How to add \hline to macro expansion?

Question

I want to add one \hline to the macro expansion of this code, that is minimally modified version of David Carlisle's answer to Generating LaTeX tabular code during compilation.

Compiling this with latexmk -pdf main.tex gives a nice tabular. I was able to add \hline to the macro expansion of the macro that generates most of the rows by adding \def\hardline{\hline} and \expandafter\hardline in \def\xlistbody in g_orig_dest_tabular1_def.tex. I was also able to add \hline in the beginning of the code by adding \noexpand\hline in \def\tablestart in the same file. But I was not able to find out how I could generate \hline between first and second row of this tabular array. It must be in \def\listheadings or in \def\xlistheadings in g_orig_dest_tabular1_def.tex. So how \hline could be added to the macro expansion that is used to create the first row of this tabular array?

Thank you :)

main.tex:

\documentclass[12pt]{article}

\usepackage[american]{babel}
\usepackage{graphicx}

\input{g_orig_dest_tabular1_def}

\begin{document}

\input{g_orig_dest_tabular1}

\end{document}

g_orig_dest_tabular1_def.tex:

\newcommand\connorigdestregionlist{ABC,DEF,GHI}

% hardline.
\def\hardline{\hline}

% listheadings.
\def\listheadings{%
\expandafter\xlistheadings\connorigdestregionlist,\relax,}

% xlistheadings.
\def\xlistheadings#1,{%
\ifx\relax#1%
\expandafter\\
\else
&\textbf{#1}%
\expandafter\xlistheadings
\fi}

% listbody.
\def\listbody{%
\expandafter\xlistbody\connorigdestregionlist,\relax,}

% xlistbody.
\def\xlistbody#1,{%
\ifx\relax#1%
\else
\textbf{#1}%
\gdef\thisrow{#1}%
\expandafter\xlistdata\connorigdestregionlist,\relax,%
\expandafter\hardline
\expandafter\xlistbody
\fi}

% xlistdata.
\def\xlistdata#1,{%
\ifx\relax#1%
\expandafter\\
\else
&\csname CONNorig\thisrow dest#1\endcsname
\expandafter\xlistdata
\fi}

% preamble.
\def\preamble{\expandafter\xpreamble\connorigdestregionlist,\relax,}
\def\xpreamble#1,{%
\ifx\relax#1%
\else
c%
\expandafter\xpreamble
\fi}

% tablestart.
\def\tablestart{%
\edef\temp{\noexpand\begin{tabular}{c\preamble}\noexpand\hline}%
    \temp}

g_orig_dest_tabular1.tex:

\input{g_orig_dest_conn_def}

\tiny
\begin{table}
    \scalebox{0.6}{
    \tablestart
    \listheadings
    \listbody
    \end{tabular}
    } % end scalebox
    \caption{Caption goes here.}
    \begin{enumerate}
        \item Item number one. 
        \item Item number two.
    \end{enumerate}
\end{table}
\normalsize

g_orig_dest_conn_def.tex:

\def\CONNorigABCdestABC{ NA }
\def\CONNorigABCdestDEF{ from ABC to DEF }
\def\CONNorigABCdestGHI{ from ABC to GHI }

\def\CONNorigDEFdestABC{ from DEF to ABC }
\def\CONNorigDEFdestDEF{ NA }
\def\CONNorigDEFdestGHI{ from DEF to GHI }

\def\CONNorigGHIdestABC{ from GHI to ABC }
\def\CONNorigGHIdestDEF{ from GHI to DEF }
\def\CONNorigGHIdestGHI{ NA }

Answer 1

To get a hline you can change the command \xlistheadings or \listheadings

The modification of \xlistheadings:

\def\xlistheadings#1,{%
 \ifx\relax#1%
  \expandafter\\\hline
 \else &\textbf{#1}%
 \expandafter\xlistheadings 
\fi}

The modification of \listheadings

\def\listheadings{%
\expandafter\xlistheadings\connorigdestregionlist,\relax,\hline}

Take your preferred way.