\documentclass{article}
\usepackage{filecontents,pgfplotstable}
\begin{filecontents*}{assocFile.dat}
0/b
1/d
2/f
3/h
\end{filecontents*}
\pgfplotstableread[header=false]{assocFile.dat}\mytable
\begin{document}
\begin{tikzpicture}
\pgfplotstablegetelem{0}{[index]0}\of\mytable
\edef\forlist{\pgfplotsretval}
\pgfplotsforeachungrouped \x in {1,2,3} {
\pgfplotstablegetelem{\x}{[index]0}\of\mytable
\edef\forlist{\forlist, \pgfplotsretval}
}
\node {\forlist};
\end{tikzpicture}
\end{document}