$. y Sparse
When $.y is displayed in the J session, only its non-zero atoms are shown, together with their indexes.
$ y=: 0 0 1 0 2.5 0 0 999 _ 0 0 0 12 ] z=: $.y 2 | 1 4 | 2.5 7 | 999 8 | _ $ z 12
To save storage when storing a sparse array.
x $. y Sparse
n&$. is a collection of functions for creating and handling sparse arrays and converting them to/from ordinary arrays.
The noun n can take one of the values 0 1 2 3 4 5 7 8 or their negations. See the entry in the J Dictionary for details of the actual function specified by n plus the form of the y-argument it expects.
Chapter 30 of 'Learning J' by Roger Stokes (also accessible via 'LJ' from the J Help pages) gently introduces to Sparse Arrays, going through various examples.