Count lines in macro argument

Question

Is it possible to define a command that takes different actions based on whether the text of its argument will occupy more than one line?

I want to have a block of text that is centered if it's only one line and justified otherwise.

Answer 1

Something like this?

\documentclass{article}

\usepackage[showframe]{geometry}% just to make it easy to see the effect

\makeatletter
\def\centerorjustify#1{%
  \setbox\@tempboxa\hbox{#1}%
  \ifdim\wd\@tempboxa>\linewidth
    #1%
  \else
    \begin{center}%
      #1%
    \end{center}%
  \fi
}

\begin{document}

\centerorjustify{Hello}

\centerorjustify{Hello world}

\centerorjustify{A long text that will be a little shorter than the text width.}

\centerorjustify{A longer text that will be longer than a line so that we can see the effect of the macro. Yeah, I don't know what else to type in here}

\end{document}

The macro puts the content in an \hbox and compares its width with \linewdith and centers if it is shorter.

Answer 2

Just for fun:

you can mis-use the caption package for a similar behaviour

\documentclass{article}

\usepackage{caption}
\usepackage{newfloat}

\DeclareCaptionFormat{empty}{#3}
\DeclareFloatingEnvironment{my}
\captionsetup[my]{format=empty}

\begin{document}

\captionof{my}{one line}

\captionof{my}{longt text over multiple lines with much longer text and even longer text so that it stretches over multiple lines}

\end{document}