UK TeX FAQ -- question label tabcellalign

### How to alter the alignment of tabular cells

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:

>     ... & \centering blah ... \\

one is liable to encounter errors that complain about a “misplaced `\``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:

>     ... & \centering blah ... \tabularnewline

(but that’s a rather verbose way of doing things).

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:

>     ... & \PBS\centering blah ... \\

or in the preamble as:

>     \begin{tabular}{...>{\PBS\centering}p{5cm}}

`array.sty`
  Distributed as part of macros/latex/required/tools (or browse the directory); catalogue entry

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