# LaTeX conditional expression

I would like to be able to set a flag at the beginning of the main flag (say output) that tells if I want to produce a paper document or an electronic document.

Then I should check this flag to define or not empty pages, hypperref, oneside or twosides, etc.

How could I do that ?

-

There are many ways.

# Plain LaTeX

Perhaps the most canonical is:

\newif\ifpaper


Then either

\papertrue % or
\paperfalse


And to use it:

\ifpaper
% using paper
\else
% electronic
\fi


However, using these primitive conditionals can lead to unexpected problems if you're not entirely familiar with the way TeX processes them, and the following solutions are recommended.

# etoolbox

A more user-friendly and modern approach is taken by etoolbox, where you'd write instead

\newtoggle{paper}


which is set with either

\toggletrue{paper}
\togglefalse{paper}


And to use it:

\iftoggle{paper}{%
% using paper
}{%
% electronic
}


Why do I say this is more friendly? You'll never run into troubles with nesting, for which LaTeX's \newif conditions can sometimes be a bit of a pain to deal with.

etoolbox also supports boolean expressions such as

\ifboolexpr { togl {paper} and togl {pdf} } {true} {false}


which can also be combined with various testing functions also provided by that package (and others):

\ifboolexpr { togl {paper} or test {\ifdef{\foo}} } {true} {false}


# expl3

Finally, for package writers (not really document authors): expl3 as part of LaTeX3 provides booleans that can be used in all sorts of interesting ways. For basic use, they look like this:

\bool_new:N \l_tmpa_bool

\bool_set_true:N \l_tmpa_bool
\bool_set_false:N \l_tmpa_bool

\bool_if:NTF \l_tmpa_bool {true} {false}
\bool_if:NT \l_tmpa_bool {true}
\bool_if:NF \l_tmpa_bool {false}


You can use boolean logic as follows:

\bool_if:nTF { ( \l_tmpa_bool || \l_tmpb_bool ) && !( \l_tmpc_bool ) } {true} {false}


Everything is expandable and lazy evaluation is used to avoid performing tests that don't need to be checked.

You can also combine boolean variables above with conditional functions such as \cs_if_eq_p:NN (‘are two control sequences equal?’) which can be tested in exactly the same way:

\bool_if:nTF { \l_tmpa_bool && \cs_if_eq_p:NN \foo \bar } {true} {false}


The overlap between etoolbox and expl3 here is rather pronounced; I think of the difference between the two as the separation between document authors (for etoolbox) and package writers (for expl3). But it's purely a matter of taste, when it comes down to it.

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Thanks ! Just perfect. –  Cedric H. Nov 24 '10 at 13:53
+1 for not mentioning the obsolete ifthen package. –  Philipp Nov 27 '10 at 10:24
"troubles with nesting" - you might link to your excellent digression/answer. –  Charles Stewart Nov 27 '10 at 11:05
your answer is very useful for me. –  xport Dec 9 '10 at 7:24
@Philipp: Waht’s the problem with ifthen I use it often an think it’s syntax is easier that that one shown by Will with etoolbox. –  Tobi Jul 7 '11 at 15:16

ifthen can be your friend also-

but Will R's solutions are more low level and possibly closer to what you want.

I use ifthen with my CV, since I have to have an internal version as well as an external version. Not to mention that it makes having a 'long' and 'short' version.

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How is etoolbox low level? –  Charles Stewart Nov 27 '10 at 11:06
I just meant the dive into straight TeX, but yes, etoolbox is decidedly not. –  flip Nov 27 '10 at 14:47

Another approach is with the optional package, which I use for many purposes, along with boolexpr, which has a nice \switch and \case command.

Here's a minimal to illustrate.

%%%my-varient-for-different-output.tex
%%%This is the file that you actually call with pdftex/lualatex/etc.
\documentclass{minimal}
%\usepackage{xifthen}
\usepackage[foo]{optional}
%\input{themain-file.tex}
%%%EOF my-varient-for-different-output.tex

%%%%START OF themain-file.tex
\usepackage{boolexpr}

\makeatletter
%Check if optional is loaded. Otherwise, you don't
%want to define these commands.
%%Don't do anything here because you've got it covered.
}{%
%%%What are the defaults if no optional class was loaded?
\usepackage[bar]{optional}
}
%%%Let's make an ifthenelse version of the \opt command that's
%%%provided by the optional package
\newcommand\ifopt[3]{%
\ifboolexpr{%
0 = \pdfstrcmp{true}{%
\opt{#1}{true}%
}%
}{#2}{#3}%
}%
%%%This command is a wrapper around the \switch command that's
%%%provided by boolexpr. It lets you quickly create a switch
%%%that checks if an option is set.
\newcommand*\switchopt[1]{%
\switch[\pdfstrcmp{\opt{#1}{#1}}]%
}
%%%I'm sure there is a way to embed the \switch
%%%command in here, but I'm not that smart. Is
%%%there some application of \noexpand?
%%%
%%%Anyway, this is more common to use than \switchopt
%%%You'll use \switch (with no optional argument)
%%%and \case[\caseopt{myoption}] Do stuff...
%%%
\newcommand*\caseopt[1]{%
\pdfstrcmp{#1}{\opt{#1}{#1}}% = 0%
}

\makeatother

\begin{document}
I'm writing some text but \opt{foo}{don't want this
unless foo is there.} If I want an else as part of
the deal, then \ifopt{bar}{bar must be there}{bar
is out to lunch} and it's time to go.

Nesting ifopt commands can be tedious. Best to use
a case if we've got a ton of definitions\ldots
%%%NOTICE THE {{ and }}. They are required when using
%%% an optional argument in \switch.
\switchopt{bar}
\case{{foo}} Foo stuff here!
\case{{bar}} Bar stuff here!
\case{{third}} Some other thingy-dingy!
\otherwise Nothing I know of was set.
\endswitch%

\switch
\case{\caseopt{foo}} Foo stuff here!
\case{\caseopt{bar}} Bar stuff here!
\otherwise Nothing I know of was set.
\endswitch%
\end{document}


Although the chosen answer is probably better, this may lead to more ideas.

-

You can at the command line do something like \def\MYFLAG{} and then test if \MYFLAG is undefined in your document (or an included style file).

This allows you to run a document in different ways without having to change it, something like command line options to a program.

Not exactly the question asked, but strongly related to the use case you give.

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How to test if \MYFLAG is undefined? –  Xiè Jìléi Feb 1 '13 at 1:11
@XièJìléi: I normally use \ifdefined\MYFLAG ... \else ... \fi. –  Peter Grill Mar 24 at 22:46