An enumerate environment in which the label is omitted when item number is just one and displayed as usual when item number is more than one

Question

How to define an enumerate environment in which the label is omitted when item number is just one and displayed as usual when item number is more than one?

Answer 1

You can typeset the environment inside a box just to get the number of items and then retypeset it:

\usepackage{environ}
\makeatletter
\NewEnviron{fenumerate}
  {\setbox0=\vbox{\enumerate\BODY\endenumerate\expandafter}%
   \expandafter\def\expandafter\@tempfenum\expandafter{\the\value{\@enumctr}}%
   \begin{enumerate}
     \ifnum\@tempfenum=\@ne\expandafter\def\csname label\@enumctr\endcsname{}\fi
     \BODY
   \end{enumerate}}
\makeatother

First of all, the infrastructure set up by environ gathers the environment's text; then it typesets it in a temporary box and at the end the value of the relevant counter (enumi, enumii, enumiii or enumiv) is stored in \@tempfenum, which is used for the "real" typesetting.

Usage:

\begin{fenumerate}
\item this won't get a number
\end{fenumerate}

\begin{fenumerate}
\item this will have a number
\item since it's followed by another item
\end{fenumerate}