How to check if a macro value is empty or will not create text with plain TeX conditionals?

Question

I have a macro which holds some text value, e.g. \macro{some text}. If it is empty, I would like for nothing to appear in the document, but it has some content of any kind which would produce text displayed in the document, I would like a message to appear, "Text: some text".

• \macro{} would be considered empty.
• \def\somedata{}, \macro{\somedata} would be considered empty.
• \macro{0} would be considered not empty.

I have tried making a TeX conditional which can check if the value is not empty, but nothing I tried works, e.g.:

\ifx #1 {}
\else
    Text: #1
\fi

\ifx#1{}
\else
    Text: #1
\fi

\ifx #1 \nil
\else
    Text: #1
\fi

\ifx#1!=""
\else
    Text: #1
\fi

What is the correct syntax for creating a conditional in plain TeX with checks if a value is not empty?

Answer 1

(1) Tests for empty token list as input

You can try

\def\temp{#1}\ifx\temp\empty
  <EMPTY>%
\else
  <NON EMPTY>%
\fi

If you know that a token, say \hfuzz, will not appear in #1, then

\ifx\hfuzz#1\hfuzz
  <EMPTY>%
\else
  <NON EMPTY>%
\fi

This, differently from the previous test, is expandable.

The safest test uses e-TeX:

\if\relax\detokenize{#1}\relax
  <EMPTY>%
\else
  <NON EMPTY>%
\fi

Using this last one shouldn't be a problem, but be warned that it doesn't work with "Knuth TeX".

Comments

You should know also that \ifx compares the two tokens that follow it and has no = sign. Similarly, \if compares the two tokens that follow it, but after having done complete expansion.

So, how does the last test work? With \detokenize{#1} the argument is transformed into a sequence of characters of category code 12, none of which is \if-equivalent to \relax. Thus with empty #1 the test would compare \relax with \relax and so return True; with #1 non empty, say abc, the code would be

\if\relax abc\relax<EMPTY>\else<NOT EMPTY>\fi

and the comparison between \relax and a returns False, so only the <NOT EMPTY> code would remain.

(2) Test for no printed output

In order to test for no printed output you can't have an expandable test:

\setbox0=\hbox{#1\unskip}\ifdim\wd0=0pt
  <EMPTY>%
\else
  <NOT EMPTY>%
\fi

This assumes that #1 doesn't contain vertical material, such as \vfill. Remove the \unskip if also "space output" should be considered. However, this can go wrong when \mymacro{\hskip1pt\hskip1pt} is called; it mostly depends on what you expect to go in the argument how to cope with limit cases.

If e-TeX is used, one can define a \foreverunspace macro that will remove all trailing spaces (assuming no whatsit appears):

\def\foreverunspace{%
  \ifnum\lastnodetype=11
    \unskip\foreverunspace
  \else
    \ifnum\lastnodetype=12
      \unkern\foreverunspace
    \else
      \ifnum\lastnodetype=13
        \unpenalty\foreverunspace
      \fi
    \fi
  \fi
}

and so the test can be

\setbox0=\hbox{#1\foreverunspace}\ifdim\wd0=0pt
  <EMPTY>%
\else
  <NOT EMPTY>%
\fi

that would remove any trailing combination of glues, kerns and penalties.

Answer 2

The etoolbox package offers \ifdefempty (and its cousin \ifcsempty for control sequence names):

\documentclass{article}

\usepackage{etoolbox}

\def\mymacroA{}
\def\mymacroB{ }
\def\mymacroC{E}
\def\mymacroD{ E }

\begin{document}

\ifdefempty{\mymacroA}{YES}{NO}\par
\ifdefempty{\mymacroB}{YES}{NO}\par
\ifdefempty{\mymacroC}{YES}{NO}\par
\ifdefempty{\mymacroD}{YES}{NO}

\end{document}

Note that like the ifmtarg package commands pointed out by Werner, but unlike Caramdir's \ifx test, \ifdefempty will expand to "true" for macro arguments consisting of at least one space and nothing else.

Answer 3

Use the ifmtarg package:

\documentclass{article}
\usepackage{ifmtarg}% http://ctan.org/pkg/ifmtarg
\makeatletter
\newcommand{\isempty}[1]{%
  \@ifmtarg{#1}{YES}{NO}}
\newcommand{\isnotempty}[1]{%
  \@ifnotmtarg{#1}{YES}}
\makeatother
\begin{document}
\verb|\isempty{}   |: \isempty{} \par
\verb|\isempty{ }  |: \isempty{ } \par
\verb|\isempty{E}  |: \isempty{E} \par
\verb|\isempty{ E }|: \isempty{ E }

\bigskip

\verb|\isnotempty{}   |: \isnotempty{} \par
\verb|\isnotempty{ }  |: \isnotempty{ } \par
\verb|\isnotempty{E}  |: \isnotempty{E} \par
\verb|\isnotempty{ E }|: \isnotempty{ E }

\end{document}

It defines \@ifmtarg{<stuff>}{<true>}{<false>} and \@ifnotmtarg{<stuff>}{<true>}.

Answer 4

One way is to do:

\makeatletter
\def\test#1{%
    \def\tmp{#1}%
    \ifx\tmp\@empty%
        Nothing here!%
    \else%
        You passed: \texttt{#1}.%
    \fi%
}
\makeatother

Here \@empty is defined by LaTeX as \def\@empty{}.