Text Formatting ⇒ ifx and ifthenelse

[heavytull]

Can anyone explain the syntax of:

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ifx&#1&%
         \else{something\fi}

?
and what is the difference with:

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\ifthenelse{\equal{#1}{}}{}{something}

?

[cgnieder]

\ifx tests if the two following tokens are equal:

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\ifx<1><2>
 true
\else
 false
\fi

Depending on that either »true« or »false« is executed.

In your example

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\ifx&#1&
 true
\else
 false
\fi

it is used to detect whether the argument #1 is empty or not. An empty #1 would mean that \ifx sees two »&« directly after one another which would lead to »true«.

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\documentclass{article}
\begin{document}

\newcommand\test[1]{%
 \ifx&#1&%
  true%
 \else
  false%
 \fi}

\test{} % true

blank is \emph{not} empty:
\test{ } % false

\test{a} % false

\end{document}

The second code does the same:

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\documentclass{article}
\usepackage{ifthen}
\begin{document}

\newcommand\test[1]{%
  \ifthenelse{\equal{#1}{}}{true}{false}}

\test{} % true

\test{ } % false

\test{a} % false

\end{document}

In many cases it won't matter which version you use but I suspect that the second one is mor robust in some cases. Since I haven't checked how \ifthenelse is defined I might be mistaken, though.