Problem with table and multicolumn

Question

I get a problem with the following table:

\begin{table}[H]  
    \newcommand{\ctab}{\centering\arraybackslash}  
    \definecolor{F1}{RGB}{255,255,255}  
    \definecolor{F0}{RGB}{210,0,0}  
    \definecolor{F2}{RGB}{0,176,80}  
    \resizebox{0.85\textwidth}{!}{\begin{tabular}{|c|c|c|c|c|c|}  
            \hline   
            \rule{0pt}{30pt}  
            \multicolumn{2}{|c|}{\cellcolor{gray!50}\huge{\textbf{a}}} &
            %\cellcolor{gray!50}\huge{\textbf{a}} &  
            %\cellcolor{gray!50}\huge{\textbf{}} &
            \cellcolor{gray!50}\huge{\textbf{b}} &  
            \cellcolor{gray!50}\huge{\textbf{C}} &  
            \cellcolor{gray!50}\huge{$\mathbf{d}$} &  
            \cellcolor{gray!50}\huge{$\mathbf{e}$}    
            \\   
            \end{tabular}}  
    \caption{XY}  
    \label{tab:XY}  
\end{table}  

Problems are with the multicolumn. If I change to:

%\multicolumn{2}{|c|}{\cellcolor{gray!50}\huge{\textbf{a}}} &  
            \cellcolor{gray!50}\huge{\textbf{a}} &  
            \cellcolor{gray!50}\huge{\textbf{}} &   

It does work...

Where is the Problem?

Thanks!

Answer 1

The problem is caused by your \rule in conjunction with \multicolumn because it is in the same cell as the \multicolumn. If you want to keep it, place it in another cell or inside the \multicolumn. If you just want decent spacing you might insert a \strut just after \huge.

Using \rule:

\documentclass[]{article}

\usepackage[table]{xcolor}

\begin{document}

\begin{table}
    \begin{tabular}{|c|c|c|c|c|c|}  
            \hline   
            \multicolumn{2}{|c|}{\cellcolor{gray!50}\huge{\textbf{a}}{\rule{0pt}{30pt}}} &
            \cellcolor{gray!50}\huge{\textbf{b}} &  
            \cellcolor{gray!50}\huge{\textbf{C}} &  
            \cellcolor{gray!50}\huge{$\mathbf{d}$} &  
            \cellcolor{gray!50}\huge{$\mathbf{e}$}    
            \\
            \hline
            f&g&h&i&j&k\\
            \hline
            \end{tabular}
    \caption{XY}  
    \label{tab:XY}  
\end{table} 
\end{document}

produces:

Using \strut:

\documentclass[]{article}

\usepackage[table]{xcolor}

\begin{document}

\begin{table}
    \centering%
    \begin{tabular}{|c|c|c|c|c|c|}  
            \hline   
            \multicolumn{2}{|c|}{\cellcolor{gray!50}\huge\strut{\textbf{a}}} &
            \cellcolor{gray!50}\huge{\textbf{b}} &  
            \cellcolor{gray!50}\huge{\textbf{C}} &  
            \cellcolor{gray!50}\huge{$\mathbf{d}$} &  
            \cellcolor{gray!50}\huge{$\mathbf{e}$}    
            \\   
            \hline
            f&g&h&i&j&k\\
            \hline
            \end{tabular}
    \caption{XY}  
    \label{tab:XY}  
\end{table} 
\end{document}

produces:

Answer 2

You can use \extrarowheight to add some padding at the top of all cells (it's hardly a luxury, considering the tightness of default row heights), or define a minimal vertical spacing at the top and bottom of cells in columns prefixed with the letter S.

I simplified the code using the \rowcolor command, and defining a \myhead command which comprises the font changes. I took the liberty to change the colour of the first row to a nicer shade of gray:

\documentclass[]{article}

\usepackage{ cellspace}
\usepackage[table, svgnames]{xcolor}
\setlength{\cellspacetoplimit}{4pt}
\setlength{\cellspacebottomlimit}{3pt}
\newcommand{\myhead}[1]{\huge\boldmath\bfseries#1}
\begin{document}

\begin{table}[!htb]
\centering
    \begin{tabular}{|*{6}{Sc|}}
            \hline
       \rowcolor{GhostWhite!80!Gainsboro} \multicolumn{2}{|c|}{\myhead{a}}&
            \myhead{b} &
            \myhead{C} &
            \myhead{$\mathbf{d}$} &
            \myhead{$\mathbf{e}$}
            \\
            \hline
            f&g&h&i&j&k\\
            \hline
            \end{tabular}
    \caption{XY}
    \label{tab:XYZ}
\end{table}

\end{document}