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Sparse Arrays

  Datatype
  Representation: indices, non-“zero” values
  

   x=: (?.3 4$10) * ?.3 4$2
   xs=: $. x

   x                xs
0 5 9 0          0 1 | 5
0 9 0 7          0 2 | 9
0 0 0 0          1 1 | 9
                 1 3 | 7