\ifthenelse as an argument to \equal

Question

In this example:

\documentclass{article}
\usepackage{ifthen}

\newcommand{\objtitle}[1]{%                                                                                                                   
\ifthenelse{\equal{#1}{a}}{avalue}{}%                                                                                                         
\ifthenelse{\equal{#1}{b}}{bvalue}{}%                                                                                                         
}

\newcommand{\counterpart}[1]{%                                                                                                                
\ifthenelse{\equal{#1}{a}}{b}{}%                                                                                                              
\ifthenelse{\equal{#1}{b}}{a}{}%                                                                                                              
}
\begin{document}
\counterpart{a}
\objtitle{a}
\objtitle{\counterpart{a}}
\end{document}

\counterpart{a} and \objtitle{a} work separately but \objtitle{\counterpart{a}} produces and error:

! Argument of \equal has an extra }.
<inserted text> 
                \par 
l.17 \objtitle{\counterpart{a}}

Using \objtitle{\protect\counterpart{a}} eliminates the error message but produces no output.

What is the problem and how to solve?

\ifthenelse is (very) fragile and can not be used in the argument of \equal — David Carlisle, Oct 11 '18 at 19:41
@DavidCarlisle using \protect produces no output. Is there a solution? — Viesturs, Oct 11 '18 at 19:45
yes \protect makes it safe but stops it executing at all so it will not be equal to anything. You do not want to use \ifthenelse at all, but you havn't given any description of what you actually want to do so not sure what to suggest. — David Carlisle, Oct 11 '18 at 19:56
related https://tex.stackexchange.com/questions/451960/nested-if-then-else-statements — , Oct 11 '18 at 21:33

score 2 · Accepted Answer · answered Oct 11 '18 at 20:39

\ifthenelse cannot be used nested. No way. Sorry, it's impossible.

If you want to do nestable case switching, you need to use more advanced tools.

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewExpandableDocumentCommand{\caseswitch}{O{}mm}
 {
  \str_case_e:nnF { #2 } { #3 } { #1 }
 }
\ExplSyntaxOff

\newcommand{\objtitle}[1]{%
  \caseswitch{#1}{
   {a}{avalue}
   {b}{bvalue}
  }%
}

\newcommand{\counterpart}[1]{%
  \caseswitch{#1}{
    {a}{b}
    {b}{a}
  }%
}

\begin{document}

\counterpart{a}

\objtitle{a}

\objtitle{\counterpart{a}}

\end{document}

You can also call \caseswitch with an optional argument; thus

\caseswitch[none of the above]{#1}{
   {a}{avalue}
   {b}{bvalue}
  }

would return “none of the above” if #1 is neither a nor b. With no optional argument, no match returns nothing.

The number of cases is arbitrary.

\ifthenelse as an argument to \equal

1 Answers1