Defining a macro with a variable number of arguments

Question

I want to define a macro that defines a series of other macros, each of which takes a variable (but in any given document fixed) number of arguments. I know that I can do this using something like this:

\documentclass{standalone}
\newcommand\MakeCommands[1]{%
  \ifcase#1\relax%
  \or\newcommand\MyCommand[1]{##1}%
  \or\newcommand\MyCommand[2]{##1, ##2}%
  \or\newcommand\MyCommand[3]{##1, ##2, ##3}%
  \or\newcommand\MyCommand[3]{##1, ##2, ##3, ##4}%
  \fi%
}
\begin{document}
  \MakeCommands{3}
  \MyCommand{one}{two}{three}
\end{document}

but I am sure that there is a better way of doing this...I get the feeling that it is time I learnt to use keys...

Does anyone have a better solution?

Answer 1

I would suggest creating a number of keys that the user can decide to use or not. It also allows them the freedom to specify only a portion of them, in any order they wish. Here's a quick example using xkeyval:

\documentclass{article}
\usepackage{xkeyval}
\makeatletter
% ========= KEY DEFINITIONS =========
\define@cmdkey{mycmd}{one}[one]{}
\define@cmdkey{mycmd}{two}[two]{}
\define@cmdkey{mycmd}{three}[three]{}
\define@cmdkey{mycmd}{four}[four]{}
% ========= KEY DEFAULTS =========
\setkeys{mycmd}{one,two,three,four}% Defaults
\newcommand{\MyCommand}[1]{%
  \begingroup%
  \setkeys{mycmd}{two=SECOND,#1}% Set defaults for this macro + new keys
  \texttt{one}: \cmdKV@mycmd@one;
  \texttt{two}: \cmdKV@mycmd@two;
  \texttt{three}: \cmdKV@mycmd@three;
  \texttt{four}: \cmdKV@mycmd@four
  \endgroup%
}
\makeatother
\begin{document}

\MyCommand{one}

\MyCommand{two=second,one=first,three=third}

\MyCommand{four=Last}

\end{document}

As reference, see How to create a command with key values?.

Answer 2

My answer is only Werner's answer by another tools: instead of usage of xkeyval, there are five lines of code with TeX primitives. The result is exactly the same as Werner's result.

\def\kv#1{\expandafter\ifx\csname kv:#1\endcsname \relax \expandafter\kvunknown
   \else \csname kv:#1\expandafter\endcsname\fi }
\def\kvunknown{???}
\def\kvscan #1#2=#3,{\ifx#1,\else \kvdef{kv:#1#2}{#3}\expandafter\kvscan\fi}
\def\kvdef#1{\expandafter\def\csname#1\endcsname}

\def\mymacro#1{\kvscan one=one, two=SECOND, three=three, four=four,,=,% implicit values
   \kvscan#1,,=,% actual values
   {\tt one}: \kv{one}; {\tt two}: \kv{two}; {\tt three}: \kv{three}; {\tt four}: \kv{four}
}

\mymacro{}

\mymacro{two=second, one=first, three=third}

\mymacro{four=Last}

If somebody will comment this as reinventing the wheel, I disagree. Writing such five lines is much more simple for me than the reading 72 pages of xkeyval documentation.

Answer 3

Similar to Werner's approach but using expl3:

\RequirePackage{expl3}
\ExplSyntaxOn
\clist_map_inline:nn { one , two ,  three , four }
  {
    \keys_define:nn {  mycmd } { #1 .tl_set:c = { l__mycmd_ #1 _tl } }
  }
\cs_new_protected:Npn \MyCommand #1
  {
    \group_begin:
      \keys_set:nn { mycmd } {#1}
      Key~values~are:
     \clist_map_inline:nn { one , two ,  three , four }
       { ~ ##1 ~ = ~  ` \tl_to_str:c  { l__mycmd_ ##1 _tl } ' }
    \group_end:
  }
\ExplSyntaxOff
\documentclass{article}
\begin{document}

\MyCommand{one = a}

\end{document}

The reason I'd favour the expl3 key module (l3keys) over xkeyval is that the behaviour of l3keys with respect to brace retention, space stripping and the category codes of , and = in arguments is all very well-defined and clear. I also find the xkyeval interface rather awkward (somewhat ironic): l3keys shares with pgfkeys.

(The above work with other formats if you use \input expl3-generic rather than \RequirePackage{expl3}. If you are definitely targetting LaTeX I'd use xparse for the user interface.)

Answer 4

I don't know in what sense this is useful, but here's an option with xparse.

\documentclass{scrartcl}
\usepackage{xparse}

\ExplSyntaxOn

\NewDocumentCommand \mycommand { g g g g g g g g g }
 {
  \seq_clear:N \l_tmpa_seq
  \IfValueT { #1 } { \seq_put_right:Nn \l_tmpa_seq { #1 } }
  \IfValueT { #2 } { \seq_put_right:Nn \l_tmpa_seq { #2 } }
  \IfValueT { #3 } { \seq_put_right:Nn \l_tmpa_seq { #3 } }
  \IfValueT { #4 } { \seq_put_right:Nn \l_tmpa_seq { #4 } }
  \IfValueT { #5 } { \seq_put_right:Nn \l_tmpa_seq { #5 } }
  \IfValueT { #6 } { \seq_put_right:Nn \l_tmpa_seq { #6 } }
  \IfValueT { #7 } { \seq_put_right:Nn \l_tmpa_seq { #7 } }
  \IfValueT { #8 } { \seq_put_right:Nn \l_tmpa_seq { #8 } }
  \IfValueT { #9 } { \seq_put_right:Nn \l_tmpa_seq { #9 } }
  \seq_use:Nn \l_tmpa_seq { , ~ }
 }

\ExplSyntaxOn

\begin{document}
\mycommand{one}{two}{three}
\mycommand{one}{two}{three}{four}{five}{six}{seven}
\mycommand{one}{two}
\end{document}

It takes up to nine arguments, and separates them with commas.