Create a label if reference undefined, else reference it (unique label)

Question

I'm trying to label items on a table. In my use, multiple labels may exist, so the initial location of the creation of the label is irrelevant (not trying to hyperlink). I want to use one command that will assign a label. So I thought the easy solution would be to check if the reference exists, if not, increment a counter and assign a new label, otherwise reference it.

I do notice that I get different results alternating compiles. I also notice that when testing the ifundefined part in isolation (i.e. print true or false), it takes two compiles. I believe this is because of the way TeX interacts with its .aux file ?

\documentclass{article}

\newcounter{myrefcnt}% my referene counter
\makeatletter
\newcommand\myref[1]{% my command
  \@ifundefined{r@#1}{%if reference doesnt exist
    \refstepcounter{myrefcnt}\themyrefcnt\label{#1}% create a label for reference
  }{%
    \ref{#1}% else reference it
  }%
}
\makeatother


\begin{document}

  \textbf{My Attempt:}

\begin{tabular}{lr}
  \hline
    Things & Applicable Things \\\hline
    Aardvark & \myref{fourlegs}, \myref{lookslikerat} \\
    Bat & \myref{lookslikerat} \\
    Cheetah & \myref{fourlegs} \\\hline
\end{tabular}

\myref{fourlegs} indicates speciment has four legs.
\myref{lookslikerat} indicates speciment looks like a rat.

  \textit{When I run the file, the labels are sometimes 1,2,3,4, but other times 5,6,6,5}

  \textbf{I want to produce the following:}

  \begin{tabular}{lr}
  \hline
    Things & Applicable Things \\\hline
    Aardvark & 1, 2 \\
    Bat & 2 \\
    Cheetah & 1 \\\hline
\end{tabular}

1 indicates speciment has four legs.
2 indicates speciment looks like a rat.

\end{document}

Answer 1

so that you can make forward references \label writes to the aux file which is read at begin document so all the r@.. macros are defined on the second run. you could make a global \my@'#1 macro that you don't record the the aux, but then test that in your macro in the same way. (You probably wouldn't need \label at all in that case.

\documentclass{article}

\newcounter{myrefcnt}% my referene counter
\makeatletter
\newcommand\myref[1]{% my command
  \@ifundefined{myr@#1}{%if reference doesnt exist
    \stepcounter{myrefcnt}\themyrefcnt\expandafter\xdef\csname myr@#1\endcsname{\themyrefcnt}% create a label for reference
  }{%
    \csname myr@#1\endcsname% else reference it
  }%
}
\makeatother


\begin{document}

  \textbf{My Attempt:}

\begin{tabular}{lr}
  \hline
    Things & Applicable Things \\\hline
    Aardvark & \myref{fourlegs}, \myref{lookslikerat} \\
    Bat & \myref{lookslikerat} \\
    Cheetah & \myref{fourlegs} \\\hline
\end{tabular}

\myref{fourlegs} indicates speciment has four legs.
\myref{lookslikerat} indicates speciment looks like a rat.

  \textit{When I run the file, the labels are sometimes 1,2,3,4, but other times 5,6,6,5}

  \textbf{I want to produce the following:}

  \begin{tabular}{lr}
  \hline
    Things & Applicable Things \\\hline
    Aardvark & 1, 2 \\
    Bat & 2 \\
    Cheetah & 1 \\\hline
\end{tabular}

1 indicates speciment has four legs.
2 indicates speciment looks like a rat.

\end{document}

You may still have problems with environments that evaluate things more than once (amsmath alignments and tabularx for example) as you need to guard the test to so that the trial runs do not make every use seem to be not the first.