Item list inside multicolumns not correctly centered

Question

I have a table with item list inside multicolumns which are not behaving as expected (vertically and horizontally centered):

\newcolumntype{C}{>{\centering\arraybackslash} m{2.3cm}}
\begin{table}
\caption{Some caption}
\label{tab:tab1}
\centering%
\begin{minipage}{\textwidth}
    \begin{tabular*}{\textwidth}{|C|C|C|C|>{\centering\arraybackslash}m{3cm}|}
        \hline
        \ & \textbf{Hellman} & \textbf{Rainbow Table} & \textbf{Proposed Method} & \textbf{Proposed Method with Compression}\\
        \hline
        Input Range & $2^{37}$ & $2^{37}$ & $2^{37}$ & $2^{37}$ \\ \hline
        Size & $1.4$ GB & $1.4$ GB & $1.4$ GB & $1.4$ GB \\ \hline
        Hash Operations & $90.3$ M & $7.4$ M & $4$ B & $3.5$ B \\ \hline
Advantages &
\multicolumn{2}{>{\centering\arraybackslash}m{4.9cm}|}{
    \begin{itemize}
        \item Few hash operations
    \end{itemize}
} &
\multicolumn{2}{>{\centering\arraybackslash}m{5.4cm}|}{
    \begin{itemize}
        \item Little disk access
        \item Disk access is serial
        \item Success is guaranteed
        \item Easily parallelizable
    \end{itemize}
} \\
\hline
    \end{tabular*}
\end{minipage}
\end{table}

Which results in:

As you can see only the third row (Hash Operations) seem to be precisely centered. The main problem, though, is the last line. The item lists are far from centered in any direction. How can I fix this?

I also don't know why the right side border is not correctly placed but I can manage to get an acceptable result playing with the cell's width (not shown in the image).

Answer 1

If I simplify your code (remove minipage, instead tabular* use just tabular) and left aesthetic point of view aside, then I obtain:

The code is now:

\documentclass[12pt]{article}
\usepackage{lmodern,caption}
\usepackage{array}
\usepackage{siunitx}

\newcolumntype{C}[1]{>{\centering\arraybackslash} m{#1}}


\begin{document}
    \begin{table}
    \centering%
\caption{Some caption}
    \label{tab:tab1}
    \begin{tabular}{|*{4}{C{2.3cm}|}C{3.1cm}|}
        \hline
        \ & \textbf{Hellman} & \textbf{Rainbow Table} & \textbf{Proposed Method} & \textbf{Proposed Method with Compression}\\
        \hline
        Input Range & $2^{37}$ & $2^{37}$ & $2^{37}$ & $2^{37}$ \\ \hline
        Size & $1.4$ GB & $1.4$ GB & $1.4$ GB & $1.4$ GB \\ \hline
        Hash Operations & $90.3$ M & $7.4$ M & $4$ B & $3.5$ B \\ \hline
Advantages &
\multicolumn{2}{C{4.9cm}|}{
    \begin{itemize}
        \item Few hash operations
    \end{itemize}
} &
\multicolumn{2}{C{5.4cm}|}{
    \begin{itemize}
        \item Little disk access
        \item Disk access is serial
        \item Success is guaranteed
        \item Easily parallelizable
    \end{itemize}
} \\
\hline
    \end{tabular}
\end{table}
\end{document}

With this your table is:

Edit: Some aesthetic improvement:

• in preamble add \setlength{\extrarowheight}{3pt}. This will give more space inabove and below text in table rows
  • for more compact itemize list replace \begin{itemize} with \begin{itemize}\firmlist and in preamlbe add definition for it:

\newcommand{\firmlist}{%
  \setlength{\itemsep}{0.5\itemsep}
  \setlength{\parskip}{0.2\parskip}%0.5
  \setlength{\listparindent}{3ex}%2em
                        }

Answer 2

This may be a visual artefact from a regular list's way of indenting itself. Perhaps creating your own tabularitemize that doesn't use much list formatting suits your needs better:

\documentclass{article}
\usepackage[margin=1in]{geometry}% Just for this example
\usepackage{array,varwidth,booktabs}
\newcolumntype{C}{>{\centering\arraybackslash} m{2.3cm}}

\newenvironment{tabularitemize}
  {\par\addvspace{\topsep}%
   \renewcommand{\item}{\par\addvspace{\itemsep}%
     \textbullet\hspace{\labelsep}\ignorespaces}%
   \begin{varwidth}{\linewidth}}
  {\par\strut\end{varwidth}}

\begin{document}

\noindent
\begin{tabular*}{\textwidth}{|C|C|C|C|>{\centering\arraybackslash}m{3cm}|}
  \hline
  & \textbf{Hellman} & \textbf{Rainbow Table} & \textbf{Proposed Method} & \textbf{Proposed Method with Compression} \\
  \hline
  Input Range & $2^{37}$ & $2^{37}$ & $2^{37}$ & $2^{37}$ \\ \hline
  Size & $1.4$ GB & $1.4$ GB & $1.4$ GB & $1.4$ GB \\ \hline
  Hash Operations & $90.3$ M & $7.4$ M & $4$ B & $3.5$ B \\ \hline
  Advantages &
  \multicolumn{2}{>{\centering\arraybackslash}m{4.9cm}|}{
    \begin{itemize}
      \item Few hash operations
    \end{itemize}
  } &
  \multicolumn{2}{>{\centering\arraybackslash}m{5.4cm}|}{
    \begin{itemize}
      \item Little disk access
      \item Disk access is serial
      \item Success is guaranteed
      \item Easily parallelizable
    \end{itemize}
  } \\
  \hline
  Input Range & $2^{37}$ & $2^{37}$ & $2^{37}$ & $2^{37}$ \\ \hline
  Size & $1.4$ GB & $1.4$ GB & $1.4$ GB & $1.4$ GB \\ \hline
  Hash Operations & $90.3$ M & $7.4$ M & $4$ B & $3.5$ B \\ \hline
  Advantages &
  \multicolumn{2}{>{\centering\arraybackslash}m{4.9cm}|}{
    \begin{tabularitemize}
      \item Few hash operations
    \end{tabularitemize}
  } &
  \multicolumn{2}{>{\centering\arraybackslash}m{5.4cm}|}{
    \begin{tabularitemize}
      \item Little disk access
      \item Disk access is serial
      \item Success is guaranteed
      \item Easily parallelizable
    \end{tabularitemize}
  } \\
  \hline
\end{tabular*}

\bigskip

\noindent
\begin{tabular*}{\textwidth}{*{5}{C}}
  \toprule
  & \textbf{Hellman} & \textbf{Rainbow Table} & \textbf{Proposed Method} & \textbf{Proposed Method with Compression} \\
  \midrule
  Input Range & $2^{37}$ & $2^{37}$ & $2^{37}$ & $2^{37}$ \\
  \addlinespace[.5ex]
  Size & $1.4$ GB & $1.4$ GB & $1.4$ GB & $1.4$ GB \\
  \addlinespace[.5ex]
  Hash Operations & $90.3$ M & $7.4$ M & $4$ B & $3.5$ B \\
  \addlinespace[.5ex]
  Advantages &
  \multicolumn{2}{>{\centering\arraybackslash}m{4.9cm}}{
    \begin{itemize}
      \item Few hash operations
    \end{itemize}
  } &
  \multicolumn{2}{>{\centering\arraybackslash}m{5.4cm}}{
    \begin{tabularitemize}
      \item Little disk access
      \item Disk access is serial
      \item Success is guaranteed
      \item Easily parallelizable
    \end{tabularitemize}
  } \\
  \bottomrule
\end{tabularx*}

\end{document}

The lists are placed inside a varwidth environment to optimize the horizontal width they take up if that is shorter than the given horizontal width.

booktabs is purely for aesthetic value.