How to know the number of elements in a clist in LaTeX3?

Question

I could not find in the LaTeX3 documentation any function to count the number of elements in a clist. So I coded my own, but it is likely to be very suboptimal.

Is there any better alternative?

To be precise, \clist_count:NN stores in its first argument (of type int) the length of its second argument (a clist)

\cs_new:Nn \clist_count:NN
{
  \int_set:Nn \l_tmpa_int {0}
  \clist_set_eq:NN \l_tmpa_clist #2
  \bool_until_do:nn 
  {
    \clist_if_empty_p:N \l_tmpa_clist
  }{
    \clist_pop:NN \l_tmpa_clist \l_tmpa_tl
    \int_add:Nn \l_tmpa_int {1}
  }
  \int_set_eq:NN #1 \l_tmpa_int
}

I am also interested in a function that would pick the n-th element of a clist.

Answer 1

EDIT (by Bruno): the correct function to use now is \clist_count:N, which essentially derives from Will's implementation below. There is also \clist_count:n which expects an explicit comma separated list as its argument rather than a list stored inside a variable. Both functions expand to an explicit integer.

---

Here's a solution that should be quite a bit faster (although I haven't tested it):

\cs_new:Npn \clist_length:N #1 {
  \int_eval:n {
    0 \clist_map_function:NN #1 \tl_elt_count_aux:n
  }
}

Here, \clist_length:N expands to the length of the comma-list; it's expandable, so you can use it inside \int_set:Nn if you like.

And here's a solution for expandably extracting the n-th item of a comma list:

\cs_new:Nn \clist_nth:Nn {
  \int_compare:nTF { \clist_length:N #1 < #2 }
  { \ERROR }
  {
    \exp_after:wN \clist_nth_aux:nn \exp_after:wN {#1} #2
  }
}
\cs_new:Nn \clist_nth_aux:nn {
  \clist_nth_aux_i:nnnw {1}{#2} #1 , \q_recursion_tail \q_recursion_stop
}
\cs_new:Npn \clist_nth_aux_i:nnnw #1#2#3, {
  \quark_if_recursion_tail_stop:n {#3}
  \int_compare:nTF {#1==#2}
  { 
    \use_i_delimit_by_q_recursion_stop:nw {#3}
  }
  {
    \clist_nth_aux_i:fnnw { \int_eval:n {#1+1} } {#2}
  }
}
\cs_generate_variant:Nn \clist_nth_aux_i:nnnw {f}

Thanks for these questions; I've been meaning to add them to l3clist for a little while. P.S. Sorry for the slow reply!