How to count the frequency of a token in a token list

Question

Is it possible to count the number of a token in a token list?

\documentclass{article}

\usepackage{expl3}

\ExplSyntaxOn

\tl_new:N \tobi_list_tl
\tl_set:Nn \tobi_list_tl {foo~bar~baz}

\begin{document}
'\tl_use:N \tobi_list_tl'~contains~?~spaces.
\end{document}

---

In a macro to typeset syllables and prosoidic symbols I need to count the number of spaces in a token list.

\cs_new_protected:Npn \tobi_metrics:nn #1 #2
 {
  \tl_set:Nn \l_tobi_words_tl { #2 }
  % change spaces into -\q_tobi_space_marker-
  \tl_replace_all:Nnn \l_tobi_words_tl { ~ } { - \q_tobi_space_marker - }
  % split the (modified) second argument at -
  \seq_set_split:NnV \l_tobi_syllables_seq { - } \l_tobi_words_tl
  % split the first argument "at nothing"
  \seq_set_split:Nnn \l_tobi_measures_seq { } { #1 }
  % AT THIS POINT I WANT TO KNOW THE NUMBER OF SPACES                          <---------
  % IN \l_tobi_words OR THE NUMBER OF \q_tobi_space_marker                  <------------
  % IN \l_tobi_syllables_seq                                                   <---------
  \int_zero:N \l_tobi_process_int
  \seq_map_inline:Nn \l_tobi_syllables_seq
   {
    \int_incr:N \l_tobi_process_int
    \tl_if_eq:nnTF { ##1 } { \q_tobi_space_marker }
     {% we had a space, step back the counter and issue a space
      \int_add:Nn \l_tobi_process_int { -1 }
      \c_space_token
     }
     {% print the syllable with the metric marker
      \tobi_print_syllable:n { ##1 }
     }
   }
 }

See https://tex.stackexchange.com/a/124531/4918.

I need the number to check whether there are enaugh symbols in the list to by combined with syllables, this is my test so far:

\int_compare:nT
 {
  \seq_count:N \l_@@_syllables_seq - 1 != \seq_count:N \l_@@_symbols_seq
 }
 {
  ERROR
 }

The -1 must be replaced by the number of spaces.

---

Maybe I should add some information on the usage. This will be part of a macro

   \metrics{_    u  u  _   _  _  | _    _  u  u   _  _   _   }
           {flos ve-te-ris vi-ni | meis na-ri-bus ob-iec-tust}

The first argument is a space separated list of metric symbols (accesd by abbreviations like u or _) and the second list contains hyphenated words. I need to check if there is a symbol for every syllable but \seq_count: doesn’t work here since spaces from the second list are treated specially. The test must be

# of symbols         = # of syllables
lengt of symbol list = lengthh of syllablelist – # of spaces 

---

This is the latest version of the complete processing macro:

\cs_new_protected:Npn \__metrix_metrics:nn #1 #2
 {
  \tl_set:Nn \l__metrix_words_tl { #2 }
  \tl_replace_all:Nnn \l__metrix_words_tl { ~ } { - \q__metrix_space_marker - }
  \seq_set_split:NnV \l__metrix_syllables_seq { - } \l__metrix_words_tl
  \seq_set_split:Nnn \l__metrix_symbols_seq { ~ } { #1 }
  \int_zero:N \l__metrix_process_int
  \seq_map_inline:Nn \l__metrix_syllables_seq
   {
    \tl_if_eq:nnT { ##1 } { \q__metrix_space_marker }
     { \int_incr:N \l__metrix_process_int }
   }
  \int_compare:nTF
   {
    \seq_count:N \l__metrix_syllables_seq -
      \seq_count:N \l__metrix_symbols_seq = \l__metrix_process_int
   }
   {
    \int_zero:N \l__metrix_process_int
    \seq_map_inline:Nn \l__metrix_syllables_seq
     {
      \int_incr:N \l__metrix_process_int
      \tl_if_eq:nnTF { ##1 } { \q__metrix_space_marker }
       {
        \int_add:Nn \l__metrix_process_int { -1 }
        \c_space_token
       }
       {
        \str_case:nnn { ##1 }
         {
          { |  } { \__metrix_l_break:  }
          { || } { \__metrix_ll_break: }
         }
         {
          \__metrix_print_syllable:n { ##1 }
         }
       }
     }
   }
   {
    \__metrix_error_msg:n { Numbers~of~symbols~and~syllables~mismatch. }
   }
 }

Every works except when one or both of the lists end with a space, so it would be great if all spaces could be cut from the right side before start processing the lists.

Answer 1

You can do it in several ways.

First way (map the syllable sequence)

%%% Code to add for checking the syllables and the measures are the same number
\int_zero:N \l_tobi_process_int
\seq_map_inline:Nn \l_tobi_syllables_seq
 {
  \tl_if_eq:nnT { ##1 } { \q_tobi_space_marker }
   { \int_incr:N \l_tobi_process_int }
 }
\int_compare:nF
 {
  \seq_count:N \l_tobi_syllables_seq -
    \seq_count:N \l_tobi_measures_seq = \l_tobi_process_int
 }
 { ERROR! }
%%% End of code to add

Second way (direct count)

This requires l3regex:

\RequirePackage{l3regex}
\cs_generate_variant:Nn \regex_count:nnN { nV }

%%% Code to add for checking the syllables and the measures are the same number
\regex_count:nVN { \c{q_tobi_space_marker} } \l_tobi_words_tl \l_tobi_process_int
\int_compare:nF
 {
  \seq_count:N \l_tobi_syllables_seq -
    \seq_count:N \l_tobi_measures_seq = \l_tobi_process_int
 }
 { ERROR! }
%%% End of code to add

Answer 2

Perhaps the simplest solution is to use a standard TeX delimited argument approach. As we are delimiting by spaces, that is slightly trickier than normal:

\documentclass{article}

\usepackage{expl3}

\ExplSyntaxOn

\tl_new:N \tobi_list_tl
\tl_set:Nn \tobi_list_tl {foo~bar~baz}
\use:x
  {
    \cs_new:Npn \exp_not:N \tl_count_spaces:n ##1
      {
        \exp_not:N \int_eval:n 
          { 
            -1 \exp_not:N \__tl_count_spaces:w ##1 ~
              \exp_not:N \q_recursion_tail \c_space_tl 
              \exp_not:N \q_recursion_stop
          }
      }
  }
\cs_new:Npn \__tl_count_spaces:w #1 ~
  {
    \quark_if_recursion_tail_stop:n {#1}
    +1
    \__tl_count_spaces:w
  }
\cs_generate_variant:Nn \tl_count_spaces:n { V }
\begin{document}
'\tl_use:N \tobi_list_tl'~contains~\tl_count_spaces:V \tobi_list_tl \c_space_tlspaces.
\end{document}

You could use a two-part approach, using \seq_set_split:Nnn to store the token list split on spaces, then \seq_count:N to get the value.