wiki page note

[P Padil]

At the bottom of https://code.jsoftware.com/wiki/Vocabulary/curlylf#dyadic,
the table "Follow a chain of variable-length records”, it says x {~^:a: y "Produces list of starting positions. Limit all values of x to at most #x, then append _1 to the end of x. Discard the final starting position of _1.”

I must be missing some context, I do not see where the _1 is coming from. Is it assuming _1 is the marker for the end of each record?

Also, the table indicates that x and y are integers, but then it says "Limit all values of x to at most #x”. Should both x and y be integer arrays? Tried several experiments but J told me I had no clue what I was doing...

Any clarifications appreciated.

Thanks
Pete

[Henry Rich]

   ]b =. 100 200 300 400 (+ i.)&.> 1 4 2 3  NB. 4 variable-length records
+---+---------------+-------+-----------+
|100|200 201 202 203|300 301|400 401 402|
+---+---------------+-------+-----------+
   ]r =. ; (>:@# , ])&.> b  NB. encode as length-then-data
2 100 5 200 201 202 203 3 300 301 4 400 401 402
   ]rr =. _1 ,~ (+ i.@#) r  NB. convert each len to index of next rcd
2 101 7 203 205 207 209 10 308 310 14 411 413 415 _1
   ]rcds =. rr {~^:a: 0   NB. find record positions
0 2 7 10 14 _1
   ]s =. 1 _1 +"1 (2) -~/\\ }: rcds  NB. convert to start/length of
each record
 1 1
 3 4
 8 2
11 3
   (,."1 s) <;.0 r  NB. extract data & box
+---+---------------+-------+-----------+
|100|200 201 202 203|300 301|400 401 402|
+---+---------------+-------+-----------+

Henry Rich

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[P Padil]

Thanks for the example! (I would have figure it out by about the same time the sun is predicted to turn into a red giant.) Maybe the example would make a good essay. Thanks again.