Centering tabular: Why does this work?

Question

About a month ago, someone asked how to align a tabular environment around the intercolumn space, which was something I'd been trying to figure out for a brief eternity. Werner's answer worked more or less fine, though I had to add extra braces around \raggedright. I was wondering, though, if someone could explain to me why it worked.

Here's the code that I'm using:

\usepackage{array}
%...

\noindent
\begin{tabular}
  {>{\raggedleft}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}|
   >{{\raggedright}}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}}
    Kindle Edition Available & k \\
    PC/Mac Edition Available & p
\end{tabular}

I think it's mostly the math for the paragraph widths that I'm not understanding, but I'd really appreciate it if someone could explain the whole thing in English. The part that I get is "insert the command \raggedright before the first paragraph-style column and \raggedleft before the second paragraph-style column"; otherwise I'm in the dark.

If I was unclear about what I'd been trying to do, here's the difference between what that block produces and what this one does:

\begin{center}
\begin{tabular}{rl}
    Kindle Edition Available & k \\
    PC/Mac Edition Available & p
\end{tabular}
\end{center}

Thanks so much for your help!

Answer 1

First of all, your >{{\raggedright}} does nothing, because {\raggedright} is inserted at the start of the cell and the effect is "do nothing", because the declaration is issued in a group. What you need is \arraybackslash (I'll explain why later on).

\begin{tabular}
  {>{\raggedleft}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}|
   >{\raggedright\arraybackslash}}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}}

You are asking for a two column alignment. Both columns are of type p and the width is

half the current line width, reduced by twice \tabcolsep and half \arrayrulewidth

You have to know that the structure of a row in your tabular is

t C t | t C t

where t denotes a space of width \tabcolsep, C denotes a cell and | denotes the vertical rule, which has width \arrayrulewidth.

Thus, since the width of C is as stated before, the tabular will be as wide as \linewidth:

t + (0.5L - 2t - 0.5a) + t + a + t + (0.5L - 2t - 0.5a) + t = L

(where L is the line width).

Why \arraybackslash? Because \raggedright redefines \\ and so this command could not be used for ending a row. The command \arraybackslash restores the correct meaning of \\. It's not required in the first column specifier, but you can of course use it also there.

Answer 2

As egreg mentions is his comment, using >{{\raggedright}} won't work as you can see when you add more text:

\documentclass{report}
\usepackage{array}

\begin{document}

\noindent
\begin{tabular}
  {>{\raggedleft\arraybackslash}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}|
   >{{\raggedright}}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}}
    Kindle Edition Available & k \\
    PC/Mac Edition Available & p text some text to test text some text to test text some text to test text some text to test text some text to test
\end{tabular}

\end{document}

Using simply >{\raggedright} for the last column and \\ to change lines will produce an error since \centering, \raggedright, \raggedleft redefine \\; the array package provides \arraybackslash to restore the meaning, so the following works:

\documentclass{report}
\usepackage{array}

\begin{document}

\noindent
\begin{tabular}
  {>{\raggedleft\arraybackslash}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}|
   >{\raggedright\arraybackslash}p{\dimexpr.5\linewidth-2\tabcolsep-.5\arrayrulewidth}}
    Kindle Edition Available & k \\
    PC/Mac Edition Available & p text some text to test text some text to test text some text to test text some text to test text some text to test
\end{tabular}

\end{document}