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When I write units in math mode, I separate them using \, eg: $3 \; kg \, m^2$
I want to create a macro that does this for me, so I used xparse like this:
\usepackage{xparse}
\NewDocumentCommand \units{O{}O{}O{}}{\;#1 \,#2 \,#3}
The problem is that if I only specify one argument, I still get 2 extra spaces because my macro skips the arguments but not the \,.
What I would like is that my macro skips the extra \, commands if I only specify 1 or 2 arguments.
Fundamentally you want to condition on the existence of your optional arguments. In xparse this is done using \IfValueTF with the TF being optional on whether or not you need a True or False branch.
\documentclass{article}
\usepackage{xparse,amsmath}
\NewDocumentCommand\units{ o o o }{%
\IfValueT{#1}{\;#1}%
\IfValueT{#2}{\,#2}%
\IfValueT{#3}{\,#3}%
}
\begin{document}
This pumpkin weighs $3\units[\text{kg}]$. That's heavy!
I can run $12\units[\text{m}][\text{s}^{-1}]$. That's fast!
Something ridiculous $1\units[a][b][c]$. That's ridiculous!
Something $5\units$ something else.
\end{document}
In general, units should be set like regular text.
A far better approach would be to use siunitx:
\documentclass{article}
\usepackage{siunitx}
\DeclareSIUnit{\aaa}{a}
\DeclareSIUnit{\bbb}{b}
\DeclareSIUnit{\ccc}{c}
\begin{document}
This pumpkin weighs $\SI{3}{\kilogram}$. That's heavy!
I can run $\SI{12}{\metre\per\second}$. That's fast!
Something ridiculous $\SI{1}{\aaa \bbb \ccc}$. That's ridiculous!
Something $5$ something else.
\end{document}
xparse, I'll definitely use siunitx from now on though.
Oct 14, 2016 at 9:08
What if you have more than three units? Here units are separated by spaces, then the macro takes care of using thin spaces.
\documentclass{article}
\usepackage{xparse,amsmath}
\NewDocumentCommand\units{>{\SplitList{ }}m}{%
\ProcessList{#1}{\addunit}%
}
\NewDocumentCommand{\addunit}{m}{%
\,\mathrm{#1}%
}
\begin{document}
This pumpkin weighs $3\units{kg}$. That's heavy!
I can run $12\units{m s^{-1}}$. That's fast!
Something ridiculous $1\units{a b c}$. That's ridiculous!
\end{document}
Of course, siunitx is far better.
xparse, but as has been mentioned by pretty much everyone siunitx is better so I'll use that instead.
Oct 14, 2016 at 9:11
siunitxpackage. But, in general to accomplish what you want you can either detect that an empty paramater was given and only add the\,spacing if the paramater is not empty, or switch to using theoorgparamter type (and then use\IfNoValueTFto detect missing parameters).