I've just discovered array/ matrix languages, and especially J. It's so a pleasure to use it. I struggle since a long time to use python or r for their mixed scalar/array style. K/J/apl are so clean and well designed in this regard.

For instance: Eratosthene's sieve to get all prime under 100 : " (-.] * /])2}.i.100 " Another great property of J is his natural language like structure :

without =: -.

multiplied_by =: */

upto =: 4 : 'x}.i.1+y'

Sieve_100 =: (2 upto 100) without (2 upto 100) multiplied_by (2 upto 100)