I think we have agreed that (32 = 3!:0) tests whether y is boxed. The original
issue was the behavior of Link ; when the right argument is an empty list of
boxes:
E =: 0$<1
3!:0 E NB. E is boxed:
32
1;E
+-++
|1||
+-++
(<1),E
+-+
|1|
+-+
The Dictionary:
x;y is (<x),y if y is boxed, and (<x),<y if y is open.
My interest:
Properties of E as the empty set, where "set" is modeled as a sorted list of
boxes, without repetitions, and "element" is the array contained in a box.
IsSet =: [: *./ (-: /:~),(-: ~.),(1 = #...@$),(32 = 3!:0)
IsSet E
1
bill lam wrote:
> On Tue, 28 Jul 2009, Dan Bron wrote:
>> Kip Murray wrote:
>>> E =: 0$<1
>>> 3!:0 E NB. E is boxed:
>>> 32
>>>
>> Raul responded:
>>> L.E
>>> 0
>> To be explicit, Raul's suggesting you change your test for "is boxed". A
>> common test for "is boxed" in J is (0<L.) . So you could rewrite your
>>
>
> I think both tests are useful depending on context. From DOJ
>
> If y is open or empty, L. y is 0; if it is boxed and
> non-empty, the level is one plus the maximum of the
> levels of the opened elements.
>
> So that L. tests for both empty and box. If it is known or assumed to
> be non-empty, then using L. will require less typing.
>
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