What's a more concise way to count the number of times an element occurs in a list in J?

Answer 1

When I do this, knowing that I'm only ever going to have a list and not a matrix, I use:

count =: 4 : '+/x=y'

Or for multiple searches in the list:

count =: 4 : '+/x=y'"0 1

Your method copies over only the elements that are equal to x, then counts the result. Summing the ones for equality is one less operation.

Answer 2

I agree with the algorithm provided by MPelletier. Since you're asking for concision, tacit phrasings may be worth looking at. Here's one such program, assigned to a name:

count =: +/ @: =

It can also be used as an anonymous verb, as in this example:

   4 (+/ @: =) 3 4 4 3 4 7 9
3

A synonym is [: +/ =

As MPelletier said, this algorithm works when what you want to count are atoms in a simple list. (A similar need that would require a different approach would be counting matrices matched in a list of similarly-shaped matrices.)