[Tex/LaTex] the best way to scan over a list of somethings

comma-separated listmacrosprogramming

It seems that a common operation needed when defining new commands is to scan over a list of arguments (separated e.g. by a comma, but maybe by something else) and do something with each argument in the list, find the last one, or whatever.

For example to write commands that look like \Command{a,b,c} or \Command{c.d.e} and
do something with each a, b, and c. See e.g. questions on defining a list of operators and ignoring even numbered inputs.

Some solutions have been posted there, but what would be the best idiom to perform such kind of operations? LaTeX's \@for looks very nice, but is it possible to use it to scan anything other than comma-separated lists?

Best Answer

I have not progressed to the level of processing a list myself but I have been using etoolbox by Philipp Lehman for some other TeX programming tasks I am doing. I believe 3.7 List processing should give you a good solution.

For example (on a recent version of etoolbox) you can write:

\DeclareListParser*{\myfor}{;}
\myfor{\fbox}{item1; item2; item3}

Related Solutions

[Tex/LaTex] How to define a command to feed the contents of a list into an \ifcase construct

This is similar to @egreg's solution and avoids \ifcase in favour of just cycling the list, but the coding is probably a bit simpler (unless you'be already loaded expl3 for other reasons)

\documentclass{article}
\usepackage{xcolor}
\def\ColorList#1{\def\xcolorlist{#1}}
\let\xsection\section
\def\section{\expandafter\xxcycle\xcolorlist,\xcolorlist\xsection}
\def\xxcycle#1,#2{%
  \ifx\relax#1\relax\else
   \color{#1}%
   \ifx\xcolorlist#2\else
    \xcycle#1,#2%
   \fi
  \fi}


\def\xcycle#1,#2\fi\fi#3\xcolorlist{\fi\fi\ColorList{#2#3#1}}



\ColorList{yellow,magenta,cyan,orange,blue,green!30,red!50!black}
%\ColorList{yellow}
%\ColorList{}

\begin{document}

\section{Title}
\section{Title}
\section{Title}
\section{Title}
\section{Title}
\section{Title}
\section{Title}
\section{Title}
\section{Title}

\end{document}

[Tex/LaTex] parse variable length comma separated list into command

Here's an option using sequences, it could probably be made a little more compact as the code from the two helper macros is almost identical.

I'm not an expert so take my advice with a grain of salt but I would just stick with using sequences. They require an extra line of code here and there, but the additional options that they provide are well worth it.

\documentclass{article}
\usepackage{xparse}
\usepackage{siunitx}
\ExplSyntaxOn
\NewDocumentCommand{\sumofsquares}{ m o }
    {
        \IfNoValueTF {#2}
            % if optional is missing call this
            {\sum_of_squares:n {#1}}
            % if not, then call this
            {\sum_of_squares:nn {#1}{#2}}
    }

\cs_new_protected:Npn \sum_of_squares:n #1
    {
        % does what the name suggests, set a sequence from the clist
        \seq_set_from_clist:Nn \l_tmpa_seq {#1}
        % applies final arg to each element of second seq and stores result in first seq
        \seq_set_map:NNn \l_tmpb_seq \l_tmpa_seq {(##1)^2}
        % \seq_use puts the items from the seq back in the input with "+" as a separator
        \sqrt{\seq_use:Nnnn \l_tmpb_seq {+}{+}{+}}
    }
\cs_new_protected:Npn \sum_of_squares:nn #1 #2
    {
        % same as above, but with the addition of units
        \seq_set_from_clist:Nn \l_tmpa_seq {#1}
        \seq_set_map:NNn \l_tmpb_seq \l_tmpa_seq {(\SI{##1}{#2})^2}
        \sqrt{\seq_use:Nnnn \l_tmpb_seq {+}{+}{+}}
    }


\ExplSyntaxOff
\begin{document}

$\sumofsquares{2,3,-4}$\par\medskip
$\sumofsquares{2,3,-4}[m/s]$

\end{document}

Here's a slightly shorter (and less clear) example using clists. The same method could have been used to shorten the code above.

\documentclass{article}
\usepackage{xparse}
\usepackage{siunitx}
\ExplSyntaxOn

\NewDocumentCommand{\sumofsquares}{ m O{} }
    {
        \sum_of_squares:nn {#1}{#2}
    }

\cs_new_protected:Npn \sum_of_squares:nn #1 #2
    {
        \tl_if_empty:nTF {#2}
            {
                \clist_set:Nn \l_tmpa_clist {#1}
                \sqrt{(\clist_use:Nnnn \l_tmpa_clist {)^2+(} {)^2+(} {)^2+(} )^2 }
            }
            {
                \clist_set:Nn \l_tmpa_clist {#1}
                \sqrt{(\clist_use:Nnnn \l_tmpa_clist {\,\si{#2})^2+(} {\,\si{#2})^2+(} {\,\si{#2})^2+(} \,\si{#2})^2}
            }
    }

\ExplSyntaxOff
\begin{document}

$\sumofsquares{2,3,-4}$\par\medskip
$\sumofsquares{2,3,-4}[m/s]$

\end{document}

Best Answer

Related Solutions

[Tex/LaTex] How to define a command to feed the contents of a list into an \ifcase construct

[Tex/LaTex] parse variable length comma separated list into command

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