How can I define a macro that adds a row to a table that has not yet been printed?

Question

I want to generate a list of technical abbreviations and acronyms that I use. Currently, I'm defining all acronyms with a macro I have designed. Now, I want the macro to call another macro, called \addtototaa, each time it is called, which adds the abbreviation to a table. This other macro is defined as

\newrobustcmd{\addtototaa}[2]{\let\OldContent\TableoftaaBody%
    \renewrobustcmd{\TableoftaaBody}{\OldContent #1 & #2 \\}}

And then I have the two additional macros

\newrobustcmd{\TableoftaaBody}{}
\newrobustcmd{\tableoftaa}[0]{\begin{longtabu}{l X}\TableoftaaBody\end{longtabu}}

where \TableoftaaBody acts as a storage container for the table contents and \tableoftaa prints the table. But when I try to compile the document I get

TeX capacity exceeded, sorry [input stack size=5000] \tableoftaa

if I call \tableoftaa (but not otherwise). Why do I get this error? I read on this blog post that this message can appear if you have an infinite recursive loop (I guess the function calls are put on the stack and that's why it's eaten up). Have I defined a recursive function here somewhere? By the way, \addtototaa is called 31 times before \tableoftaa is called, because that's how many acronyoms and abbreviations I have defined, and the two arguments I send to \addtototaa are always just literals, and no additional macros.

Answer 1

You can use the etoolbox package to append content to an existing macro using \gappto (works similar to \g@addto@macro). I've simplified your code snippets so I can make a MWE, but it would most likely be easy to extend to your usage:

\documentclass{article}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\usepackage{tabularx}% http://ctan.org/pkg/tabularx
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\let\TableoftaaBody\relax
\newcommand{\tableoftaa}{\begin{tabularx}{\textwidth}{l X}\TableoftaaBody\end{tabularx}}

\newcommand{\addtototaa}[2]{%
  \gappto\TableoftaaBody{#1 & #2 \\}%
}
\begin{document}
\addtototaa{1}{\lipsum*[1]}
\addtototaa{2}{\lipsum*[2]}
\addtototaa{3}{\lipsum*[3]}
\noindent\tableoftaa
\end{document}

Answer 2

@Werner has shown how to do what you want, so I'll answer the other question: Yes you have defined an infinite recursive loop.

\newrobustcmd{\TableoftaaBody}{}

\newrobustcmd{\addtototaa}[2]{\let\OldContent\TableoftaaBody%
\renewrobustcmd{\TableoftaaBody}{\OldContent #1 & #2 \\}}

so \TableoftaaBody is eempty then you do

 \addtototaa{1}{2}

so \OldContent is \let to \TableoftaaBody so is defined empty and \TableoftaaBody is defined to be

\OldContent 1 & 2 \\

Then you go

 \addtototaa{3}{4}

so \OldContent is \let to \TableoftaaBody so is defined to be \OldContent 1 & 2 \\ (oooops!!!!) and \TableoftaaBody is defined to be

\OldContent 3 & 4 \\

so at this point if you expand \TableoftaaBody the first token you see is \OldContent so that expands but the first token of that expansion is \OldContent again and so you are in an infinite loop. Each time you expand \OldContent you see \OldContent again but push further tokens on the input stream after that, so finally you fill up all available memory.