How to use siunitx with non numerical values?

Question

I understand that something like \SI{5}{\meter\per\second} defines a certain space between the number (5) and units (m/s). What should I use if instead of a numerical quantity I want a variable: R m/s? \SI{R}{\meter\per\second} fails because it expect a number in first parameter, and R \si{\meter\per\second} or $R$ \si{\meter\per\second} show different space.

Which is the correct use?

\documentclass{standalone}
\usepackage{siunitx}
\sisetup{per-mode=symbol}
\begin{document}
\begin{tabular}{r}
\SI{5}{\meter\per\second}\\
$R$ \si{\meter\per\second}\\
R \si{\meter\per\second}\\
\end{tabular}
\end{document}

Answer 1

As others have commented, this is partly a style question. In terms of non-numerical input, you've got a couple of choices. You could add R as a 'symbol' to those that siunitx knows, but that will not always print in math mode. So I'd prefer the approach of turning off the number parser:

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\SI[parse-numbers = false]{R}{\metre\per\second}
\end{document}

As egreg points out, to get italic output you'll need to set the math-rm option

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\SI[number-math-rm = \mathnormal, parse-numbers = false]{R}{\metre\per\second}
\end{document}

Turning off the parser forces math mode, but retains font control and also consistent spacing (so if you alter the setting for number-unit-separator it will be updated).

Answer 2

I think you are raising two questions: First, how to set the spacing between the "R" and its unit (m/s) and second, whether to typeset the "R" in upright or italic shape.

Regarding the former: I believe what siunitx does in the \SI macro is to insert a thinspace width between the quantity and its unit; one can generate this spacing manually with the \, command. Regarding the latter question: I believe it is (near-)universal practice to typeset variables in italic shape. Hence, I'd recommend you use the following command:

$R$\,\si{meter\per\second}

Answer 3

\NewDocumentCommand{\varSI}{O{}}{\SI[math-rm=\mathnormal,parse-numbers=false,#1]}

\varSI{R}{\meter\per\second}
\varSI[math-rm=\mathbf]{R}{\meter\per\second}

Answer 4

As suggested above, you can declare the following:

\NewDocumentCommand\varSI{omm}{%
  \IfNoValueTF{#1}%
  {\SI{#2}{#3}}%
  {$#1$\,$=$\,\SI{#2}{#3}}%
}

The physical quantity comes as the optional argument and is displayed (if specified) in math mode, whereas the numerical value and the unit come as mandatory arguments. Hence, use wherever in the text \varSI[Quantity]{Value}{Unit with siunitx} or \varSI{Value}{Unit with siunitx}

For example, you can use either \varSI[E_K]{3}{\electronvolt} to display E_K = 3 eV and only \varSI{3}{\electronvolt} to write 3 eV depending on the context.

Answer 5

What you could do for instance is to introduce the \si part into the mathmode part:

\documentclass{standalone}
\usepackage{siunitx}
\sisetup{per-mode=symbol}
\begin{document}
\begin{tabular}{r}
\SI{5}{\meter\per\second}\\
$R\,\si{\meter\per\second}$\\
\end{tabular}
\end{document}

Then you have control of the spacing and you can choose a different spacing than \, if you like.