Non-integer chapters

Question

I am trying to get a chapter number that is not an integer. My current code looks like this:

\chapter*{Example\vspace{-75pt}\\\huge Chapter 0.5}\vspace{50pt}
\addcontentsline{toc}{chapter}{0.5 Example}

However, the code is very messy, and every section needs a similar change for it to stay consistent (Thankfully I don't have any sections in the chapter but it is a problem).

What I am looking for is a solution that solves the first (or both) problems noted above.

EDIT: For clarification, 0.5 is just an example. More examples:

\chapter*{Example\vspace{-75pt}\\\huge Chapter Q}\vspace{50pt}
\addcontentsline{toc}{chapter}{Q Example}

\chapter*{Example\vspace{-75pt}\\\huge Chapter Zero point five}\vspace{50pt}
\addcontentsline{toc}{chapter}{Zero point five Example}

\chapter*{Example\vspace{-75pt}\\\huge Chapter Chapter}\vspace{50pt}
\addcontentsline{toc}{chapter}{Chapter Example}

\chapter*{Example\vspace{-75pt}\\\huge Chapter $\frac12$}\vspace{50pt}
\addcontentsline{toc}{chapter}{$\mathbf{\frac12}$ Example}

Answer 1

Not going to explain too much, but the idea is to define \additionalchapter that does a few things:

1. the chapter number is stepped down (so when \chapter is called the main number is the same as the previous one)

2. \thechapter is redefined to include the desired additional part;

3. \chapter is issued, but in its original form.

Why do we need \standardchapter? Because we have to add to \chapter the code for restoring the standard meaning of \thechapter.

Finally, a code for “textual” fractions is added.

Production notes. I use openany and geometry just to make a smaller picture of the result.

\documentclass[openany]{book}
\usepackage{amsmath} % for \text

\usepackage[a6paper]{geometry} % just to make smaller pictures

\NewDocumentCommand{\additionalchapter}{mO{#3}m}{%
  % #1 = suffix to add
  % #2 = optional argument for toc and header, as usual
  % #3 = title
  \addtocounter{chapter}{-1}%
  \renewcommand{\thechapter}{\standardthechapter#1}%
  \nonstandardchapter[#2]{#3}%
}

\AtBeginDocument{%
  \NewCommandCopy{\standardthechapter}{\thechapter}%
  \NewCommandCopy{\nonstandardchapter}{\chapter}%
  \AddToHook{cmd/chapter/before}{%
    \RenewCommandCopy{\thechapter}{\standardthechapter}%
  }%
}

\NewDocumentCommand{\chapfrac}{mm}{$\frac{\text{#1}}{\text{#2}}$}

\begin{document}

\chapter{First}\label{ch:one}

\ref{ch:one}, \ref{ch:oneandhalf}, \ref{ch:oneplus}, \ref{ch:two}

\additionalchapter{\chapfrac{1}{2}}{First and a half}\label{ch:oneandhalf}

\additionalchapter{\chapfrac{2}{3}}{First and more}\label{ch:oneplus}

\chapter{Second}\label{ch:two}

\end{document}

Answer 2

This creates an array of chapter "numbers" to be used for \thechapter.

\documentclass{report}

\renewcommand{\thechapter}{\csname chapt\arabic{chapter}\endcsname}

\makeatletter
\@namedef{chapt1}{1}
\@namedef{chapt2}{1.5}
\@namedef{chapt3}{II}
\@namedef{chapt4}{C}
\@namedef{chapt5}{$\pi$}
\makeatother

\begin{document}
\chapter{First}
\chapter{Second}
\chapter{Third}
\chapter{Fourth}
\chapter{Fifth}
\end{document}