Is there a reverse \alph?

The backtick in numerical context can be used to get the character code of a letter. This can be used to calculate the position of the letter in the alphabet.

• The definition range for the argument of \inversalph is a to z and A to Z for \inverseAlph.

• The letter can be hidden inside a macro (or even nested macros).

\documentclass{article}
\newcommand*{\inversalph}[1]{%
  \the\numexpr(\expandafter`\romannumeral-`\x#1-`a+1)\relax
}
\newcommand*{\inversAlph}[1]{%
  \the\numexpr(\expandafter`\romannumeral-`\x#1-`A+1)\relax
}
\begin{document}
  a: \inversalph{a},
  b: \inversalph{b},
  k: \inversalph{k},
  z: \inversalph{z}

  A: \inversAlph{A},
  B: \inversAlph{B},
  K: \inversAlph{K},
  Z: \inversAlph{Z}

  \def\lettera{a}
  \def\letterz{z}
  \def\letteraa{\lettera}
  \def\letterzz{\letterz}
  \lettera: \inversalph{\lettera} = \inversalph{\letteraa},
  \letterz: \inversalph{\letterz} = \inversalph{\letterzz}

   Page: \inversalph{\alph{page}}
\end{document}

Remarks:

• \numexpr is used for the calculations.
• \romannumeral-`\x is a trick to expand the following token multiple times.

A quick solution with \int_from_alph:n or \int_from_roman:n from expl3. Note that a and A does not matter here, as well as uppercase and lowercase roman figures are identical here.

The \setcounter{section}{\alphtonumber{w}} example was used to show that the macros are expandable.

\documentclass{article}


\usepackage{xparse}

\ExplSyntaxOn
\newcommand{\alphtonumber}[1]{%
  \int_from_alph:n{#1}
}


\newcommand{\romantonumber}[1]{%
  \int_from_roman:n{#1}
}

\ExplSyntaxOff

\usepackage{pgffor}


\begin{document}


\alphtonumber{a} -- \alphtonumber{z}

And \alphtonumber{M}\ is a pretty number and \romantonumber{MMXVI} is a good year (more or less), as well as \romantonumber{mdcccclxxiv} was a good year. 


\setcounter{section}{\alphtonumber{w}}

\section{Foo}

\end{document}

The number is always stored in the counter while \alph and the like are only for displaying the value.

For example if you define

\newcounter{foo}
\renewcommand{\thefoo}{\alph{foo}}

with

\thefoo

you get a, b, c, …

but with

\value{foo}

you can always access the integer value of the counter.