I don't know what you mean, but filtering and selection can be accomplished in
many ways in J. The most fundamental, in my
opinion, is # . To wit:
] some_numbers =. 5 ? 10
2 7 1 9 5
greater_than_4 =. 4 < ]
] mask =. greater_than_4 some_numbers
0 1 0 1 1
mask # some_numbers
7 9 5
(#~ 4 < ]) some_numbers
7 9 5
Here, mask was a boolean list which indicated which elements of some_numbers
to keep and which to discard. That is, told #
which elements to filter.
My mnemonic for was (when I started), that # looks like a little net or
sieve. When you put things in a net, you lose whatever
lies over a hole, and you keep everything else. In a boolean list, the 0s
(little circles, absences) are the holes, and the 1s
the lines (presences), which together make up the net.
-Dan
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