How to expand a string macro using a counter?

Question

I try to expand a string using a counter. I will use a very simple example:

\def\st{}
\setcounter{i}{1}
\expandafter\def\expandafter\st\expandafter{\st{ } \the\numexpr\value{i}}
\addtocounter{i}{1}
\texttt{\st}

One would expect the result to be '1'. Yet, the result is '2' because the counter increased after the expansion. So, I undestand that 'st' is always "interpreted" using the final value of 'i'. So, in an iteration of the counter 'i' one would get n n n n ... (where 'n' is the final value of 'i'). But I would like to get 1 2 ... n. How can this be done?

(I spent a lot of time trying to achieve this reault, alas, in vain!)

Answer 1

You need full expansion. Also, you're adding unwanted braces and spaces.

\documentclass{article}

\def\st#1{}
\newcounter{i}
%\renewcommand{\thei}{\arabic{i}} % it's default

\begin{document}

\count255=0
\loop
\ifnum\count255<10
  \advance\count255 1
  \stepcounter{i}
  \edef\st{\st\space\thei}
\repeat

--\st--

\end{document}

At the first iteration, \st will gobble \space.

A different implementation with etoolbox

\documentclass{article}
\usepackage{etoolbox}

\def\st{}
\newcounter{i}
%\renewcommand{\thei}{\arabic{i}} % it's default

\begin{document}

\count255=0
\loop
\ifnum\count255<10
  \advance\count255 1
  \stepcounter{i}
  \eappto\st{\ifdefempty\st{}{ }\thei}
\repeat

--\st--

\end{document}