X =: (&{::)(@:[)
delitem =: ;@:((1 X {. ]) ; (0 X + 1 X) }. ]) NB. len idx :X str :Y
delitemG =: delitem^:(#@] >: +/@[) NB. guarded: do nothing if out of bounds
(delitemG~ 2, 0 i.~ 2&(+/\))(^:_) 1 3 _3 3 5
1 3 5
On Friday, December 6, 2019, 07:09:57 p.m. EST, Henry Rich
<[email protected]> wrote:
join =: 4 : 0
neq =. (- |. x) i.&0@:=&((x <.&# y)&{.) y NB. len of max prefix of y
that matches suffix of x
((-neq)}.x) , neq}.y
)
ddup =. (({. join&$: }.)~ <.@-:@#)^:(1<#)NB. split & recur
ddup 2 1 1 _1 2 _2 _1
2
Henry Rich
On 12/6/2019 6:26 PM, Louis de Forcrand wrote:
> Not particularly J-ish, but (array-shuffling aside) linear solution:
>
> s=: ,`(1}.])@.(= -@{.)/
> s 1 _1 2 _2{~100?.@#4
> 2 1 2 1 _2 _1 2 1 1 2 2 2 _1 2 _1 2 1 2 2 2 1 1 _2 _1 2 1 _2 _1 _2 _2 _2 _2
> _1 _2 _1 _1 _2 _2 1 1 1 2 1 _2 1 _2 _1 _1 _1 2
>
> Since the reduced form of the input list is unique, we are free to perform
> reductions in any order we please; in particular, we can start simplifying
> from the back, which is what s does.
> Might do strange stuff on an empty input list.
>
> Cheers,
> Louis
>
> ----Original Message----
> From : [email protected]
> Date : 06/12/2019 - 23:51 (CEST)
> To : [email protected]
> Subject : Re: [Jprogramming] Removing annihilating pairs
>
> If the answer to Jimmy's question is no, then the uniqueness of the resulting
> array has to do (surprisingly) with free groups
> (https://en.wikipedia.org/wiki/Free_group).
> Indeed we can view a vector of numbers as a word over the alphabet [0,∞) of
> positive real numbers (where negative numbers / zero are inverses of letters
> in our alphabet). The fact that all maximal reductions (reductions which
> contain no adjacent inverses) of a given word are equal means exactly that
> our reduced list is unique.
> For a proof see:
> https://math.stackexchange.com/a/2425147
>
> I'll look into this problem, it's interesting!
> Cheers,
> Louis
>
> ----Original Message----
> From : [email protected]
> Date : 06/12/2019 - 23:36 (CEST)
> To : [email protected]
> Subject : Re: [Jprogramming] Removing annihilating pairs
>
> Hi,
>
> is the expected output of transforming 1 3 _3 3 5 to be 1 3 5 or 1 5 ?
>
> On Fri, Dec 6, 2019 at 7:15 AM R.E. Boss <[email protected]> wrote:
>
>> Given an array with zero or more annihilating pairs, i.e., two subsequent
>> numbers which add up to zero, the question is to clean up the array by
>> deleting all annihilating pairs such that no such pairs are left.
>> I do have a solution that is both elegant and efficient (I believe), but I
>> am curious about other thoughts.
>>
>> foo 2 1 1 _1 2 _2 _1
>> 2
>>
>> foo 1 _1 2 _2{~100?.@#4 NB. Notice (?.)
>> 2 1 2 1 _2 _1 2 1 1 2 2 2 _1 2 _1 2 1 2 2 2 1 1 _2 _1 2 1 _2 _1 _2 _2 _2
>> _2 _1 _2 _1 _1 _2 _2 1 1 1 2 1 _2 1 _2 _1 _1 _1 2
>>
>>
>> R.E. Boss
>> ----------------------------------------------------------------------
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>>
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