No grouping for one index using \imakeidx

Question

I have a large project with multiple indexes in LuaLaTeX. Is there a way to use \imakeidx perhaps with a .ind file to have just one of these indexes 'turn off' grouping so that each level 0 item is spaced from the last item or subitem?

For this index, the specific level 0 items do not have a page number so I'd like it to look like:

Item
   Subitem            page
   Subitem            page
   Subitem            page

Item
   Subitem            page

Item
   Subitem            page
   Subitem            page

etc.

Because everything is already working except this, I'd like to stick with just makeindex/imakeidx and style files, not xindy, texindy or anything else.

Here is an example of what I'd like, but it redefines \item, \subitem etc. in the preamble, so only works if it is applied to all indexes. I need it for just a single index, leaving the others alone:

How to set the space between two index entries with the same character

Here's an example in texindy, but I don't know how to use texindy and I have multiple other indexes I'm not keen on redoing:

texindy style file: adding space between grouped entries

Can this be done in just plain regular \makeindex/\imakeidx with a style file? It seems I need an element like item_10 but this does not appear to exist. Or is there some other simple, direct way?

Answer 1

You can apply the customization only to the thematic index.

\documentclass{article}
\usepackage{imakeidx}

\makeindex
\makeindex[title=Thematic index,name=thematic,columns=1]

% customize the thematic index
\makeatletter
\newcommand{\thematiccustom}{%
  \def\@idxitem{\par\addvspace{10\p@ \@plus 5\p@ \@minus 3\p@}\hangindent 40\p@}%
  \def\subitem{\par\hangindent 40\p@ \hspace*{20\p@}}%
  \def\subsubitem{\par\hangindent 40\p@ \hspace*{30\p@}}%
  \def\indexspace{}%
}
\makeatother

\begin{document}

Text.

\index{something}
\index{another}

\index[thematic]{Fruit!Apple}
\index[thematic]{Fruit!Banana}
\index[thematic]{Fruit!Cherry}

\index[thematic]{Animal!Whale}
\index[thematic]{Animal!Gnu}

\index[thematic]{Automobile!Chevrolet}
\index[thematic]{Automobile!Jaguar}

\index[thematic]{Miscellaneous!Beauty}
%\index[thematic]{Miscellaneous!Happiness}

\printindex

\begingroup
\thematiccustom
\printindex[thematic]
\endgroup

\end{document}

Note: columns=1 is just to better show the separation between items.

Answer 2

Amended answer

For each thematic "category", manually add (and keep track of) a separate sorting character (s@t, where s = sorting string, t = index entry text, with "a@" for "cats", "b@" and "c@" for canids and carrypacks respectively, and "z@" for misc, so the index entry would look like, for example, \index[theme]{a@Cats!leopard}). "Miscellaneous" would logically be at the end in a theme list.

An .ist file with a dotfill command would give you right justification: if it is called plain.ist, invoking it would be by: \makeindex[name=theme,title=Thematic Index,options=-s plain].

One-column (columns=1 option in the \makeindex command):

Default two-columns:

MWE

\documentclass{article}
\usepackage[splitindex]{imakeidx}
\makeindex[name=fel,title=Cats]%a
\makeindex[name=can,title=Canids]%b
\makeindex[name=equ,title=Carrypacks]%c
\makeindex[name=msc,title=Misc]%z
\makeindex[name=theme,title=Thematic Index,options=-s plain]
\begin{document}
lion\index[fel]{lion}\index[theme]{a@Cats!lion}
coyote\index[can]{coyote}\index[theme]{b@Canids!coyote}
zebra\index[equ]{zebra}\index[theme]{c@Carrypacks!zebra}
moose\index[msc]{moose}\index[theme]{z@Miscellaneous!moose}
\newpage
wolf\index[can]{wolf}\index[theme]{b@Canids!wolf}
gazelle\index[equ]{gazelle}\index[theme]{c@Carrypacks!gazelle}
tiger\index[fel]{tiger}\index[theme]{a@Cats!tiger}
mouse\index[msc]{mouse}\index[theme]{z@Miscellaneous!mouse}
\newpage
guinea pig\index[msc]{guinea pig}\index[theme]{z@Miscellaneous!guinea pig}
leopard\index[fel]{leopard}\index[theme]{a@Cats!leopard}
dingo\index[can]{dingo}\index[theme]{b@Canids!dingo}
horse\index[equ]{horse}\index[theme]{c@Carrypacks!horse}
\newpage
\printindex[can]
\printindex[equ]
\printindex[fel]
\printindex[msc]
\printindex[theme]

\end{document}

The plain.ist file:

delim_0 "\\space\\dotfill\\space "
delim_1 "\\space\\dotfill\\space "
delim_2 "\\space\\dotfill\\space "
delim_n ", "
delim_r "--"
delim_t ""
encap_prefix "\\"
encap_infix "{"
encap_suffix "}"
line_max 1000

---

Original answer

If I have understood correctly, do you mean adding something like a summary or thematic index, using level 0 as grouping?

It's a separate index and has its own \index[...]{...} entries.

A script could go through the .idx files and derive a theme .idx file, getting, say, \indexentry{lion}{1} (from the -fel.idx) and outputting \indexentry{Cats!lion}{1} into the -theme.idx but this would be partway through the automatic toolchain, and in any case, a mapping to match the sub-entry (lion) to the grouping (Cats) would still need to be done/defined somewhere beforehand. Perhaps a regex could work, after the mapping is established.

MWE

\documentclass{article}
\usepackage[splitindex]{imakeidx}
\makeindex[name=fel,title=Cats]
\makeindex[name=can,title=Canids]
\makeindex[name=msc,title=Misc]
\makeindex[name=theme,title=Thematic Index]
\begin{document}
lion\index[fel]{lion}\index[theme]{Cats!lion}
coyote\index[can]{coyote}\index[theme]{Canids!coyote}
moose\index[msc]{moose}\index[theme]{Miscellaneous!moose}
\newpage
wolf\index[can]{wolf}\index[theme]{Canids!wolf}
tiger\index[fel]{tiger}\index[theme]{Cats!tiger}
mouse\index[msc]{mouse}\index[theme]{Miscellaneous!mouse}
\newpage
guinea pig\index[msc]{guinea pig}\index[theme]{Miscellaneous!guinea pig}
leopard\index[fel]{leopard}\index[theme]{Cats!leopard}
dingo\index[can]{dingo}\index[theme]{Canids!dingo}
\newpage
\printindex[can]
\printindex[fel]
\printindex[msc]
\printindex[theme]

\end{document}