Creating lists of figures based on groups

Question

Friends, consider the following code:

\documentclass{article}
\usepackage{graphicx}
\usepackage{lipsum}
\begin{document}
\listoffigures
\newpage
\lipsum[2]
\begin{figure}[h]
\centering
\includegraphics{imgs/shark.png}
\caption{A shark.}
\label{fig:shark}
\end{figure}
\lipsum[2]
\begin{figure}[h]
\centering
\includegraphics{imgs/duck.png}
\caption{A duck.}
\label{fig:duck}
\end{figure}
\lipsum[2]
\begin{figure}[h]
\centering
\includegraphics{imgs/elephant.png}
\caption{An elephant.}
\label{fig:elephant}
\end{figure}
\lipsum[2]
\end{document}

Basically, I have some text and three figures, as seen in the output:

I also have a \listoffigures:

I was wondering if it's possible to create lists of figures based on groups. For instance, group those animals by their habitat:

First list of figures

Animals that live in water

1 . A shark. ................. 2

Second list of figures

Animals that live in land

2 . A duck. ................. 2

3 . An elephant. ........ 2

I'm also wondering if it's possible to have intersections, e.g., the duck could be in both lists. I was thinking if there is some magical command like this:

...
\magicalcommandhere{water}
\includegraphics{imgs/shark.png}
...
\magicalcommandhere{water,land}
\includegraphics{imgs/duck.png}
...    
\magicalcommandhere{land}
\includegraphics{imgs/elephant.png}
...
\magicallist{water}
\magicallist{land}
...

I was thinking on the float package for my first attempt, but I'm not sure if it's the right choice.

Any ideas?

Answer 1

With the help of the caption package you can achieve what you desire. The idea is to declare a new caption type for each category, and then to use \captionof and \addcontentsline to decide in which list(s) should the corresponding entry go. A little example:

\documentclass{article}
\usepackage{caption}

\DeclareCaptionType[fileext=wat]{water}[Figure][Animals that live in water]
\DeclareCaptionType[fileext=lan]{aland}[Figure][Animals that live in land]

\begin{document}
\listoffigures
\listofwaters
\listofalands

\begin{figure}[!ht]
  \centering
  A shark
  \captionof{water}{A shark}
  \addcontentsline{lof}{figure}{\protect\numberline{\thewater}A shark}
  \label{fig:shark}
  \stepcounter{aland}
  \stepcounter{figure}
\end{figure}

\begin{figure}[!ht]
  \centering
  A duck
  \captionof{aland}{A duck}
  \addcontentsline{wat}{figure}{\protect\numberline{\thealand}A duck}
  \addcontentsline{lof}{figure}{\protect\numberline{\thealand}A duck}
  \label{fig:duck}
  \stepcounter{water}
  \stepcounter{figure}
\end{figure}

\begin{figure}[!ht]
  \centering
  An elephant
  \captionof{aland}{An elephant}
  \addcontentsline{lof}{figure}{\protect\numberline{\thealand}An elephant}
  \label{fig:elephant}
  \stepcounter{water}
  \stepcounter{figure}
\end{figure}

\end{document}

Notice that you will have to increase the counters manually to keep the numbering schema.

Answer 2

\documentclass[a4paper]{book}

\makeatletter
\newcommand\newkind[2]{%
  \protected\@namedef{div@#1}{\addto@kind{#1}}%
  \@namedef{div@head@#1}{#2}}
\protected\def\div@water{}
\protected\def\div@land{}
\def\divcaption#1#2{\caption{#2}% generate the caption and the number
  \def\div@list{#1}%
  \@for\next:=\div@list\do
    {\edef\div@write{\write\tf@divided{\csname div@\next\endcsname{%
       \unexpanded{\noexpand\contentsline{figure}}
       {\unexpanded{\noexpand\numberline}{\thefigure}{\ignorespaces
        \unexpanded{#2}}}{\unexpanded{\thepage}}}}}\div@write}
}
\newcommand{\categorize}[1]{\def\div@list{#1}%
  \@for\next:=\div@list\do{\global\@namedef{div@kind@\next}{}}
  \chapter{\listfigurename}
  \@input{\jobname.div}
  \@for\next:=\div@list\do{\expandafter\div@print\expandafter{\next}}
  \if@filesw
    \newwrite\tf@divided
    \immediate\openout\tf@divided=\jobname.div\relax
  \fi
}
\newcommand{\addto@kind}[2]{%
  \expandafter\g@addto@macro\csname div@kind@#1\endcsname{#2}}
\newcommand{\div@print}[1]{\section*{\@nameuse{div@head@#1}}\csname div@kind@#1\endcsname}
\makeatletter

\newkind{water}{Animals living in water}
\newkind{land}{Animals living on land}

\begin{document}
\frontmatter
\categorize{water,land}

\mainmatter
\chapter{Pippo}
\begin{figure}
\divcaption{water}{Whale}
\end{figure}
\begin{figure}
\divcaption{water,land}{Duck}
\end{figure}
\end{document}

Of course this needs some refinement.

It works by defining a new output stream into which we write an entry similar to that of the .lof file, but given as argument to a suitable macro; for instance, \divcaption{land}{Tiger} would write

\div@land{\contentsline{figure}{\numberline{1}{\ignorespaces Tiger}{2}}

assuming it's figure 1 on page 2. For \divcaption{water,land}{Frog} we'd get two lines:

\div@water{\contentsline{figure}{\numberline{2}{\ignorespaces Frog}{4}}
\div@land{\contentsline{figure}{\numberline{1}{\ignorespaces Frog}{4}}

The file is read by the \categorize command in the same way that's read the list of figures. This command takes as argument the comma separated list of "kinds", in the order in which we want them listed. The commands \div@water and \div@land add their argument to a container for each kind and, when the file is completely read in, the containers are expanded, which cause the printing of a header, followed by the entries. The kind is defined beforehand with \newkind{name}{header}.