[Tex/LaTex] Optional argument to \item

Question

I am typesetting an ordered list where the entries may appear out of order, but should be assumed in order otherwise. I don't want to have to jump through \setcounter hoops to do this. That is, I want something like

\begin{enumerate}
  \item First
  \item Second
  \item[8] Eighth!
  \item Ninth!
  \item[4] Fourth
\end{enumerate}

to give the following Good output, instead of the Bad output:

A minimum working example follows.

\documentclass{article}
\usepackage{enumitem} % preferably something compatible with this guy
\begin{document}

Good:

\begin{enumerate}
  \item[1.] First
  \item[2.] Second
  \item[8.] Eighth!
  \item[9.] Ninth!
  \item[4.] Fourth
\end{enumerate}

Bad:

\begin{enumerate}
  \item First
  \item Second
  \item[8] Eighth!
  \item Ninth!
  \item[4] Fourth
\end{enumerate}

\end{document}

It should be noted that I don't want to have to specify the period afterwards, in case my list using a different numbering, e.g. Roman. Compatibility with enumitem is preferred, but not required.

What is the correct way of going about this?

Answer 1

You don't want to abuse the optional argument to \item; better defining a \nextitem command:

\documentclass{article}

\makeatletter
\newcommand\nextitem[1]{%
  \setcounter{\@enumctr}{#1}%
  \addtocounter{\@enumctr}{-1}%
}
\makeatother

\begin{document}
\begin{enumerate}
\item First
\item Second
\nextitem{8}
\item Eighth!
\item Ninth!
\nextitem{4}
\item Fourth
\end{enumerate}
\end{document}