Macro that calls ifthen's \equal doesn't work in section heading. Why?

Question

In the following MWE, I have a macro \mytest that tests whether its argument is a or b, returning "yes"/"no", respectively. This macro uses (a) \ifthenelse and (b) \equal from the if then package.

Works fine, except…

When I place the call in a section heading, I get:

Undefined control sequence. \equal {a}{a} l.9 ...tion{Troublesome construction: \mytest{a}.}

\documentclass{article}
\usepackage{ifthen}
\newcommand{\mytest}[1]{%
    \ifthenelse{\equal{#1}{a}}{yes}{%
    \ifthenelse{\equal{#1}{b}}{no}{%
}}}
\begin{document}
Troublesome construction: \mytest{a}.
%\section{Troublesome construction: \mytest{a}.}
\end{document}

I've commented out the problematic \section{Troublesome construction: \mytest{a}.} so that it will compile.

Why is this not working in a section heading?

Answer 1

As discussed in the comment the problem is that \ifthenelse is not expandable, so it does not yield the expected result when it is expanded in an \edef (or \write). That makes the entire macro \mytest fragile, which means it can't be used in moving argument like section headings or captions. Much more background can be found in What is the difference between Fragile and Robust commands?

One solution would be to \protect the macro or define it to be robust in the first place. See xstring functions in section/paragraph headers. That should work in many cases, but it can be a problem for macros that depend not only on their input, but also on some global 'variable'. For example

\documentclass{article}
\newcounter{myfoo}
\begin{document}
\tableofcontents
\stepcounter{myfoo}
\section{Foo \protect\themyfoo}
\end{document}

gives

while the unprotected \section{Foo \themyfoo} would do the right thing (because \themyfoo is expandable) and would show "Foo 1" in both instances.

In this case the unprotected (and expandable) macro is "evaluated" immediately and its expansion is used in all later steps. If the macro is protected it is not immediately expanded, instead it is passed on in its macro form until it really needs to be evaluated for typesetting. But the context for the two evaluations (one in the TOC the other in the heading) differs and then so does the result.

So if your macro relies on more than just its input to determine its output, you might be better off trying to create an expandable version. Instead of ifthen's conditionals you can use etoolbox's \ifstrequal

\documentclass{article}
\usepackage{etoolbox}
\newcommand{\mytest}[1]{%
  \ifstrequal{#1}{a}
    {yes}
    {\ifstrequal{#1}{b}
       {no}
       {}}}

\begin{document}
\tableofcontents
\section{Nothing problematic here}
Troublesome construction: \mytest{a}.
\section{Troublesome construction: \mytest{a}.}
\end{document}

Answer 2

The command \ifthenelse is fragile. What's a good definition of the term? A command is fragile when it produces uncanny error messages when used in a moving argument. Moving arguments are:

1. the mandatory argument to a title (\chapter, \section and similar) or to \caption, unless the optional argument is used;
2. the optional argument to a title or to a \caption, if used.

The standard method for coping with fragile commands in moving arguments is to prefix them with \protect.

A user defined command can be made robust by defining it with \DeclareRobustCommand.

\documentclass{article}
\usepackage{ifthen}

\newcommand{\mytest}[1]{%
    \ifthenelse{\equal{#1}{a}}{yes}{%
    \ifthenelse{\equal{#1}{b}}{no}{%
}}}
\DeclareRobustCommand{\myrobusttest}[1]{%
    \ifthenelse{\equal{#1}{a}}{yes}{%
    \ifthenelse{\equal{#1}{b}}{no}{%
}}}

\begin{document}

Troublesome construction: \mytest{a} and \mytest{b}.

Troublesome construction: \myrobusttest{a} and \myrobusttest{b}.

\section{Troublesome construction: \protect\mytest{a} and \protect\mytest{b}.}

\section{Troublesome construction: \myrobusttest{a} and \myrobusttest{b}.}

\end{document}

For hyperref, you can use \protected\def instead:

\documentclass{article}
\usepackage{ifthen}
\usepackage{hyperref}

\protected\def\mytest#1{%
    \ifthenelse{\equal{#1}{a}}{yes}{%
    \ifthenelse{\equal{#1}{b}}{no}{%
}}}

\begin{document}

Troublesome construction: \mytest{a} and \mytest{b}.

\section{Troublesome construction: \mytest{a} and \mytest{b}.}

\end{document}

If the argument to \mytest is expected to be a string (no special character or command), the following will work also with hyperref:

\documentclass{article}
\usepackage{xparse}
\usepackage{hyperref}

\ExplSyntaxOn
\NewExpandableDocumentCommand{\mytest}{m}
 {
  \str_case:nn { #1 }
   {
    {a}{yes}
    {b}{no}
   }
 }
\ExplSyntaxOff

\begin{document}

Troublesome construction: \mytest{a} and \mytest{b}.

\section{Troublesome construction: \mytest{a} and \mytest{b}.}

\end{document}

The advantage is that you get no warning at all from hyperref about Token not allowed and that the list of cases is easily extended to three or more.

With a further level of abstraction, you can define as many similar tests as you want.

\documentclass{article}
\usepackage{xparse}
\usepackage{hyperref}

\ExplSyntaxOn
\NewDocumentCommand{\definetest}{mm}
 {
  \NewExpandableDocumentCommand{#1}{m}
   {
    \str_case:nn { ##1 } { #2 }
   }
 }
\ExplSyntaxOff

\definetest{\mytest}
 {
  {a}{yes}
  {b}{no}
 }

\begin{document}

Troublesome construction: \mytest{a} and \mytest{b}.

\section{Troublesome construction: \mytest{a} and \mytest{b}.}

\end{document}

Answer 3

With a simple \ifx:

\documentclass{article}
\newcommand\mytest[1]{%
    \ifx#1a\relax yes\else\ifx#1b\relax no\fi\fi}
\begin{document}
Troublesome construction: \mytest{a}.
\section{Troublesome construction: \mytest{a}.}

Troublesome construction: \mytest{b}.
\section{Troublesome construction: \mytest{b}.}

Troublesome construction: \mytest{c}.
\section{Troublesome construction: \mytest{c}.}
\end{document}

If more than one letter is possible, but only the first one is important for the if clause:

\newcommand\mytest[1]{\myTEST#1!}
\def\myTEST#1#2!{%
    \ifx#1a yes\else\ifx#1b no\fi\fi}

And if a whole word should be tested then use:

\def\Bee{b}\def\Aih{a}%% Change to your "words"
\newcommand\mytest[1]{%
  \def\TEMP{#1}%
  \ifx\TEMP\Aih yes\else\ifx\TEMP\Bee no\fi\fi}