a/an substitution in LaTeX

Question

I was wondering if there is a conditional command that I can use in LaTeX (e.g., \ifthenelse) that would allow me to, in one case, use the word 'a' and in another use the word 'an' based on the whether or not the following word starts with a vowel.

Answer 1

I agree with Caramdir that checking for a vowel is insufficient for determining which article to use. Nonetheless, if you really want to do this, this is how I will do this in ConTeXt.

\def\placearticle#1%
  {\doifinsetelse{#1}{a,e,i,o,u}{an}{a} #1}

% Alternative definition, but a bit slower
% \def\placearticle#1%
%   {\doifinstringelse{#1}{aeiou}{an}{a} #1}

\starttext
\placearticle apple,
\placearticle orange,
\placearticle banana,
\placearticle strawberry
\stoptext

I don't know which LaTeX package provides functionality similar to \doifinsetelse or \doifinstringelse.

Answer 2

To do this in plain TeX (works in LaTeX too) I would say something like:

\newtoks\vowellist % make a new empty token register named vowellist
\vowellist{\\a\\e\\i\\o\\u} % put vowels in it delimited by \\
                            % Redefining \\ will allow us to map over the list.
\def\placearticle#1{
   \edef\tempa{#1}                % In case #1 is a command, fully expand it.
   \def\tempb#1#2.${\let\tempc#1} % make \tempc the first symbol of tempa
   \expandafter\tempb\tempa.$     %
   \def\\##1{\ifx\tempc##1 n\fi}  % Check if the current vowel is the same as the letter in \tempc
   a\the\vowellist\            % Map \\ over \vowellist 
   \tempa                      % Insert #1; use tempa to make sure #1 isn't evaluated twice.
}

This should work with any reasonably sane input, but I haven't te