horizontal alignmenttables
With the following code:
\documentclass[a4paper]{report}
\begin{document}
\begin{center}
\halign{\vrule \hfil # \hfil & \hfil # \hfil \vrule \cr
a & b \cr\noalign{\hrule}
c & d \cr\noalign{\hrule}
}
\end{center}
\end{document}
I'd expect to get the simple table produced with \halign centered in the page. However, it is not, as the picture below shows.
\centering also has no effect. Why does that happen?
The problem is that the \begin{...} and \end{...} pair commands automatically create a "group" so that, in effect, the & and \\ are "out of scope" for the tabular, while inside the "production" environment they just show up at a place where the compiler is not expecting them.
A second problem with your definitions is that, even if they would work, they would be adding an extra \\ at the end of the tabular, adding an unwanted space at the end. Perhaps some more appropriate definitions would be
\documentclass[a4paper,10pt]{article}
\newcommand{\production}[1]{#1 ::= &}
\newenvironment{grammar}{\tabular{p{3cm}l}}{\endtabular}
\begin{document}
\begin{grammar}
\production{XmlStartTag} ... \\
\production{XmlOtherTag} ... \\
\production{XmlEndTag} ...
\end{grammar}
\end{document}
Note no \\ at the end of the last production. Also in the definition of the grammar you don't need to repeat the work of \begin/\end, and you can instead directly use \tabular and \endtabular.
When using \multicolumn{1}, you don't need to respecify the column width via m{<len>}, since it requires a cumbersome interface when trying to centre the column entry. Rather just use \multicolumn{1}{c}{...}.
Here is an updated version of your code snippet:
\documentclass{article}
%\usepackage[landscape]{geometry}% http://ctan.org/pkg/geometry
\usepackage{array}% http://ctan.org/pkg/array
\newcommand{\stackcell}[2][c]{%
\begin{tabular}{@{}#1@{}}
#2
\end{tabular}%
}
\begin{document}
\begin{table*}
\centering
\caption{Results on Xilinx Virtex 5 Devices}
\label{tab:XilinxResults}
\begin{tabular}{|>{\bfseries\arraybackslash}l|*{8}{>{\raggedleft\arraybackslash}m{13mm}|}}
\cline{2-9}
\multicolumn{1}{c|}{} & % blank box
\multicolumn{2}{c|}{\bfseries \stackcell{Clock Frequency\\(MHz)}} &
\multicolumn{2}{c|}{\bfseries \stackcell{Throughput\\(Mbit/s)}} &
\multicolumn{2}{c|}{\bfseries \stackcell{Area\\(CLB Slices)}} &
\multicolumn{2}{c|}{\bfseries \stackcell{Throughput /\\Area}} \\ \hline
\multicolumn{1}{|c|}{\bfseries Architecture} &
\multicolumn{1}{c|}{\bfseries 512-bit} &
\multicolumn{1}{c|}{\bfseries 256-bit} &
\multicolumn{1}{c|}{\bfseries 512-bit} &
\multicolumn{1}{c|}{\bfseries 256-bit} &
\multicolumn{1}{c|}{\bfseries 512-bit} &
\multicolumn{1}{c|}{\bfseries 256-bit} &
\multicolumn{1}{c|}{\bfseries 512-bit} &
\multicolumn{1}{c|}{\bfseries 256-bit}
\tabularnewline\hline\hline
BMW x1 & 4.89 & 8.14 & 5004 & 4168 & 12039 & 6164 & 0.42 & 0.68 \\ \hline
BMW x1-PPL18 & 46.51 & 70.97 & 47628 & 36335 & 24564 & 11610 & 1.84 & 2.96 \\ \hline
BMW /32(h) & 44.27 & 69.41 & 1416 & 1110 & 4001 & 2211 & 0.35 & 0.50 \\ \hline
CubeHash x1 & 152.70 & 151.93 & 2443 & 2430 & 745 & 672 & 3.28 & 3.62 \\ \hline
CubeHash x1-PAR5 & 151.26 & 151.56 & 12101 & 12125 & 3742 & 3490 & 3.23 & 3.47 \\ \hline
CubeHash x1-PAR17 & N/A & 150.49 & N/A & 40933 & N/A & 11360 & N/A & 1.91 \\ \hline
CubeHash x1-PAR33 & 150.65 & N/A & 79542 & N/A & 24570 & N/A & 3.24 & N/A \\ \hline
CubeHash /2(h) & 173.52 & 184.71 & 1388 & 1477 & 733 & 624 & 1.89 & 2.37 \\ \hline
\end{tabular}
\end{table*}
\end{document}
Hopefully this improves some of the formatting and readability of code. Some improvements include:
*{<num>}{<col spec>} which repeats <col spec> a total of <num> times;\raggedleft at the "column specification" level to avoid cluttering the tabular contents with \raggedleft;\arraybackslash within >{<stuff>} in the column specifier, which restores the traditional use of \\ as \tabularnewline;\stackcell[<col spec>]{<stuff>} which adds a stackable tabular within a cell (although you may not need/require this).Finally, as a recommendation for professional-looking tables, consider using the booktabs package. The horizontal alignment/structure of a table lends itself to avoid using vertical rules, something promoted by/when using booktabs.
(geometry was loaded in landscape mode to make the table fit on the page when compiling the MWE.)
Best Answer
One might be tempted of using low level commands also in LaTeX; however there are some subtleties in them that are not easy to grasp at the beginning.
One of these subtleties is that
\halignis a vertical command, so when found it causes vertical mode to start; if TeX is in horizontal mode when it scans\halign, it quits it by issuing\par.This is proved by the following test file (Plain TeX):
that outputs
The temporary redefinition of
\paris aimed at showing that\paris effectively executed.When TeX is in vertical mode, it simply stacks boxes without making paragraphs; therefore
\centering(which is in effect incenter) does nothing, because it only influences paragraphs. Thus your\halignwill be printed flush with the left margin, since\tabskipis normally zero. The\tabskipglue in force at the moment\halignis scanned is added the left of every box formed by\halign(one box for every alignment row) and between any two alignment cells unless another\tabskipis specified. Read chapter 22 in the TeXbook for more information, but it's not easy.How can one make
\halignpart of a paragraph? By hiding it inside something that can and that can also contain it: a\vboxis usually employed (the most common device for centering tables in Plain TeX is to use$$\vbox{\halign{...}}$$; don't try it in LaTeX). While\vboxdoesn't by itself start a paragraph, it doesn't interrupt it either, sowill center your alignment.
Use
tabular; you'll be on the safe side.