How to determine whether a list of figures is empty?

Question

I want to check whether the document has any figures so as to decide whether or not to have a list of figures. Is this possible? Feasible without doing something like parsing the lof file?

Note that checking whether the lof file is empty is not a good enough solution for me, as some code may be writing things other than table lines into it (e.g. babel).

Answer 1

The idea behind this attempt is to use standard LaTeX references in order to detect whether a figure was used in the document. Here are the steps involved (with some modifications by @egreg):

1. Conjure up a label name that you won't use in your document, ever. Something like fig:was:used:in:doc.

2. Patch the figure environment to define an empty command (say) \there@is@a@figure at the end of every figure. This is possible using the etoolbox package which provides \AtEndEnvironment. More specifically, the command

   \AtEndEnvironment{figure}{\gdef\there@is@a@figure{}} 
   

   will be perpetually redefined until the last figure in the document.

3. At the end of the document, we check whether \there@is@a@figure exists (is defined), and define a label \label{fig:was:used:in:doc}. It doesn't really matter what this label contains (it could be the last used figure, section, table, or whatever):

   \AtEndDocument{\ifdefined\there@is@a@figure\label{fig:was:used:in:doc}\fi}
   

4. Write your own "conditional LoF" command using

   \newcommand{\conditionalLoF}{\@ifundefined{r@fig:was:used:in:doc}{}{\listoffigures}}%
   

   The command \conditionalLoF checks whether the macro \r@fig:was:used:in:doc is defined or not. This is the standard LaTeX referencing mechanism at work, since each label <lab> has an associated macro \r@<lab> defined. \@ifundefined{<cs>}{<undef code>}{<def code>} allows execution of <undef code> if <cs> is undefined, otherwise execute <def code>.

5. Use \conditionalLoF instead of \listoffigures.
6. Compile your document at least 3 times.

The main idea behind (1) is to have the label fig:was:used:in:doc only exist when using a figure, and not having to worry about what it is called. (2) provides the hook to the figure environment and allows a global (re)definition of \there@is@a@figure. (3) delays the definition of the fig:was:used:in:doc label until the end of the document in order to avoid causing "Multiply defined label" warnings. (4) and (5) provides a clean interface to the conditional LoF. (6) may be the only drawback. However, this comes standard with label referencing, requiring at least two compiles to work in general.

Here's a minimal working example (MWE). It uses graphicx (with the demo package option) and lipsum merely for show and is therefore actually not necessary. Compiling this MWE

\documentclass{article}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\usepackage[demo]{graphicx}% http://ctan.org/pkg/graphicx
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\makeatletter
\AtEndEnvironment{figure}{\gdef\there@is@a@figure{}} 
\AtEndDocument{\ifdefined\there@is@a@figure\label{fig:was:used:in:doc}\fi} 
\newcommand{\conditionalLoF}{\@ifundefined{r@fig:was:used:in:doc}{}{\listoffigures}}%
\makeatother
\begin{document}
\conditionalLoF% Conditionally insert List of Figures
\section{First section}
\lipsum[1]
%\begin{figure}
%  \centering\includegraphics{figure}
%  \caption{This is a caption}
%\end{figure}
\end{document}

as-is produces

Uncommenting the figure environment and compiling a couple of times produces