help with nomenclature groups and ifthenelse

Question

I am grouping my nomenclature symbols. I'm using the answer from this post: How to make section in Nomenclature?

The syntax I've used is this:

\renewcommand{\nomgroup}[1]{%
    \ifthenelse{\equal{#1}{C}}{\item[\textbf{Variables}]}{%
        \ifthenelse{\equal{#1}{V}}{\item[\textbf{sets}]}{}}
}

If I set prefixes to C and V, the grouping works well. However, it does not work when I try to use numeric prefixes such as 5 and 6, i.e.

\renewcommand{\nomgroup}[1]{%
    \ifthenelse{\equal{#1}{6}}{\item[\textbf{Variables}]}{%
        \ifthenelse{\equal{#1}{5}}{\item[\textbf{sets}]}{}}
}

I am not sure why this is the case. I cannot use letters because I have many letters, in both capitals case and lower case. For my document, numbers makes a lot more sense.

The documentation suggest the quote below but I don't know what this means.

Note that for symbols and numbers you have to check for the strings “Symbols”and “Numbers”. http://texdoc.net/texmf-dist/doc/latex/nomencl/nomencl.pdf

How do I overcome this issue?

EDIT: 04/04/2019 1941GMT MWE:

\documentclass[a4paper,openany]{book}

\usepackage{siunitx}
\usepackage{nomencl}
\usepackage{ifthen}
\makenomenclature
\setlength{\nomlabelwidth}{1.45cm}
\renewcommand{\nompreamble}{text here.}

\renewcommand{\nomgroup}[1]{%
    \ifthenelse{\equal{#1}{6}}{\item[\textbf{Variables}]}{%
        \ifthenelse{\equal{#1}{V}}{\item[\textbf{sets}]}{}}
}

\begin{document}
test
\printnomenclature
\nomenclature[C1]{$\overline{\text{C}_{\text{F}}}$}{up, [1/s]}

\nomenclature[V2]{$\text{V}_{\text{D}}$}{down, [$\SI{}{\meter\per\second}$]}

\nomenclature[W]{$\overline{\text{We}}$}{left}
\nomenclature[O]{$\overline{\text{Oh}}$}{right}

\nomenclature[6K1]{$k$}{in}

\end{document}

Answer 1

This is a limitation of makeindex. I experimented with

\index{6@66}
\index{6k@6k}

and the first entry is listed among “numbers”, whereas the second entry appears among “symbols”. Only completely numeric parts before the @ will be sorted among “numbers” in the index. Besides, what's recorded in the .nlo file is

\nomenclatureentry{6K1$k$@[{$k$}]\begingroup in\nomeqref {0}|nompageref}{2}

and before the @ there will always be something that's not a number.

You're out of luck, sorry: only 27 different groups can be used, 26 for the letters and one for symbols. MakeIndex does not distinguish between uppercase and lowercase when dividing in groups.

I can only offer a less painful way to redefine \nomgroup when many letters are involved.

% arara: pdflatex
% arara: nomencl
% arara: pdflatex

\documentclass[a4paper,openany]{book}

\usepackage{siunitx}
\usepackage{nomencl}
\usepackage{xparse}
\makenomenclature
\setlength{\nomlabelwidth}{1.45cm}
\renewcommand{\nompreamble}{text here.}

\ExplSyntaxOn
\NewExpandableDocumentCommand{\checkletter}{mmm}
 {
  \str_case:nnF { #1 } { #2 } { #3 }
 }
\ExplSyntaxOff

\renewcommand{\nomgroup}[1]{%
  \checkletter{#1}{
    {A}{\item[\textbf{aaa}]}
    {B}{\item[\textbf{bbb}]}
    {C}{\item[\textbf{ccc}]}
    % other groups
    {O}{\item[\textbf{ooo}]}
    % other groups
    {V}{\item[\textbf{sets}]}
    {W}{\item[\textbf{www}]}
    {X}{\item[\textbf{xxx}]}
    {Y}{\item[\textbf{yyy}]}
    {Z}{\item[\textbf{zzz}]}
  }{\item[\textbf{Variables}]}
}

\begin{document}
test

\printnomenclature

\nomenclature[C1]{$\overline{\text{C}_{\text{F}}}$}{up, [1/s]}

\nomenclature[V2]{$\text{V}_{\text{D}}$}{down, [$\SI{}{\meter\per\second}$]}

\nomenclature[W]{$\overline{\text{We}}$}{left}
\nomenclature[O]{$\overline{\text{Oh}}$}{right}

\nomenclature[6K1]{$k$}{in}

\end{document}

This produces