define variables using calculations

Question

How can I define a variable, which is calculated from different variables, something like this:

\newcommand \varA {3}
\newcommand \varB {2}
\newcommand \varC {\varB*\varA}

So that \varC will conain 6 and not 3*2. This is important, since \varC will be used as a variable in text and in a pgfplot.

Use \pgfmathparse. Have a look at https://tex.stackexchange.com/questions/337831/pgfmathparse-basic-usage — Raven, Nov 24 '20 at 09:16
I don't think so. Because I need the variable also in outside of a pgfplot. And it doesn't seem to work there — flobue, Nov 24 '20 at 09:23

score 3 · Accepted Answer · answered Nov 24 '20 at 09:45

If your aim is to use floating point numbers, pgfmath is not good. Look at the following example.

\documentclass{article}
\usepackage{xfp}
\usepackage{pgfmath}
\ExplSyntaxOn
\NewDocumentCommand{\setvar}{mm}
 {
  \fp_zero_new:c { l_flobue_var_#1_fp }
  \fp_set:cn { l_flobue_var_#1_fp } { #2 }
 }
\NewExpandableDocumentCommand{\usevar}{m}
 {
  \fp_use:c { l_flobue_var_#1_fp }
 }
\ExplSyntaxOff
\begin{document}
\section{PGF}
\pgfmathsetmacro\varA{123.45}
\pgfmathsetmacro\varB{0.012345}
\pgfmathsetmacro\varC{\varA/\varB}
\verb|\varA| is \varA \
\verb|\varB| is \varB \
\verb|\varC| is \varC
\section{xfp}
\setvar{A}{123.45}
\setvar{B}{0.012345}
\setvar{C}{\usevar{A}/\usevar{B}}
\verb|A| is \usevar{A} \
\verb|B| is \usevar{B} \
\verb|C| is \usevar{C}
\setvar{B}{0.0012345}
\setvar{C}{\usevar{A}/\usevar{B}}
\noindent
Now \verb|C| is \usevar{C}; try with pgfmath
\end{document}

There are several advantages in the xfp approach with \setvar and \usevar:

greater accuracy
larger range
no \pgfmathsetmacro that does no check about the command being defined
greater flexibility

The fourth point is worth an example: if you need to use the result of the previous division, you can simply do

\fpeval{\usevar{A}/\usevar{B}}

and don't need to set another variable for getting the result expandably.

define variables using calculations

1 Answers1