Moving an object to the right margin

Question

I need a macro that aligns an object against the right margin, on the current line if this leaves a space between the text and the object, and on the next line otherwise. No other material should appear on the same line as the object in the latter case.

My first attempt (\tmarksi) seems to work, but it causes underfull hbox warnings if it occurs at the end of a paragraph (I don't understand why). The second attempt (\tmarksii) seems to insert an unwanted paragraph break. Therefore I tried locally setting \parskip to zero in the third attempt, but this doesn't work either.

Surely this should not be difficult; what am I missing?

\documentclass{article}
\setlength\parindent{0pt}
\setlength\parskip{2cm} %For illustration purposes
%
\newcommand\tmarksi[1]{\hspace*{\fill}~\mbox{[#1]}\newline}
\newcommand\tmarksii[1]{\hspace*{\fill}~\mbox{[#1]}\vskip0pt}
\newcommand\tmarksiii[1]{\hspace*{\fill}~\mbox{[#1]}\bgroup\parskip=0pt\parindent=0pt\par\egroup}
%
\begin{document}
The quick brown fox jumps over the lazy dog. \tmarksi{1}
The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over \tmarksi{1}
the lazy dog.
\end{document}

Answer 1

I assume you meant that \tmarksi generates an underfull hbox not an overfull one if it comes at the end of the paragraph. The reason for this behavior is that the \newline in that definition always starts another line in that paragraph and then the final line is fully empty. This is why in LaTeX \\ or \newline are introduced as line separators not as line endings (though many people try to use \\ \\ to get empty lines or extra space).

So, this means either you write two different commands for the two cases or you try something like this:

\newcommand\tmarksi[1]{\hspace*{\fill}~\mbox{[#1]}\penalty -9999 }

What is different here is that I used a penalty which is not quite an "always break here" so that the final breakpoint at the end of the paragraph will overwrite it. However, within the paragraph it should normally be sufficient, that is as long as you do not use explicit \linebreak commands. So take your pick.

Answer 2

You can use a variant of the "Bourbaki problem" in the TeXbook

\documentclass[convert,border=2]{standalone}

\newcommand{\tmark}[1]{%
  {{\unskip\nobreak\hfil\penalty50
  \hskip2em\vadjust{}\nobreak\hfil[#1]%
  \parfillskip=0pt \finalhyphendemerits=0 \par
  \penalty 10000 \parskip=0pt\noindent}}\ignorespaces}

\begin{document}
\begin{minipage}{150pt}\parindent=20pt\parskip=20pt
The quick brown fox jumps over the lazy dog.\tmark{1}
The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over\tmark{1}
the lazy dog.

The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy \tmark{1}
the lazy dog.

The quick brown fox jumps over the lazy dog.
The quick brown fox jumps over the lazy brown dog \tmark{1}

Another
\end{minipage}

\end{document}

This introduces a paragraph break, but also issues \noindent and \penalty 10000 that inhibits a page break. I've added also \parskip=0pt to cover our tracks. The minimum space between the end of text and the mark is 2em, play with it.

Note: the \parskip is set in the minipage just to show the effect. Also \parindent needs to be set, otherwise it's zero.

To get the text flush right in case the mark is moved to the next line, change the first \hfil into \hskip0pt. For a normal interword space as the minimum, change \hskip2em into \space.

The following version implements this and also avoids page breaks in case the mark is moved to the next line:

\newcommand{\tmark}[1]{%
  {{\unskip\nobreak\vadjust{\nobreak}\hskip0pt\penalty50 \space
  \vadjust{}\nobreak\hfil[#1]%
  \clubpenalty=0 \widowpenalty=0 \brokenpenalty=0
  \parfillskip=0pt \finalhyphendemerits=0 \par
  \penalty 10000 \parskip=0pt\noindent}}\ignorespaces}

There is a price to pay: we have less control of widow and club lines, but as Frank Mittelbach remarks in the comments it seems quite hard to comply with both necessities.