Write 4 : 'x&{.&.;: y' tacitly

Question

Conor Hoekstra recently solved a Leetcode problem in APL https://youtu.be/QtvvQ7MdwKY The problem is to take the first x words from a character string y

In J, using &. (Under) and ;: (Words) I can come up with a nice explicit one liner

solve =. 4 : 'x&{.&.;: y'         NB. Box words in y -> take first x-> unbox words in result while retaining spaces between
   s=. 'Hello how are you Contestant'
   4 solve s
Hello how are you

The trouble I am having is finding the tacit version that still includes &., mainly because I believe x needs to be bound to {. during the creation of the verb. This is also an example where the magic 13 : conversion is not helpful

   13 : 'x&{.&.;: y'
4 : 'x&{.&.;: y'

I can solve it tacitly by using ;:^:_1 to create the inverse of ;:

   solve2=. (;:^:_1)@:({. ;:)
   4 solve2 s
Hello how are you

But that is not nearly as pretty as the tacit version of 4 : 'x&{.&.;: y' could be.
Anyone have a pretty tacit solution for 4 : 'x&{.&.;: y'?

score 8 · Answer 1 · answered Apr 06 '21 at 20:50

8

With semiduals (introduced with J902 or J903?) you can write 4 {.&.(a:`;:) s. Then ;: gets only applied to the right argument, while still doing the inverse later. u&.(f`a:) would apply f only on the left argument.

answered Apr 06 '21 at 20:50

xash

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Wow, I did not know this. Every time I turn around this language surprises me and I have been playing with it for 20 years! Off to play some more. Thank you xash – bob Apr 07 '21 at 22:24

score 3 · Accepted Answer · answered Apr 06 '21 at 06:24

3

mainly because I believe x needs to be bound to {. during the creation of the verb.

If creating a verb with an input is the key, shouldn't you use an adverb?

   solve =: 1 : 'm&{.&.;:'
   4 solve
4&{.&.;:
   (4 solve ,: 2 solve) 'Hello how are you Contestant'
Hello how are you
Hello how

solve itself isn't tacit (or a verb), but 4 solve is tacit verb.

answered Apr 06 '21 at 06:24

Julian Fondren

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Yes, thank you Julian. I was not thinking of adverbs as a valid tacit solution. Great answer. – bob Apr 06 '21 at 06:29
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...and `_3 grab_words 'are you ready to go'` if you want to get words from end of string. – user3234709 Apr 06 '21 at 10:04

score 1 · Answer 3 · answered May 27 '22 at 17:14

1

I don't think you can write this as a tacit verb, but it's easy to do as a tacit adverb: ]:&{.&.;:.

answered May 27 '22 at 17:14

Omar Antolín-Camarena

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Yes, you are quite correct, although at the time that the question was asked, tacit modifiers had not been reintroduced to the language. – bob May 30 '22 at 16:37

Write 4 : 'x&{.&.;: y' tacitly

3 Answers3