How to process a list and ignore spaces?

Question

This question is part of a new package:
metrix. Typeset metric/prosodic symbols standalone and above syllables.

I’m wrinting a new package and I need to process a list (using etoolbox at the moment but that can change …). Everything works but I want the list to do two things that don’t work:

1. Ignore spaces, because xstring doesn’t get the right strings otherwise.
2. Process an empty list element. It is ignored at the moment.

\documentclass{article}

\usepackage{etoolbox}
\usepackage{xstring}

\setlength{\fboxsep}{0pt}
\DeclareListParser{\dolist}{-}
\newcommand{\mylist}[1]{%
    \renewcommand{\do}[1]{%
        \IfStrEqCase{##1}{%
            {}{0}%
            {a}{1}%
            {b}{2}%
            {c}{3}%
        }[?]%
    }%
    \dolist{#1}%
}

\begin{document}
    1. \mylist{a-b-c--a}

    2. \mylist{ a- b- c- - a}

    3. \mylist{ a - b  - c  -  - a }
\end{document}

The output is

1. 1231
2. 1231
3. ????

but it should be the following in all three cases

12301

---

What I really wan’t to know ;-)

I’m implementing a package to typeset the metrics of (latin) verses, and I’d like to have a macro like

\metrics{_    u  u  _   }
        {quid co-me-dent}

that renders as

What I’ve got so far is a macro that works with this syntax

\metrics{_-\ -u-u-_}
        {quid-\ -co-me-dent}

Obviously there are two differences the space between two words must be separated with hyphen as if it is a syllable and it must be escaped with \. It would be great if the first list (the symbols) can be a space separated list and the second on a combination of hyphen and space separated.

You may download my full code from my web site: http://tweh.de/texsx/metrics.tex

Answer 1

You can first replace the space with a special marker, say -\q_tobi_space_marker- so you can split at hyphens and have a way to check whether the item was a space. Here's an implementation.

Plan of attack:

1. We change spaces in the way outlined before
2. The two lists are split into sequences
3. The "syllables" sequence is mapped item by item, stepping a counter each time;
   1. if the item is the space marker, step back the counter and issue a space
   2. otherwise use the syllable and put over it the right marker, looking at the "measures" sequence.

The symbol corresponding to the brevis or longa is obtained by defining two functions \tobi_u_mark: and \tobi___mark: so we can build it by using the item in the "measures" sequence.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\metrics}{mm}
 {
  \tobi_metrics:nn { #1 } { #2 }
 }

\tl_new:N \l_tobi_words_tl
\seq_new:N \l_tobi_syllables_seq
\seq_new:N \l_tobi_measures_seq
\quark_new:N \q_tobi_space_marker
\int_new:N \l_tobi_process_int

\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 }
  \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 }
     }
   }
 }
\cs_new_protected:Npn \tobi_print_syllable:n #1
 {% Just a rough approximation, use your method
  \leavevmode\vbox{\offinterlineskip
    \halign{##\cr\hidewidth \tobi_print_meter: \hidewidth \cr #1 \cr}}
 }
\cs_new_protected:Npn \tobi_print_meter:
 {
  \use:c {tobi_\seq_item:Nn \l_tobi_measures_seq { \l_tobi_process_int }_mark:}
 }
\cs_new:Npn \tobi_u_mark: { \Large \u{} }
\cs_new:Npn \tobi___mark: { \Large -- }
\ExplSyntaxOff

\begin{document}
\metrics{_    u  u  _   }
        {quid co-me-dent}
\end{document}

Probably one should add some error checks on whether the lengths of the sequences agree. See How to count the frequency of a token in a token list.

Answer 2

\documentclass{article}
\usepackage{stringstrings}

\newcommand{\mylist}[1]{%
  \edef\thestring{#1-}%
  \noblanks[q]{\thestring}%
  \convertword[q]{\thestring}{--}{-0}%
  \convertword[q]{\thestring}{a-}{1}%
  \convertword[q]{\thestring}{b-}{2}%
  \convertword[q]{\thestring}{c-}{3}%
  \convertword{\thestring}{d-}{4}%
}

\begin{document}
    1. \mylist{a-b-c--a}

    2. \mylist{ a- b- c- - a}

    3. \mylist{ a - b  - c  -  - a }
\end{document}