Recently I have found that there is a useful TeX command \phantom{Phantom Text} which behaves just like it would typeset its argument except that nothing is typeset and there is a blank space of the corresponding dimensions.

When I have been using it I imagined it would be sometimes even more useful to have a command which can put some other stuff in the reserved space instead of the phantom stuff.

Luckily, some smart guys have already written such a command.

Here is a modified version of their minimal example which illustrates nicely the use of this command:


\documentclass{article}
\usepackage{calc} % for the \phantomas command

% The following command is a better version of \phantom and requires the "calc" package
% Credits: Jean-Côme Charpentier & Scott Pakin
% Source: comp.text.tex, "Phantom-ish command"
% usage: \phantomas[l]{phantom words which will be overwritten}{with these words}
% the optional parameter [l] says that the words "with these words" will appear aligned left to the reserved space
% another optional parameter is [r] for aligning the words right
% if no optional parameter is given, the words will be centred in the reserved space
\newcommand*\phantomas[3][c]{%
\ifmmode
\makebox[\widthof{$#2$}][#1]{$#3$}%
\else
\makebox[\widthof{#2}][#1]{#3}%
\fi
}

\begin{document}

A long entry another long entry and our last long entry\par
\phantomas{A long entry}{centred} \phantomas[l]{another long entry}{on the left}
\phantomas[r]{and our last long entry}{on the right}

$\sin^2 x + \cos^2 x = 1$\par
$\phantomas{\sin^2 x + \cos^2 x}{f(x)} = 1$\par
$\phantomas[l]{\sin^2 x + \cos^2 x}{f(x)} = 1$\par
$\phantomas[r]{\sin^2 x + \cos^2 x}{f(x)} = 1$

\end{document}