One often needs to alter the alignment of a tabular p (‘paragraph’) cell, but problems at the end of a table row are common. With a p cell that looks like:
one is liable to encounter errors that complain about a “misplaced... & \centering blah ... \\
\noalign” or “extra alignment tab”, or the like.
The problem is that the command \ means different things in
different circumstances: the tabular environment
switches the meaning to a value for use in the table, and
\centering, \raggedright and \raggedleft all change the
meaning to something incompatible. Note that the problem only
arises in the last cell of a row: since each cell is set into a box,
its settings are lost at the & (or \ ) that
terminates it.
In the old days, the actual value of \ that the
tabular environment uses was only available as an
internal command. Nowadays, the value is a public command, and you
can in principle use it explicitly:
(but that’s a rather verbose way of doing things).... & \centering blah ... \tabularnewline
The array package provides a command \arraybackslash
which restores \ to its correct (within table) meaning;
the command may be used in array’s “field format” preamble
specifications:
\begin{tabular}{... >{\centering\arraybackslash}p{50mm}}
...
The \tabularnewline and \arraybackslash commands are
(somewhat) modern additions to LaTeX and the array
package, respectively. In the unlikely event that neither is
available, the user may try the (old) solution which preserves the
meaning of \ :
\newcommand\PBS[1]{\let\temp=\\%
#1%
\let\\=\temp
}
which one uses within a table as:
or in the preamble as:... & \PBS\centering blah ... \\
\begin{tabular}{...>{\PBS\centering}p{5cm}}
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=tabcellalign