Frequently Asked Question List for TeX
Programming
Finding if you’re on an odd or an even page
Another question discusses the issue
of getting \marginpar commands to put their output in the correct
margin of two-sided documents. This is an example of the general
problem of knowing where a particular bit of text lies: the output
routine is asynchronous, and (La)TeX will usually process quite a
bit of the “next” page before deciding to output any page. As a
result, the page counter (known internally in LaTeX as
\c@page) is normally only reliable when you’re actually in
the output routine.
The solution is to use some version of the \label mechanism to
determine which side of the page you’re on; the value of the page
counter that appears in a \pageref command has been inserted in
the course of the output routine, and is therefore safe.
However, \pageref itself isn’t reliable: one might hope that
\ifthenelse{\isodd{\pageref{foo}}}{odd}{even}
would do the necessary, but both the babel and
hyperref packages have been known to interfere with the
output of \pageref; be careful!
The changepage package needs to provide this functionality
for its own use, and therefore provides a command \checkoddpage;
this sets a private-use “label”, and the page reference part of that
label is then examined (in a hyperref-safe way) to set a
conditional \ifoddpage true if the command was issued on an odd
page. (The memoir class has the same command.) LaTeX
users who are unfamiliar with TeX’s \if... commands may use
the ifthen package:
\usepackage{ifthen,changepage}
...
\checkoddpage
\ifthenelse{\boolean{oddpage}}{<odd page stuff>}{<even page stuff>}
Of course, the “label” contributes to LaTeX’s “Rerun to get cross-references right” error messages…
The Koma-Script classes have an addmargin* environment
that also provides the sorts of facilities that the changepage
offers. Koma-Script’s supporting command:
\ifthispageodd{<true>}{<false>}
executes different things depending on the page number.
The package ifoddpage is designed to provide the same
facility; crucially, it can behave “sensibly” even if you are
typesetting for one-side printing only; like the changepage
it uses a “check” command \checkoddpage. The conditional “side”
flags are set using (Plain) TeX conditionals; they are defined
locally, so that you can minimise their use of TeX workspace —
see the package documentation for the somewhat tricky sequence
involved. In addition the package provides a command
\ifoddpageoroneside, which is true on odd pages of a two-side
document, or on all pages of a one-side document. Usage is:
\checkoddpage
\ifoddpage
odd-side text
\else
even-side text
\fi
The author’s recommended usage (trickily) includes the whole operation in a box; this has the advantage that your test will always work, but the usual disadvantage that boxes may not split. In common uses, the whole work will be done inside a box (as, for example, in the case of a float), so the elaborate work proposed by the author is not necessary.