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Amend with multiple indices per substitution in J

In J, how do you idiomatically amend an array when you have:

substitution0    multipleIndices0
...
substitutionN    multipleIndicesN

(not to be confused with:

substitution0    multipartIndex0
...
substitutionN    multipartIndexN

)

For example, my attempt at the classic fizzbuzz problem looks like this:

   i=.3 :'<@I.(,*./)(=<.)3 5%~"(0 1)y'
   d=.1+i.20
   'fizz';'buzz';'fizzbuzz' (i d)};/d
|length error
|   'fizz';'buzz';'fizzbuzz'    (i d)};/d

I have created the verb m} where m is i d which is 3 boxes containing different-sized lists of 1-dimensional indices, whereas I think } expects m to be boxes containing single lists that each represent a single index with dimensions at least as few as the rank of the right argument.

How is this generally solved?

Solution 
  • 'fizz';'buzz';'fizzbuzz' (i d)};/d

    This has a few problems:

    1. the x param of } is 'fizzbuzz', not a list of boxes, as the } happens before the ;s on the left. You mean
    ('fizz';'buzz';'fizzbuzz') (i d)};/d
    1. the boxed numbers in the m param of } are not interpreted as you expect:
       _ _ (1 1;2 2) } i.3 3
0 1 2
3 _ 5
6 7 _
   _ _ (1 1;2 2) } ,i.3 3  NB. your length error
|length error
|   _ _    (1 1;2 2)},i.3 3
    1. If you raze the m param you get the right kind of indices, but still don't have enough members in the x list to go around:
       _ _ (;1 3;5 7) } i.9
|length error
|   _ _    (;1 3;5 7)}i.9
   _ _ _ _ (;1 3;5 7) } i.9
0 _ 2 _ 4 _ 6 _ 8

    These work:

       NB. raze m and extend x (fixed)
   i=.3 :'<@I.(,*./)(=<.)3 5%~"(0 1)y'
   d=.1+i.20
   ((;# each i d)#'fizz';'buzz';'fizzbuzz') (;i d)};/d
+-+-+----+-+----+----+-+-+--------+----+--+----+--+--+----+--+--+--------+--+----+
|1|2|fizz|4|fizz|buzz|7|8|fizzbuzz|buzz|11|fizz|13|14|buzz|16|17|fizzbuzz|19|fizz|
+-+-+----+-+----+----+-+-+--------+----+--+----+--+--+----+--+--+--------+--+----+


   NB. repeatedly mutate d
   i=.3 :'<@I.(,*./)(=<.)3 5%~"(0 1)y'
   d=.;/1+i.20
   ('fizz';'buzz';'fizzbuzz'),.(i ;d)
+--------+--------------+
|fizz    |2 5 8 11 14 17|
+--------+--------------+
|buzz    |4 9 14 19     |
+--------+--------------+
|fizzbuzz|14            |
+--------+--------------+
   0$(3 : 'd =: ({.y) (1{::y) }d')"1 ('fizz';'buzz';'fizzbuzz'),.(i ;d)
   d
+-+-+----+-+----+----+-+-+----+----+--+----+--+--+--------+--+--+----+--+----+
|1|2|fizz|4|buzz|fizz|7|8|fizz|buzz|11|fizz|13|14|fizzbuzz|16|17|fizz|19|buzz|
+-+-+----+-+----+----+-+-+----+----+--+----+--+--+--------+--+--+----+--+----+