Misplaced \noalign if using \ifstrequal

Question

I get a "misplaced \noaling" error within an tabularx environment if I am using \ifstrequal.

This is working:

\documentclass{article}

\usepackage{tabularx}
\usepackage{etoolbox}

\begin{document}

\begin{tabularx}{\textwidth}{rrrrr}
    I & am & a & useless & table \\
    \ifstrequal{a}{a}{
        I & am & a & useless & table \\
    } {
        I & am & a & useless & table \\
    }
\end{tabularx}

\end{document}

This not:

\documentclass{article}

\usepackage{tabularx}
\usepackage{etoolbox}

\begin{document}

\begin{tabularx}{\textwidth}{rrrrr}
    I & am & a & useless & table \\
    \ifstrequal{a}{a}{
        \hline
        I & am & a & useless & table \\
        \hline
    } {
        I & am & a & useless & table \\
    }
\end{tabularx}

\end{document}

Answer 1

You could resort to the good old \ifx:

\documentclass{article}

\usepackage{tabularx}
\usepackage{booktabs}

\newcommand{\stringa}{a}
\newcommand{\stringb}{b}

\begin{document}

\begin{tabularx}{\textwidth}{rrrrr}
    I & am & a & useless & table \\
    \ifx\stringa\stringa
        \midrule
        I & am & a & useless & table \\
        \midrule
    \else
        I & am & a & useless & table \\
    \fi
\end{tabularx}

\end{document}

(I'm using rules from the booktabs package instead of \hline because they give better spacing)