Kip Murray wrote:
>
> Here is my latest attempt to model set theory in J. All sets have
> distinct
> elements and are ordered by /:~ so that match -: determines whether two
> sets are
> the same. Sets must be created by the verb set or by provided operations.
> The
> intention is theoretical not practical! --Kip Murray
>
> ]A =: set 0;'b';2 NB. elements 0 'b' 2 are put in boxes preceding the
> last
> ┌─┬─┬─┬┐
> │0│2│b││
> └─┴─┴─┴┘
> ]B =: set 2;'b';'b';0 NB. same elements so same set
> ┌─┬─┬─┬┐
> │0│2│b││
> └─┴─┴─┴┘
>
> A -: B
> 1
>
> ]C =: set 2;'b';'c';'d'
> ┌─┬─┬─┬─┬┐
> │2│b│c│d││
> └─┴─┴─┴─┴┘
>
> B sand C NB. intersection, "set and"
> ┌─┬─┬┐
> │2│b││
> └─┴─┴┘
> B sor C NB. union
> ┌─┬─┬─┬─┬─┬┐
> │0│2│b│c│d││
> └─┴─┴─┴─┴─┴┘
>
> (A sand B sor C) -: (A sand B) sor (A sand C) NB. distributive law
> 1
>
> pwrset A NB. A has 3 elements, power set has 2^3, including the empty
> set
> ┌──────┬────────┬────┬──────┬────┬──────┬──┬────┬┐
> │┌─┬─┬┐│┌─┬─┬─┬┐│┌─┬┐│┌─┬─┬┐│┌─┬┐│┌─┬─┬┐│┌┐│┌─┬┐││
> ││0│2││││0│2│b││││0││││0│b││││2││││2│b│││││││b││││
> │└─┴─┴┘│└─┴─┴─┴┘│└─┴┘│└─┴─┴┘│└─┴┘│└─┴─┴┘│└┘│└─┴┘││
> └──────┴────────┴────┴──────┴────┴──────┴──┴────┴┘
> NB. Elements are contained in boxes preceding the last which is always
> NB. the Boxed Empty a: (Ace). The use of a: permits a unique and
> visible
> NB. empty set, viz
>
> (,a:) -: E =: A less A NB. see verb less below
> 1
> E
> ┌┐
> ││
> └┘
> a:
> ┌┐
> ││
> └┘
> E -: a:
> 0
>
> NB. Definitions
>
> E =: ,a: NB. empty set
> set =: a: ,~ [: /:~ ~. NB. create set from boxed list y
> NB. each box of y encloses an element
> get =: { }: NB. get boxed elements (from curtail because
> NB. elements are inside boxes of curtail)
> isin =: e. }: NB. Do boxes in list x contain elements of y?
> less =: a: ,~ -.&}: NB. remove elements of y from x
> sand =: [ less less NB. intersection, "set and"
> sor =: a: ,~ [: /:~ [: ~. ,&}: NB. union, "set or"
> diff =: less sor less~ NB. symmetric difference
> card =: [: # }: NB. count elements: cardinality
> issubs =: [ -: sand NB. Is x a subset of y?
> pwrset =: a: ,~ [: /:~ ] (<@#~) 1 (,~"1) 2 (#"1~ {:)@#:@i...@^ #...@}:
> NB. pwrset by Raul Miller, adapted
> islist =: 1 = #...@$ NB. islist through isunique from validate.ijs
> isboxed =: 0 < L.
> issorted =: -: /:~
> isunique =: -: ~.
> isset =: islist *. isboxed *. (a: -: {:) *. issorted@:}: *. isunique@:}:
> NB. isset y asks, is array y a set?
> iselement =: <@[ isin ] NB. Is array x an element of set y?
>
> NB. End
>
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>
There is a caveat:
set =: a: ,~ [: /:~ ~.
E =: ,a:
set E
+++
|||
+++
set 1;E
+-+++
|1|||
+-+++
set E;1
+-+--++
|1|++||
| |||||
| |++||
+-+--++
set E;1;E
+-++--++
|1||++||
| ||||||
| ||++||
+-++--++
--
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