In APL, how can I compute the lowest unused positive integer from a given set of integers?

Question

For example, given 1 8 4 9 0 2 , return 3. Thanks.

Answer 1

  {⍬⍴(⍳1+⍴⍵)~⍵} 1 8 4 9 0 2

3

Answer 2

The case where they are all used seems to have "one plus the largest" as the answer, but whether that is "from" the list is another question.

@Paul, are you using APL385 as your browser's font? That seems to have cleared up many things from this end.

Answer 3

Try

((⍳⍴n)∊n)⍳0

or

((iota rho n) element n) iota 0

This is more or less the standard idiom for choosing the next available file tie number.