How to refer to number of items in a list before the list appears in the .tex file?

Question

I would like LaTeX to "know" how many items there are in a certain list. I want this number to be stored in a variable. In such a way I hope that this variable is sampled during the first compilation and available for the next compilations. Then I can refer to the number of items in my text before the list appears in the same text. How can I accomplish that? My minimum example does not work. Thanks in advance :)

\documentclass[10pt,a4paper,twosided]{article}
\usepackage{etoolbox}

\begin{document}

%The following list contains \NumberOfItems\ items

\begin{enumerate} 

\item item A
\item item B
\item item C

\end{enumerate}
\newcommand{\NumberOfItems}{\value{enumi}}

The previous list contains \NumberOfItems\ items


\end{document}

Answer 1

If you plan to use this only for first level lists and don't plan to use the enumitem package for personalizing the environment, using a \label in the last item is sufficient.

However a general solution that allows for rearrangement of items without bothering with the position of \label, is compatible also with enumitem and also works for nested list can be the following:

\documentclass[a4paper]{article}

\makeatletter
\newenvironment{enumeratecount}[1]
  {\def\thisenumeratecountlabel{#1}\enumerate}
  {\edef\@currentlabel{\number\value{\@enumctr}}%
   \label{\thisenumeratecountlabel}\endenumerate}
\makeatother

\begin{document}

The following list contains~\ref{firstlist} items, while
the nested list contains~\ref{innerlist}.

\begin{enumeratecount}{firstlist}
\item item A
\item item B
  \begin{enumeratecount}{innerlist}
  \item sub A
  \item sub B
  \end{enumeratecount}
\item item C
\end{enumeratecount}

The previous list contains \ref{firstlist} items, while the
inner list contains~\ref{innerlist}.

\end{document}

You just use \begin{enumeratecount}{<label>} for stating the label to be used (before or after the environment) for getting the number of items.

If you use enumitem you can even say

\begin{enumeratecount}{<label>}[<enumitem settings>]

so, for instance,

\begin{enumeratecount}{mylist}[label=\Alph*]

would work correctly (which wouldn't with the "simple method" outlined above and in Ulrike's answer).

---

The solution uses a couple of facts: a \label command refers to the current value of \@currentlabel, so before doing \endenumerate we define \@currentlabel to be the complete expansion of \value{\@enumctr}. The macro \@enumctr expands to enumi, enumii, enumiii or enumiv depending on the nesting level of the current enumerate environment; thus we're guaranteed to get the right number at all levels. Then we say \label{\thisenumeratecountlabel} that will set the label name to the original argument to enumeratecount. Locality of environments does the rest.

---

An extension

Suppose we want to say "the following list contains 1 item" or "the following list contains 3 items", adding the "s" conditionally.

This can be done with the help of the refcount package.

\usepackage{refcount}
\newcommand{\addphrase}[3]{% #1 = label, #2 = text if number >1, #3 = text if number =1
  \ifnum\getrefnumber{#1}>1
    #2%
  \else
    #3%
  \fi}

So the text above can be generated by

the following list contains \ref{firstlist}~\addphrase{firstlist}{items}{item}

Answer 2

In your example already \ref gives the correct output:

\documentclass[10pt,a4paper,twosided]{article}
\usepackage{etoolbox}


\begin{document}

The following list contains \ref{a} items

\begin{enumerate} 

\item item A
\item item B
\item\label{a} item C

\end{enumerate}
\end{document}

Answer 3

Q: How can I count the number of items and store this number in a command by Q:\newcommand?

A: Add a counter to item

\let\olditem\item
\AtBeginEnvironment{enumerate}{
 \renewcommand{\item}{%
 \olditem%
 \stepcounter{mytempnumber}%
}}

As always one cannot redefine a macro recursive (that's why we need \let). I would like to recommend to use a counter for a number, not a macro.

Q: How can this be done, so that the value of this variable carries over to the next compilation where it can be available for "My conditional text" ?

A: Write it out! You can do this as value only, or as assignment:

\AtEndDocument{%
 \openoutputfile{\jobname.num}{number}
 \addtostream{number}{%%
 \noexpand\setcounter{mynumber}{\themytempnumber}} 
 \closeoutputstream{number}
}

We need to handle expansion carefully here, since everything is expanded when wrote to a file.

Here's the complete code:

\documentclass{article}
\usepackage{etoolbox}

\usepackage{newfile}

\newoutputstream{number}

\newcounter{mynumber}
\newcounter{mytempnumber}

\let\olditem\item
\AtBeginEnvironment{enumerate}{
 \renewcommand{\item}{%
 \olditem%
 \stepcounter{mytempnumber}%
}}

\AtEndDocument{%
\openoutputfile{\jobname.num}{number}
\addtostream{number}{%%
 \noexpand\setcounter{mynumber}{\themytempnumber}} 
\closeoutputstream{number}
}

\AtBeginDocument%
{%
\IfFileExists{\jobname.num}{\input{\jobname.num}}{\message{NO \jobname.num rerun!}}%
}

\begin{document}

\themynumber% use as normal counter

\begin{enumerate} 
\item Issue A
\item Issue B
\item Issue C
\item 
\end{enumerate}

\end{document}

I used etoolbox to hook into the enumerate environment and newfile to do the i/o stuff but more convenient. A interested user will notice, that the file <main file name>.num contents only one line of code: \setcounter{mynumber}{4}. So it's important to include the file before the usage of the counter.

Answer 4

I used the ifthen package instead of etoolbox as I am more familiar with the former; there is likely a similar construct in the latter. Create a new counter, say itemnum. Set the value of the counter at then end of your enumerate command. Then a little trickery: take 1 from the counter, then refstepcounter it, which adds one back and allows you to save its value in a label. The \ifthenelse command uses its value and compares it to 1.

This, of course, takes 2 runs to take effect as does any \label & \ref combination.

\documentclass[10pt,a4paper,twosided]{article}
\usepackage{ifthen}

\newcounter{itemnum}

\begin{document}

This report will addresses the following 
\ifthenelse{\ref{label:itemnum}=1}{issue}{issues}.


\begin{enumerate} 

\item Issue A
\item Issue B
\item Issue C
\setcounter{itemnum}{\value{enumi}}
\addtocounter{itemnum}{-1}
\refstepcounter{itemnum}\label{label:itemnum}

\end{enumerate}



\end{document}