# Increment (or decrement) a parameter in LaTeX

1. Nov 14, 2009

### 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

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?

2. Nov 16, 2009

### 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