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.

 

[oswaler]

To increment or decrement a parameter in LaTeX, you can use the \advance command. This command allows you to perform basic arithmetic operations on a parameter. In your case, you can use it to decrement the first parameter passed to your environment foo.

To do this, you can use the following code within your newenvironment definition:

\advance#1 by -1

This will decrement the value of #1 by 1. So, for example, if you pass the value 4 as the first parameter, #1 will now have a value of 3.

In your specific case, you can use this code to decrement the first parameter and then use the decremented value to set the number of columns in your tabular environment. So, your newenvironment definition would look like this:

\newenvironment{foo}[1]{\advance#1 by -1 \begin{tabular}{*{#1}c*{#1-1}r}}{\end{tabular}}

This will allow you to pass only one parameter (n) and still produce a table with n + (n - 1) columns as in the environment foo.

I hope this helps!