Creating variables with index (using counters)

Question

Is there a way to add an index to a command? For example:

\newcounter{count}
\loop<condition>
  \def\var\Roman{count}{some data}
\repeat

The \var\Roman{count} should become the variable \varI, \varII, \varIII and so on.

Background: I need to get some data from external files and store it into variables (which I will later use to create a document). Normally every \varI should be the same as every \varII, but I need to check whether it really is the case or not. If not I should return a warning or error.

---

Solution for the first problem is in the comments, but I have a second problem.

Suppose I wrote this:

\newcommand\addindex[3]{  % #1 as name of var, #2 as index and #3 as content
  \expandafter\def\csname #1\Roman{#2}\endcsname{#3}  % thanks to egreg
}

\newcounter{count}
\loop\unless\ifnum\value{count}=5
  \stepcounter{count}
  \addindex{var}{count}{\arabic{count}}
\repeat

All \var<index> have now as value 5, because they are defined as \arabic{count}, and not their value at the moment. Adding \expandafter before \arabic{count} does not seem to work for some reason...

Answer 1

The method is quite simple, but I add a trick that can be useful in your application:

\newcounter{tempcount} % for temporary usage

\newcommand\addindex[4][]{% #2 as name of var, #3 as index and #4 as content
  \csname#1def\expandafter\endcsname\csname #2\Roman{#3}\endcsname{#4}%
}

Now we want to define \varI, \varII, \varIII and \varIV to expand to some text:

\setcounter{tempcount}{0}

\loop\ifnum\value{tempcount}<4
  \stepcounter{tempcount}
  \addindex{var}{tempcount}{some text}
\repeat

If instead you want that \varI expands to 1 and so on, here's the role of the optional argument:

\setcounter{tempcount}{0}

\loop\ifnum\value{tempcount}<4
  \stepcounter{tempcount}
  \addindex[e]{var}{tempcount}{\arabic{tempcount}}
\repeat

By passing e as the optional argument, what's executed is \csname edef\endcsname, that is \edef that expands completely the replacement text before doing the definition. With no optional argument only \def is used.

A more efficient definition would be of \vari, \varii and so on (with lowercase roman numbers):

\newcommand\addindex[4][]{% #2 as name of var, #3 as index and #4 as content
  \csname#1def\expandafter\endcsname\csname #2\roman{#3}\endcsname{#4}%
}

It's more efficient because \Roman{tempcount} has to scan each character in the expansion of \roman{tempcount} and change it to its uppercase counterpart, while \roman{tempcount} expands to \romannumeral\c@tempcount using the TeX primitive \romannumeral, which is quite fast.