# Create macro using xparse that creates spaces between arguments

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.

• do you really set kg in math italic? – David Carlisle Oct 13 '16 at 14:18
• Why don't you use siunitx? – Ulrike Fischer Oct 13 '16 at 14:45
• Welcome to TeX.SE. For units you should defintely use siunitx package. 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 the o or g paramter type (and then use \IfNoValueTF to detect missing parameters). – Peter Grill Oct 13 '16 at 15:41

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}

• Thank you for the help with xparse, I'll definitely use siunitx from now on though. – Sebastian Oct 14 '16 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}{%
}
\,\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.

• I think I prefer your way of using xparse, but as has been mentioned by pretty much everyone siunitx is better so I'll use that instead. – Sebastian Oct 14 '16 at 9:11