jsoftware.com> wrote:
>
>> I thought that perhaps this could be fast:
>>
>> A=. ?. 1e8#1e3
>>
>> timespacex '100 (2 i.~ +/\)@:E. A' NB. or = instead of E.
>> 0.460527 1.20796e9
>>
>>
>> This application happens to everyone at least once, if
>>
>> (u { I.@E.)
>>
>> had special code where the result of u (or simply m) is presumed (for
>> speed optimization) to be a list/atom of top (rather than last) occurrences
>> of matches.
>>
>>
>> Here is a dyad that returns the xth occurrences (indexes) of x in y if
>> they exist. where x is in format x (, or ;) m where m is the list of
>> indexes requested.
>>
>> It is a bit slow due to the y =. n }. y step.
>>
>> xm =: {. ,&boxopen }. NB. split arguments as head ; tail. assignment as
>> 'h t' =. xm y
>>
>> findxth =: 4 : 0
>>
>> 'x m' =. xm x NB. m is sorted list of find indexes. x is match value.
>> if. 0 = #m do. m =. 0 end.
>> acc =. i.0
>> oset =. 0
>>
>> for_i. i. >./ m do.
>>
>> idx =. x i.~ y
>>
>> if. idx = # y do. acc return. end.
>>
>> if. i e. m do. acc =. acc , oset + idx end.
>>
>> oset =. oset + >: idx
>>
>> y =. (>: idx) }. y
>>
>> end.
>>
>> acc , oset + x i.~ y
>>
>> )
>>
>> 1 1 3 4 6 findxth 0 1 0 0 1 0 1 1 1
>>
>> 4 7 8
>>
>> (1 ; 1 3 4 6) findxth 0 1 0 0 1 0 1 1 1
>>
>> 4 7 8
>>
>> 1 findxth 0 1 0 0 1 0 1 1 1
>>
>> 1 NB. m is 0 (first item) if omitted.
>>
>>
>> 5 {. 100 I.@E. A
>>
>> 990 1599 2797 4550 5073
>>
>> 100 0 1 4 findxth A
>>
>> 990 1599 5073
>>
>>
>> On Friday, August 26, 2022 at 05:27:46 p.m. EDT, Devon McCormick <
>> devon...@gmail.com> wrote:
>>
>>
>>
>>
>>
>> I get these timings on J 9.04:
>>
>> A=. ?1e8#1e3
>> ts '100 find2 A'
>> 2.4e_6 1536
>> ts '100 (1{I.@E.) A'
>> 0.19722 8.39853e6
>> ts '100 ({.@}.@(I.@E.)) A'
>> 0.200139 8.39866e6
>> (100 find2 A) -: 100 (1{I.@E.) A
>> 1
>> find2
>> ([: >: i.~) + [ i.~ ] }.~ [: >: i.~
>>
>>
>> On Fri, Aug 26, 2022 at 3:53 PM 'Mike Day' via General <
>> gene...@jsoftware.com> wrote:
>>
>> > Does that include drop, }. ? I suppose it can, since we only need to
>> > move the pointer to the start... I’ll check on the laptop, once I’ve
>> done
>> > my Listener xwd.
>> >
>> > (Last week’s was the quarterly numeric puzzle, an ingenious
>> construction
>> > including among all the digits a few decimal points and solidus ( / )
>> for
>> > rationals!)
>> >
>> > Cheers,
>> > Mike
>> >
>> >
>> >
>> > Sent from my iPad
>> >
>> > > On 26 Aug 2022, at 20:36, Raul Miller <rauldmil...@gmail.com> wrote:
>> > >
>> > > Updating arrays without generating a new copy was introduce in J805 --
>> > > https://code.jsoftware.com/wiki/System/ReleaseNotes/J805
>> > >
>> > > So in J701, that approach would indeed be slower (since it's creating
>> > > a complete copy of the array).
>> > >
>> > > Also, virtual blocks (which speed up the }. approach) were introduced
>> > > in J807. I guess I need to roll up my sleeves and do some
>> > > benchmarking...
>> > >
>> > > Thanks,
>> > >
>> > > --
>> > > Raul
>> > >
>> > > On Fri, Aug 26, 2022 at 3:20 PM 'Mike Day' via General
>> > > <gene...@jsoftware.com> wrote:
>> > >>
>> > >> These seem simpler and are possibly quicker, at least in J701 on
>> this
>> > oldish iPad:
>> > >> 100 (1{I.@E.) A NB. Fails if there’s no (second) 100 in A
>> > >> 719
>> > >> 100 ({.@}.@(I.@E.)) A NB. Returns 0 in that case
>> > >> 719
>> > >> Easy to correct for such errors, of course.
>> > >>
>> > >> I tried
>> > >> A =. ?1000000#1000
>> > >> find2 =: 13 : 'F + ((F=.>:y i. x) }. y) i. x' NB. No direct defs in
>> > J701
>> > >> ts'100 find2 A'
>> > >> 0.020555 8.39091e6
>> > >> ts'100 (1{I.@E.) A'
>> > >> 0.011503 76160
>> > >> ts'100 ({.@}.@(I.@E.)) A'
>> > >> 0.007626 84864
>> > >>
>> > >> Speed a bit better, space quite a lot, if happy with time & space
>> > tests.
>> > >>
>> > >> Only you know if it’s wise to overwrite the first occurrence of V in
>> A!
>> > >>
>> > >> Cheers,
>> > >>
>> > >> Mike
>> > >>
>> > >>
>> > >>
>> > >> Sent from my iPad
>> > >>
>> > >>> On 26 Aug 2022, at 19:36, Raul Miller <rauldmil...@gmail.com>
>> wrote:
>> > >>>
>> > >>> i. returns the index of the first occurrence of a value within an
>> > array.
>> > >>>
>> > >>> So, when implementing an algorithm which needs the index of the
>> second
>> > >>> occurrence of the value within a (large) array, we need to do some
>> > >>> additional work.
>> > >>>
>> > >>> let's say that our array is A, the value is V
>> > >>>
>> > >>> F=: A i. V NB. the index of the first occurrence of V
>> > >>>
>> > >>> What's the most efficient way of finding the second occurrence?
>> > >>>
>> > >>> One possibility is
>> > >>> S=: (1+F) + ((1+F) }. A) i. V
>> > >>>
>> > >>> Another possibility, assuming that V is numeric and not zero, would
>> be
>> > >>> S=: (0 F} A) i. V
>> > >>>
>> > >>> But A is large, so perhaps a faster approach would be:
>> > >>> S=: {{ while. V~:y{A do. y=. y+1 end. y }} F
>> > >>>
>> > >>> (Which has me wishing that S=: A i.!.F V would do the job, though
>> I'm
>> > >>> not sure that that's completely appropriate...)
>> > >>>
>> > >>> But, we can probably eliminate the need to generate a copy of A
>> with a
>> > >>> little extra work:
>> > >>>
>> > >>> A=: 0 F} A
>> > >>> S=: A i. V
>> > >>> A=: V F} A
>> > >>>
>> > >>> It seems to me that this is probably going to be the fastest
>> approach.
>> > >>>
>> > >>> Can anyone think of a faster approach (or something with comparable
>> > >>> speed which isn't quite so unwieldy?)
>> > >>>
>> > >>> Thanks,
>> > >>>
>> > >>> --
>> > >>> Raul
>> > >>>
>> ----------------------------------------------------------------------
>> > >>> For information about J forums see
>> http://www.jsoftware.com/forums.htm
>> > >>
>> ----------------------------------------------------------------------
>> > >> For information about J forums see
>> http://www.jsoftware.com/forums.htm
>> > > ----------------------------------------------------------------------
>> > > For information about J forums see
>> http://www.jsoftware.com/forums.htm
>> > ----------------------------------------------------------------------
>> > For information about J forums see http://www.jsoftware.com/forums.htm
>> >
>>
>>
>> --
>>
>> Devon McCormick, CFA
>>
>> Quantitative Consultant
>>
>> ----------------------------------------------------------------------
>> 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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