Simple way to multiply two values

Question

I have a macro to define the scale of my figures:

\newcommand{\myscale}{0.35}

Now I have a figure that needs to be scaled twice as big, so I tried to use something like this:

\begin{figure}[hbt]
  \includegraphics[scale=2\myscale]{my-image}
  \caption{My Caption}
  \label{fig:my-label}
\end{figure}

This results a very big image, as 2\scale expands to 20.35...

I tried/found:

• Using scale=2*\myscale, but still get errors
• Some more things using calc, again did not work
• \real{}, but it does not seem to work also
• intcalc, but this only seems to work for integers, which is not always the case for \myscale.
• pgf package, but it seems a complex for a simple multiplication
• fp package works, but still somewhat complex

The easiest way (that works for me), is this fp-based implementation:

\begin{figure}[hbt]
  \FPeval\calculatedScale{2*\myscale}
  \includegraphics[scale=\calculatedScale]{my-image}
  \caption{My Caption}
  \label{fig:my-label}
\end{figure}

Am I missing something? Is there a more simple/elegant solution? Preferably directly usable with the \includegraphics command without creating the additional \calculatedScale.

Answer 1

The l3fp package allows expandable calculations with floating point numbers. Expandability means that you don't need to store the result in a temporary variable.

\documentclass{article}
\usepackage{graphicx} % 

%%%%
% Provide the command \fpeval as a copy of the code-level \fp_eval:n.
\usepackage{expl3}[2012-07-08]
\ExplSyntaxOn
\cs_new_eq:NN \fpeval \fp_eval:n
\ExplSyntaxOff
%%%%

\begin{document}
\newcommand{\myscale}{0.53}
\includegraphics[scale=\fpeval{2*\myscale}]{your-image}
\end{document}

Answer 2

Maybe simply (second version is with the scaling):

\documentclass{article}
\usepackage{graphicx}


\begin{document}

\includegraphics[scale=0.2]{it}

\scalebox{2}{\includegraphics[scale=0.2]{it}

\end{document}

Answer 3

Bruno's answer above was super helpful, thanks for that. I didn't even know about this facility in latex3 and was trying to use pgfmath operations which was not working.

Here is an overleaf project https://www.overleaf.com/read/kbbhfkpywfnp that has sample of making this table chart

The latex for this is

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{xcolor}
% makes a table with bars that scale to desired maximum percent or degrees values
% based on https://tex.stackexchange.com/questions/115558/simple-way-to-multiply-two-values
%%%%
% Provide the command \fpeval as a copy of the code-level \fp_eval:n.
\usepackage{expl3}
\ExplSyntaxOn
\cs_new_eq:NN \fpeval \fp_eval:n
\ExplSyntaxOff
%%%%
\begin{document}
\pagestyle{empty}
\newcommand{\barwidth}{10} % cm
\newcommand{\percentscale}{100} % max scale for percent bars
\newcommand{\degscale}{90} % max scale for degree bars
\def\pcbar#1{%%
  #1s% & {\color{red}\rule{\fpeval{#1/\percentscale\barwidth} cm}{8pt}}%
}
\def\degbar#1{%%
  #1s$^\circ$ & {\color{red}\rule{\fpeval{#1/\degscale\barwidth} cm}{8pt}}%
}
\begin{table*}
\begin{tabular}{rl}
 \pcbar{90}\
 \pcbar{60}\
 \pcbar{30}\
\degbar{10}\
\degbar{50}\
\degbar{89}\
\end{tabular}
\end{table*}
\end{document}