How can I indent all text after the label in my example environment

Question

I have built my own example environment for a paper I am working on. I have created a counter to label the resulting blocks of text. Now I want to indent the Text after the label, like so:

(1)   Lorem ipsum dolor sit amet, consectetur adipiscing elit.
      Nullam hendrerit tempor purus in interdum. Ut nec augue
      ipsum.

(2)   Sed volutpat aliquet mauris non porta. Vivamus sagittis
      ante dapibus tellus vehicula ullamcorper. Quisque tris-
      tique commodo nisi et semper.

...

(10)  Sed volutpat aliquet mauris non porta. Vivamus sagittis
      ante dapibus tellus vehicula ullamcorper. Quisque tris-
      tique commodo nisi et semper.

This is the code I have written so far:

\newcounter{examplectr}

\newenvironment{example}[1][]{
    \ignorespaces
    \refstepcounter{examplectr}
    (\theexamplectr)
    \hfill
}{}

But unfortunately, the resulting output looks more like this:

(1)   Lorem ipsum dolor sit amet, consectetur adipiscing elit.
Nullam hendrerit tempor purus in interdum. Ut nec augue ipsum.

(2)   Sed volutpat aliquet mauris non porta. Vivamus sagittis
ante dapibus tellus vehicula ullamcorper. Quisque tristique
commodo nisi et semper.

Does anyone know how to achieve that?

Answer 1

The following answer builds closely on the OP's original code, only adding some low-level TeX code in the preamble to set the width of the hanging indents. Basically, it defines a length variable called \alength and then sets its value to the width of a "box" filled by "(\theexamplectr)" (plus a minute fudge factor, determined empirically). The environment example is instructed to hang-indent all but the first line of its paragraph by the amount of \alength. Note that the amount of the indent is adjusted automatically as the counter variable switches from 1 to 2, 3, etc digits.

This code is, by design, not as "elegant" as simply setting some of the options of the enumitem package. In contrast, it has the advantage (IMHO) of not depending on this package and, instead, demonstrating how one may go about achieving the OP's objective by setting the \hangafter and \hangindent parameters. :-)

Addendum: I edited the code, following the OP's comment, to allow the possibility that the maximum number of examples is known in advance, so that the bodies of the examples may all be set to the same (hanging) indent. This is implemented by defining a counter variable named maxexamplectr; if this variable is greater than zero, its value is taken to be the largest number of any example environment. Conversely, if its value is zero the value of the hang-indent is calculated separately for each label.

\documentclass{article}
\usepackage{lipsum,ifthen}

\newcounter{maxexamplectr}  %% largest possible number of examples
\setcounter{maxexamplectr}{1002} 

\newcounter{examplectr} %% counter of current example environment
\setcounter{examplectr}{0}  %% initialize this counter

\newlength\alength   

\makeatletter
% if maxexamplectr>0, calculate \alength only once 
\ifthenelse{\themaxexamplectr>0}{
    \sbox\@tempboxa{(\themaxexamplectr)\ \ }
    \setlength{\alength}{\the\wd\@tempboxa}}{}

\newenvironment{example}{%
    \refstepcounter{examplectr}%
    \ifthenelse{\themaxexamplectr=0}{% calculate \alength if maxexamplectr=0
       \sbox\@tempboxa{(\theexamplectr)\ \ }
       \setlength{\alength}{\the\wd\@tempboxa}}{}
    % start a "generic list" with a single, numbered item
    \begin{list}{{}(\theexamplectr)}%
       {\leftmargin\alength
        \labelwidth\alength
        \topsep0pt}
        \ignorespaces\item }
{\unskip \end{list}}
\makeatother

\begin{document}
\noindent
The quick brown fox \ldots

\begin{example}
\lipsum[2]
\end{example}

\setcounter{examplectr}{20}
\begin{example}
\lipsum[2]
\end{example}

\setcounter{examplectr}{1001}
\begin{example}
\lipsum[2]
\end{example}

\noindent
The quick brown fox \ldots
\end{document}

Answer 2

Using the enumitem package you can easily create a list-like environment to achieve the desired result:

\documentclass{article}
\usepackage{enumitem}
\usepackage{lipsum}

\newlist{example}{enumerate}{4}
\setlist[example,1]{label=(\arabic*),leftmargin=*,resume,ref=\arabic*}

\begin{document}

\begin{example}
\item \lipsum[1]
\end{example}
\lipsum[1]
\begin{example}
\item \lipsum[1]
\item \lipsum[1]
\end{example}

\end{document}

EDIT: you can wrap the list-like environment inside another one, using the series=, resume= options:

\documentclass{article}
\usepackage{enumitem}
\usepackage{lipsum}

\newlist{example}{enumerate}{4}
\setlist[example,1]{label=(\arabic*),leftmargin=*,ref=\arabic*}
\newenvironment{myexample}
  {\begin{example}[series=lafter,resume=lafter]\item}
  {\end{example}}

\begin{document}

\begin{myexample}
\lipsum[1]
\end{myexample}
\lipsum[1]
\begin{myexample}
\lipsum[1]
\end{myexample}

\end{document}

Answer 3

Below is a solution using ntheorem and the changepage package.

\documentclass{report}
\usepackage{ntheorem}
\usepackage{lipsum}
\usepackage[showframe]{geometry}
\usepackage{changepage}

\makeatletter
\newtheoremstyle{marginjustnumber}%
{\item[\theorem@headerfont \llap{(##2) }]}%
{\item[\theorem@headerfont \llap{(##2)} (##3)]}%
\makeatother

\newlength{\examplelabel}
\theoremstyle{marginjustnumber}
\theorembodyfont{}
\theoremseparator{}
\theoremprework{%
\settowidth{\examplelabel}{(\themyexample)\quad}%
\begin{adjustwidth}{\examplelabel}{}}
\theorempostwork{\end{adjustwidth}}
\newtheorem{myexample}{}

\begin{document}

\begin{myexample}
\lipsum[1]
\end{myexample}
\begin{myexample}
\lipsum[2]
\end{myexample}
\begin{myexample}
\lipsum[3]
\end{myexample}

\end{document}