[Tex/LaTex] How to reduce the spacing between columns in a table

tablestabularx

There are other similar questions, all with different answers, but I could not find one which achieves what I want. In the table below, the spacing between the numbers is too wide. How can I shrink it a bit?

\newcommand{\testmode}[2] {
    \begin{tabular}{c}
        \textbf{#1} \\ \textbf{d=#2}
    \end{tabular}
}

\begin{table}[ht]
\begin{tabular}{l | c c c | c c c |}
\cline{2-7}
          & \multicolumn{3}{c|}{\textbf{Single}} & \multicolumn{3}{c|}{\textbf{Double}} \\
\cline{2-7}
          & \testmode{Scalar}{1} & \testmode{SSE-4}{4} & \testmode{AVX-2}{8} & \testmode{Scalar}{1} & \testmode{SSE-4}{2} & \testmode{AVX-2}{4} \\
\hline
\multicolumn{1}{|c|}{\textbf{Eytzinger}                        } &     209.86 &     370.18 &     411.25 &     180.85 &     245.37 &     262.61 \\
\multicolumn{1}{|c|}{\textbf{Classic Offset}                    } &     213.28 &     363.35 &     476.10 &     209.04 &     247.02 &     208.63 \\
\hline
\end{tabular}
\caption{Throughput in millions of searches per second with vector $X$ of size 15}
\label{tab:results0}
\end{table}

Best Answer

In your example you have doubled the column separation using a tabular inside a tabular. Note: LaTeX adds a distance of \tabcolsep before and after each column. In your case you have such a distance before and after the column of the inner tabular of \testmode and before and after the column of the outer tabular. To avoid this, you should add @{} before the first column of the inner tabular and after the last column of the inner tabular:

\documentclass{article}

\begin{document}
\newcommand{\testmode}[2] {%
    \begin{tabular}{@{}c@{}}% Avoid doubling \tabcolsep
        \textbf{#1} \\ \textbf{d=#2}
    \end{tabular}%
}

\begin{table}[ht]
\begin{tabular}{l | c c c | c c c |}
\cline{2-7}
          & \multicolumn{3}{c|}{\textbf{Single}} & \multicolumn{3}{c|}{\textbf{Double}} \\
\cline{2-7}
          & \testmode{Scalar}{1} & \testmode{SSE-4}{4} & \testmode{AVX-2}{8} & \testmode{Scalar}{1} & \testmode{SSE-4}{2} & \testmode{AVX-2}{4} \\
\hline
\multicolumn{1}{|c|}{\textbf{Eytzinger}                        } &     209.86 &     370.18 &     411.25 &     180.85 &     245.37 &     262.61 \\
\multicolumn{1}{|c|}{\textbf{Classic Offset}                    } &     213.28 &     363.35 &     476.10 &     209.04 &     247.02 &     208.63 \\
\hline
\end{tabular}
\caption{Throughput in millions of searches per second with vector $X$ of size 15}
\label{tab:results0}
\end{table}
\end{document}

enter image description here

To explain some more: By default LaTeX adds \hskip\tabcolsep before it starts a column and after it has finished a column. So you have the distance \tabcolsep before the first and after the last column and 2\tabcolsep between two columns. If you use @{…} the code of the argument of @ is added instead of the distance. So @{} just removes the distance.

Additional note: I would recommend to avoid vertical rules in tables (see the manual of package booktabs) and to use package siunitx for number columns. And I would not center the elements of the first table row but left-align them. And because d=… in the table head is something like math, you should set it in math mode. In this case I also would not make it bold (using, e.g., \boldmath), because font attributes in math have mostly a semantic (e.g, bold is often used for vectors or sets).

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