there are more ways than one to skin a cat
In J many operations that are traditionally grouped into specific libraries (string operations, list operations) are generalized for broader range of arrays and distinction can be blurred between character or numerical data.
The cut u;.n conjunction represents a family of partitioning operations taken by selector n, much like circle x o. functions. As other J traditions go, the subset is not merely returned, but a verb in u is applied to them; also the arguments, indices and subsets are in general with multidimensional arrays in mind.
Ranges u;.0
This is generalization of most common operation substring or more generally, selecting a range from an array.
The range is given in x argument by starting point and extent (size). But because it is generalized to higher arrays, these are given not as scalars but as index and shape vectors. To accomodate this pair we need a 2×N table where N is rank (number of dimensions) of our indices.
A=. 'abcdefghij'
(,:~ {:@":"0@i.@#) A NB. indices of items
0123456789
abcdefghij
(3,:4) ];.0 A NB. 4 items from 3
defgNegative position counts from back index of tail. Negative size reverses result.
(_3,:4) ];.0 A NB. 4 items with 3rd from end efgh (3,:_4) ];.0 A NB. reverse result gfed
Multiple differently shaped subsets in one operation are possible. The left rank of cut is 2 and normal expansion for higher rank is used.
((2,:3),:(3,:4)) <;.0 A NB. multiple boxed cuts +---+----+ |cde|defg| +---+----+
Here is an example with a table.
]B=: 4 6$a.{~97+i.24 NB. 4×6 alphabet table
abcdef
ghijkl
mnopqr
stuvwxRank of indices gives the frame of operation, cells are complemented to argument rank.
(1,:2) ];.0 B NB. two rows from 1 ghijkl mnopqr (1 2,:2 3) ];.0 B NB. 2×3 block from (1,2) ijk opq