[Tex/LaTex] Problem with hline in new tabular environment

Question

When I write truth tables I like a bit more space in the cells, so I assign new values to arraystretch and tabcolsep, like this …

\documentclass[12pt]{article}
\begin{document}
\renewcommand{\arraystretch}{1.5}
\renewcommand{\tabcolsep}{0.2cm}
\begin{tabular}{| c  | c | c | c | c |}
    \hline
    $Q$ & $R$ & $Q \lor R$ \\ \hline \hline
    T & T & T \\
    T & F & T \\
    F & T & T \\
    F & F & F \\
    \hline
\end{tabular}
\end{document}

… and all is well.

Eventually I made a new environment for truth tables:

\newenvironment{truthtabular}[1]{%
    \renewcommand{\arraystretch}{1.5}
    \renewcommand{\tabcolsep}{0.2cm}
    \begin{tabular}{#1}{%
    }
}{\end{tabular}}

The environment worked fine unless I wanted the hline at the top, in which case I would get the following errors:

Misplaced \noalign. \hline
You can't use `\hrule' here except with leaders. \hline
Missing number, treated as zero. \hline
Illegal unit of measure (pt inserted). \hline

Attempting to diagnose the problem, I removed the reassignments from the definition of the new environment, thus:

\newenvironment{truthtabular}[1]{%
    %\renewcommand{\arraystretch}{1.5}
    %\renewcommand{\tabcolsep}{0.2cm}
    \begin{tabular}{#1}{%
    }
}{\end{tabular}}

The compilations still fails, with the same errors, when beginning with an hline. Example of failure follows:

\documentclass[12pt]{article}
\begin{document}

\newenvironment{truthtabular}[1]{%
    \begin{tabular}{#1}{%
    }
}{\end{tabular}}

\begin{truthtabular}{| c  | c | c | c | c |}
    \hline
    $Q$ & $R$ & $Q \lor R$ \\ \hline \hline
    T & T & T \\
    T & F & T \\
    F & T & T \\
    F & F & F \\
    \hline
\end{truthtabular}
\end{document}

Why does the leading hline cause problems in the defined environment? What can be done to avert the problems?

Answer 1

You have a spurious pair of braces in your definition.

I slightly improved your table, using \\hhline for the double line, and I propose another layout:

 \documentclass[12pt]{article}
\usepackage{array, hhline, booktabs} 
\usepackage[x11names, table]{xcolor} 
\newenvironment{truthtabular}[1]{%
    \renewcommand{\arraystretch}{1.5}
    \setlength{\tabcolsep}{0.2cm} \tabular{#1}
}{\endtabular}

\begin{document}

\begin{truthtabular}{| c | c | c | }
    \hline
    $Q$ & $R$ & $Q \lor R$ \\ %
    \hhline{:=:=:=:}
    T & T & T \\
    T & F & T \\
    F & T & T \\
    F & F & F \\
    \hline
\end{truthtabular}

{ % begin box
\setlength\aboverulesep{0pt}
\setlength\belowrulesep{0pt}
\arrayrulecolor{LightSteelBlue3}
 \begin{truthtabular}{cc!{\color{LightSteelBlue3}\vrule width1.5pt}c }
 $Q$ & $R$ & $Q \lor R$ \\ %
 \midrule
 T & T & T \\
 T & F & T \\
 F & T & T \\
 F & F & F 
\end{truthtabular}
} % end box

\end{document}