The array and verb were an example; didn't want to copy anybody's array and
boolean mask.
A general purpose solution, to me, is a verb (that generates the mask), not an
explicitly defined noun, lines above. It's how I've saved the snippet in my
snippets file, and how I use it. Plug-and-play, or hot-swappable come to mind.
({~ ([: <"1 $ #: [: I. 0 = 3 | ,)) i.2 3 4
A or the trivial predicate would be something like 1:"0, generating all the
array's 'full' (rank 0? its atoms'? don't know proper wording) indices.
Maybe I shouldn't've replied.
Dec 11, 2023, 20:01 by [email protected]:
> Sorry, Viktor, but I don't see your point.
> (>:i.8)* 8 8 $ 1j2 # 1
> generates an integer array, not a Boolean.
>
> I agree (5&=) yields a boolean, but is it in any way a general-purpose
> condition?
>
> You might note that Henry Rich proposed the same solution to LdBeth's enquiry.
> He (HR) might understand what you're driving at!
>
> Cheers,
>
> Mike
>
> On 11/12/2023 17:09, 'Viktor Grigorov' via Programming wrote:
>
>> A fully tacit solution would encapsulate the predicate too, here
>> parenthesized for emphasis:
>>
>> ($ #: [: I. [: (5&=) ,) (>:i.8)* 8 8 $ 1j2 # 1
>> 4 1
>> 4 4
>> 4 7
>>
>>
>>
>> Dec 11, 2023, 17:01 [email protected]:
>>
>>> I see Pablo has just replied, but here's a similar/same approach, as a
>>> tacit
>>> expression:
>>>
>>> b =. 0=3|i.2 3 4
>>> ($#:I.@,) b
>>> 0 0 0
>>> 0 0 3
>>> 0 1 2
>>> 0 2 1
>>> 1 0 0
>>> 1 0 3
>>> 1 1 2
>>> 1 2 1
>>> Might be of use,
>>>
>>> Mike
>>>
>>> Sent from my iPad
>>>
>>>> On 11 Dec 2023, at 10:06, LdBeth<[email protected]> wrote:
>>>>
>>>> I've been running to too many situations that I uses 4$.$.y a lot to
>>>> get indexes of all non zero elements to an array, I wonder if there
>>>> are any alternative ways to do that in J, especially when y is a
>>>> boolean array.
>>>>
>>>> Also monadic I. only works for 1D array, while ⍸ from APL works
>>>> for higher dimension cases, how to implement this in J?
>>>>
>>>> LdBeth
>>>>
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