\expandafter parameters that are not commands

Question

I'm using a custom command to format all tables in a document. Some tables share properties other than style, such as column definitions and headers, which is why this answer inspired me to even more reduce repeated code.

Inserting header data into the table command did not cause any problems, while for inserting column definitions I had to find out how to expand the column definitions before insertion. \expandafter solved that problem.

Now, I'd like to combine parameters given as commands and others given as literals, e.g. the table's data. If the to-be-expanded command that contains the column definitions happens to be the first argument, this works fine. What do I need to do if conversely a literal is the first argument? How do I make \expandafter bypass a pair of curly braces instead of a command?

\documentclass{article}
\usepackage{booktabs}

\begin{document}

    \newcommand{\colDef}{{cc}}  
    \newcommand{\tableBody}{a & b\\}

    \newcommand{\myTabA}[2]{%
        \begin{tabular}{#1}
            1 & 2\\\midrule
            #2
        \end{tabular}       
    }

    %same command with #1 and #2 switched
    \newcommand{\myTabB}[2]{%
        \begin{tabular}{#2}
            1 & 2\\\midrule
            #1
        \end{tabular}       
    }       

    %draw table A
    \expandafter\myTabA\colDef{\tableBody}
    \expandafter\myTabA\colDef{c & d\\} 

    %draw table B
    \expandafter\myTabB\expandafter\tableBody\colDef

    %produces error
    %\expandafter\myTabB\expandafter{c & d\\}\colDef    

\end{document}

The answers to this question will help you out I think: http://tex.stackexchange.com/q/22288/14100 — Scott H., May 22 '12 at 17:08
True, that's it. Thanks, Scott and Hendrik. Make it an answer and I'll accept it. — dgs, May 22 '12 at 17:32
I wish etextools's documentation did not look like a geocities website! All that is missing is a <blink> tag. — Aditya, May 22 '12 at 17:52

score 10 · Answer 1 · answered May 22 '12 at 18:03

It's simpler of you push the expansion control into the macro, so it always expands the first token of #2 once, then you don't need \expandafter in the document.

\documentclass{article}
\usepackage{booktabs}

\begin{document}

    \newcommand{\colDef}{{cc}}  
    \newcommand{\tableBody}{a & b\\}

    \newcommand{\myTabA}[2]{%
        \begin{tabular}{#1}
            1 & 2\\\midrule
            #2
        \end{tabular}       
    }

    %same command with #1 and #2 switched
    \newcommand{\myTabB}[2]{%
        \def\temp{\begin{tabular}}%
           \expandafter\temp\expandafter{#2}
            1 & 2\\\midrule
            #1
        \end{tabular}       
    }       

    %draw table A
    \expandafter\myTabA\colDef{\tableBody}
    \expandafter\myTabA\colDef{c & d\\} 

    %draw table B
    \myTabB\tableBody\colDef

    %produces error (not now!)
    \myTabB{c & d\\}\colDef    

\end{document}

Thanks David, that's less redundant and a lot more elegant. – dgs May 22 '12 at 22:01 — dgs, May 22 '12 at 22:01

score 8 · Accepted Answer · answered May 22 '12 at 17:48

The etextools package provides some tools for doing this, for example

\ExpandNextTwo{code}{arg 1}{arg 2}

as used below:

\documentclass{article}
\usepackage{booktabs}
\usepackage{etextools}

\begin{document}

   \newcommand{\colDef}{{cc}}  
   \newcommand{\tableBody}{a & b\\}

   \newcommand{\myTabA}[2]{%
    \begin{tabular}{#1}
        1 & 2\\\midrule
        #2
    \end{tabular}       
}

%same command with #1 and #2 switched
\newcommand{\myTabB}[2]{%
    \begin{tabular}{#2}
        1 & 2\\\midrule
        #1
    \end{tabular}       
}       

%draw table A
\expandafter\myTabA\colDef{\tableBody}
\expandafter\myTabA\colDef{c & d\\} 

%draw table B
\expandafter\myTabB\expandafter\tableBody\colDef

% no longer produces error
\ExpandNextTwo{\myTabB}{c & d\\} \colDef

\end{document}

score 2 · Answer 3 · answered May 22 '12 at 21:59

2

I've ended up using an etextools command inside the tabular definition, thus drawing from both all answers.

\newcommand{\myTabB}[2]{%
    \expandnext{
    \begin{tabular}}{#2}
        1 & 2\\\midrule
        #1
    \end{tabular}       
}

answered May 22 '12 at 21:59

dgs

2,741

\expandafter parameters that are not commands

3 Answers3