How do I declare and pass conditions to commands?

Question

I'm new to LaTeX so I'm not quite sure how to proceed with this. I'm trying to have different versions of the same document, depending on which variables are set. What I want is to change the whole document changing one line.

So far, I've been trying something I found in a template, but I haven't been able to get it to work.

\newif\if@thing\@thingfalse
\newcommand*{\displaything}{\@thingtrue}

That way, when I write \displaything, the variable should be set. If someday I don't want the variable, I'd just comment this line. Then, I want to pass this variable to another command, like this:

\newcommand*{\foo}[2]{
  \ifthenelse{#1}
  {
    % do something if set, using #2
  }
  {
    % do something else if not set, using #2 (default)
  }
}

The idea is to have multiple \foo calls with more arguments, and different flag-things, and then use them like this:

\foo{\@thing}{arg}, \foo{@thing2}{arg}

Maybe this isn't even the right way to do it, any thoughts?

Answer 1

I'd use etoolbox for this, rather than ifthen.

\documentclass{article}
\usepackage{etoolbox}

\newtoggle{thing}

\newcommand{\foo}[2]{%
  \iftoggle{#1}
    {%
     --#2--% do something if set, using #2
    }%
    {%
     \fbox{#2}% do something else if not set, using #2 (default)
    }%
}

\begin{document}

% thing is initially false

\foo{thing}{baz}

\toggletrue{thing}

\foo{thing}{baz}

\end{document}

Answer 2

There are different ways of defining conditionals. When you use or set the value of a conditional, you need to use the right syntax for the definition you've chosen.

In addition to the etoolbox method demonstrated in egreg's answer (which I've never used), there are at least two other very common methods. Your problem is caused by trying to use the syntax for one of these methods when handling a conditional defined using the other method.

The following example demonstrates the two methods:

\documentclass{article}
\newif\iffoo% new conditional defined using method 1
\footrue
\newcommand*{\fooboo}{%
  \iffoo
    {\Huge FOO!\par}
  \else
    {\tiny fooless\dots\par}
  \fi}
\usepackage{ifthen}% method 2 requires ifthen
  \newboolean{bar}% new conditional defined using method 2
  \setboolean{bar}{true}
  \newcommand*{\barboo}{%
    \ifthenelse{\boolean{bar}}{\Huge BAR!\par}{\tiny barless\dots\par}}

\begin{document}
\fooboo
\foofalse% note syntax for foo
\fooboo
\barboo
\setboolean{bar}{false}% note syntax for bar
\barboo
\end{document}