Array functionality in TeX?

Question

I've been using LaTeX for a while, but only recently I made myself a mission to create a class. I needed to learn the details of TeX, and I couldn't find answers to some of my questions. One of them is:

How can I implement a dynamically allocated array in TeX? One of the environments I defined takes a single parameter such as {1234}. I need to split this into characters, which can be done this way. Then I need to list the numbers such that array=[1,2,3,4].

Then I want to loop the elements of this array and place a single-row tabular environment with i columns for each i in the array.

To make things clearer, here's is my purpose. One will define a choices environment such that:

\begin{choices}{32}
\choice ch1
\choice ch2
\choice ch3
\choice ch4
\choice ch5
\end{choices}

is going to create multiple choices in two rows, 3 for the first and 2 for the second (3+2=5):

A) ch1  B) ch2  C) ch3
    D) ch4  E) ch5

This notation is going to help create more flexible choices, with a maximum number of 9 choices per line.

Answer 1

For small-scale arrays, the usual solution is to use a \csname-based approach

\expandafter\def\csname my@array@1\endcsname{content-for-key-1}

which can then be read back just using

\csname my@array@1\endcsname

Depending on the use case, you may of course want to have the index as a LaTeX counter or TeX count

\csname my@array@\the\my@array@index\endcsname

The problem with this is that TeX's hash algorithm is not great, so with large arrays performance is poor. This can be solved by 'packing' the array into a single macro, as is done for example by the prop module of expl3.