Increment (or decrement) a parameter in LaTeX

[robbins]

How can I increment or decrement a parameter in LaTeX?

For example, suppose I create an environment foo as follows:

\newenvironment{foo}[2]{\begin{tabular}{*{#1}c*{#2}r}}{\end{tabular}}

I can write

\begin{foo}{4}{3}
<tabular data in seven columns>
\end{foo}

But in my application, if the first parameter is n, then second is always n-1. Is there a way to write this that passes only one parameter (in this case n), but which will produce the table with n + (n - 1) columns as in the environment foo?

Decrementing the parameter #1 inside the newenvironment -- if it is possible -- would surely work, but I don't know how to do it.

Note: The snippet
\let\temp#1
\advance\temp by -1
doesn't work because if I pass the parameter 4, \temp takes the value 'the character 4', which can't follow the \advance command.

I'm stuck. Any help?

 

[robbins]

This solution was posted by Joseph Wright on LaTeX Community:

"You don't want \let, you want to do things with numbers proper. The thing is that they then [need] to be assigned to TeX counters. You seem to want something like

\newcount\mycount
\newenvironment{foo}[1]
{%
\mycount #1\relax
\advance\mycount -1\relax
\begin{tabular}{*{#1}c*{\the\mycount}r}%
}
{\end{tabular}}

Here, I'm using a TeX count register to do the maths. You can do the same with a LaTeX counter, but I find this route a bit easier for going downward. TeX assigns count registers locally, so the above should work in most cases."

Thanks, Joseph.