If Then Else for odd page/even page

Question

How to use if-then-else structure in LaTeX? I need a example of odd and even page.

if odd then
  command 1
else
  command 2

Answer 1

Use the ifthen package:

\documentclass{article}
\usepackage{ifthen}
\begin{document}
 \ifthenelse{\isodd{\thepage}}
   {Odd}
   {Even}
\end{document}

Answer 2

Use the changepage package:

\usepackage{changepage}
\strictpagecheck
...
\checkoddpage
\ifoddpage
  ...
\else
  ...
\fi

If you use the memoir class, then this feature is built in automatically.

Answer 3

As Harald has pointed out already, the use of \thepage isn't foolproof to decide whether we are on an even or odd page. Try this extended version of Ryan's example:

\documentclass{article}
\usepackage{ifthen}
\newcounter{mycount}

\begin{document}
\noindent\whiledo{\themycount<130}{%
  \ifthenelse{\isodd{\thepage}}%
    {Odd\\}%
    {Even\\}%
  \stepcounter{mycount}%
}
\end{document}

You will read `Odd' on every page.

Instead, it is safer to use a \label+\pageref approach:

\documentclass{article}
\usepackage{ifthen}
\newcounter{mycount}

\begin{document}
\noindent\whiledo{\themycount<130}{%
  \label{mylabel\themycount}%
  \ifthenelse{\isodd{\pageref{mylabel\themycount}}}%
    {Odd\\}%
    {Even\\}%
  \stepcounter{mycount}%
}
\end{document}

Answer 4

For the sake of completeness: KOMA-Script offers the command \ifthispageodd{<true>}{<false>}. It can be used in the standard classes by loading scrextend:

\documentclass{article}
\usepackage{scrextend}

\begin{document}
\pagenumbering{roman}
\ifthispageodd{I'm odd}{I'm even}% I'm odd
\newpage
\ifthispageodd{I'm odd}{I'm even}% I'm even
\newpage
\pagenumbering{arabic}
\ifthispageodd{I'm odd}{I'm even}% I'm odd
\newpage
\ifthispageodd{I'm odd}{I'm even}% I'm even
\end{document}

memoir also offers a similar conditional \ifoddpage ... \else ... \fi which needs to be preceded by \checkoddpage. Additionally one should call \strictpagecheck so the page numbers get written to the aux file.

\documentclass{memoir}
\strictpagecheck
\makeatletter
\newcommand*\ifthispageodd{%
  \checkoddpage
  \ifoddpage
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}
\makeatother
\begin{document}
\pagenumbering{roman}
\ifthispageodd{I'm odd}{I'm even}% I'm odd
\newpage
\ifthispageodd{I'm odd}{I'm even}% I'm even
\newpage
\pagenumbering{arabic}
\ifthispageodd{I'm odd}{I'm even}% I'm odd
\newpage
\ifthispageodd{I'm odd}{I'm even}% I'm even
\end{document}

Answer 5

If you want an environment, which automatically right or left justifies depending on page odd/evenness, I use something like the below.

For example, I have boxes for my equations or key derivations, done with tikzpicture, and wrapping that inside the below environment does the expected. I wouldn't, however, use it around regions that can page-break mid way, like paragraphs of text, since a variable is stored (def\mylr{...}) at the start of the environment, that variable remains as is.

\newenvironment{autoLR}{
    \ifthenelse{\isodd{\value{page}}}%
        {\def\mylr{flushright}}%
        {\def\mylr{flushleft}}%
    \begin{\mylr}
}{%
    \end{\mylr}
}