Arithmetic operation between command arguments and latex parameters

Question

I'm defining a new command that places two images one next to the other and I want to add an horizontal space between the two images which corresponds to one third of the space that remains blank. The command definition reads:

\newcommand{\twofig}[4]{%
\begin{center}\includegraphics[width=#2\columnwidth]{#1.png}%
\hspace{(1-#2-#4)/3 \columnwidth }%
\includegraphics[width=#4\columnwidth]{#3.png}%
\end{center} }

I'm missing what I should use in order to get this operation solved. I've tried with \dimexpr but I always got errors. Is it not the right solution? When should I use \dimexpr and when \numexpr? The etex manual also was not so helpful for me.

Answer 1

as cfr says, no calculation is necessary, but if #2\columnwidth works then #2 must be a factor but \numexpr needs an integer and \dimexpr needs a length so neither can calculate (1-#2-#4)

\makebox[\textwidth]{%
\hfill
\includegraphics{...}%
\hfill
\includegraphics{...}%
\hfill}

should do what you want.

In general you can do

\makebox[\textwidth]{%
\hspace{\stretch{1}%
\includegraphics{...}%
\hspace{\stretch{2}%
\includegraphics{...}%
\hspace{\stretch{3}%
 }

which will stretch the glue in the ratio 1:2:3, the example above is of course equivalent to having each argument of \stretch be 1.