Defining a command with arbitrary number of parameters

Question

I've done a few searches, and found possible solutions to my question. However, those that I saw were somehow more complicated than what I think I really need. So please excuse me for asking again. Here's the scenario:

I want to define a math command that gives the long exact sequence of the given parameters. So for example, if I write \longexactsequence{A}{B}{C}{D} then I expect to have something like 0 \longrightarrow A \longrightarrow B \longrightarrow C \longrightarrow D \longrightarrow 0.

Similarly, if I just write \longexactsequence{A}, then I would just get 0 \longrightarrow A \longrightarrow 0.

How can I do this? I'm also pushing this one step further, as I would doubly appreciate a "fix" for breaking the line if the output equation is already too long.

Thanks a lot, and all comments, hints, etc would be greatly appreciated!

Answer 1

An implementation using \kernel@ifnextchar for an input syntax like \longexactsequence{A}{B}{C}{D} and an implementation using LaTeX’s \@for macro for an input syntax like \longexactsequence{A, B, C, D} can be found in the history of this answer.

When you want to use , as a separator for the entries, you can use one of many packages that provide loops. One of this is the quite powerful \foreach macro of PGF, it has its own tag: foreach.

If you want to apply a second value for a superscript of the arrow you can use \foreach, too, but need to use / for the delimiter of those arguments.

An arrow that should not be labeled need to be input as /<entry>.

There are other ways to input this, say

$ \longexactsequence{[f]A, [g]B, C} $

making the arrow label optional but this will need a more sophisticated implementation.

A poor man’s version of this has been implemented by using \ifx that checks whether both following macros are equal (which \superscript and \entry are if you don’t provide the /). This can be surpassed by adding another space (D/ D), an empty group (D{}/D) or \relax (D\relax/D) in one of both parameters.

The examples should show this clearly.

Code

\documentclass{article}
\usepackage{extarrows,pgffor}
\newcommand*{\longexactsequence}[2][0]{%
  #1 \longrightarrow
  \foreach \superscript/\entry in {#2} {%
    \entry \xlongrightarrow{\ifx\superscript\entry\else\superscript\fi}%
  } #1%
}

\begin{document}
$ \longexactsequence{A, B, C, D, E} $

$ \longexactsequence{f/A, g/B, C, D/ D} $ % a space before or after / makes the Ds unequal

$ \longexactsequence[Z]{a/A, {a, b}/B} $

$ \longexactsequence{} $
\end{document}

Output

Answer 2

The \getargsC macro parses a single argument into "words" that are placed in \argi, \argii, \argiii, \argiv, etc. The total number of words are set in the string \narg. A word with spaces or ending in a macro, can be isolated by surrounding it in braces (i.e., the contents of the inner braces becomes a word. Then I just regurgitate the words one by one, with the proper decorations surrounding them.

In the loop, \csname arg\roman{index}\endcsname just becomes successive arguments (\argi, \argii, \argiii, \argiv)

\documentclass{article}
\usepackage{readarray}
\newcounter{index}
\newcommand\longexactsequence[1]{%
  \getargsC{#1}%
  0\longrightarrow%
  \setcounter{index}{0}
  \whiledo{\value{index} < \narg}{%
    \stepcounter{index}%
    \csname arg\roman{index}\endcsname\longrightarrow%
  }
  0%
}
\begin{document}
\(
\longexactsequence{A B C D}
\)

To show with other than simple characters

\(
\longexactsequence{A_1 {\alpha} {B + C}}
\)
\end{document}