Unit column in table with square brackets and siunitx

Question

The header might be a bit weird. But I would like to add a specified column type that is reserved to units.

For instance, take a look at the following working example:

\documentclass[10pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{array}
    \newcolumntype{M}{>{$} l <{$}}

\begin{document}

\begin{align}
R &= \sqrt{Q\: \frac{t}{\pi\: b\: n_{eff}}}
\end{align}

Where:\\

\begin{tabular}{M |l l}
R & Radius for BNBO & $\left[\si{\meter}\right]$ \\
Q & Extraction rate & $\left[\si{\cubic\meter\per\second}\right]$ \\
t & Time frame for BNBO & $\left[\si{\second}\right]$ \\
b & Aquifer thickness & $\left[\si{\meter}\right]$ \\
n_{eff} & Effective porosity & $\left[\si{--}\right]$ \\
\end{tabular}
\end{document}

This should return this:

Now, what I would like to do, is to be free of writing all the stuff before and after the unit. This means that instead of writing

$\left[\si{\cubic\meter\per\second}\right]$

I would simply just write

\cubic\meter\per\second

as the only thing for the unit description of Q.

As you can see, I have already defined a column with respect to math, the column M. I have tried to do it in a similar way with units, which returned an error. The code I tried was

\newcolumntype{U}{>{$\left[\si{ l <}\right]$}}

Inserting U into the third column and deleting math and siunitx environment, like this

\begin{tabular}{M |l U}
    R & Radius for BNBO & \meter \\
    Q & Extraction rate & \cubic\meter\per\second \\
    t & Time frame for BNBO & \si{\second \\
    b & Aquifer thickness & meter \\
    n_{eff} & Effective porosity & - \\
\end{tabular}

the following error occurs:

Package array Error: >{..} at wrong position: token ignored. \begin{tabular}{M |l U}

I can't really figure it out. Please help me!

Answer 1

1. Please unlearn your habit of using \left ... \right all over

2. Drop the [], they do nothing to the interpretation of that column

3. use the s column, and in that column just write say \meter

In code

\documentclass[10pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{array,booktabs}
\newcolumntype{M}{>{$} c <{$}}
\newcommand\mc[1]{\multicolumn{1}{c}{#1}}
\begin{document}

\begin{tabular}{M l s } %|
\toprule
\mc{Variable} & Meaning & \mc{Unit} \\
\midrule
R & Radius for BNBO & \meter\\
Q & Extraction rate & \cubic\meter\per\second\\
t & Time frame for BNBO & \second\\
b & Aquifer thickness & \meter\\
n_{\textup{eff}} & Effective porosity & \mc{---} \\
\bottomrule
\end{tabular}
\end{document}

Answer 2

This can be done with the help of the collcell package (loads array).

Just define your column as

\newcolumntype{U}{>{$[\collectcell\si} l <{\endcollectcell]$}}

and you're done.

MWE

\documentclass[10pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{siunitx}
\usepackage{collcell} % loads array
    \newcolumntype{M}{>{$} l <{$}}
    \newcolumntype{U}{>{$[\collectcell\si} l <{\endcollectcell]$}}

\begin{document}

\begin{equation}
R = \sqrt{Q\: \frac{t}{\pi\: b\: n_{eff}}}
\end{equation}

Where:\bigskip

\begin{tabular}{M |l U}
    R & Radius for BNBO & \meter\\
    Q & Extraction rate & \cubic\meter\per\second\\
    t & Time frame for BNBO & \second\\
    b & Aquifer thickness & \meter\\
    n_{\textit{eff}} & Effective porosity & -\\
\end{tabular}
\end{document}

Output

Notice that the right way to add some spacing is not \\ but something like \bigskip. Also, usign align when there is nothing to align is not the best choice, use equation instead.

If you really want, but I would avoid it, you can even add \left and \right

\newcolumntype{U}{>{$\left[\collectcell\si} l <{\endcollectcell\right]$}}

Answer 3

You add no meaning with the units in brackets; in my opinion it's even wrong, as usually brackets denote abstract dimensions, such as “length·time^–1” or “force·length”.

The s column type is what you're looking for. I also removed all \: spacing commands, which are wrong, and set “eff” in upright type.

Note also that Where:\bigskip would allow a page break after it.

\documentclass[10pt,a4paper]{report}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage{siunitx}

\newcolumntype{M}{>{$} l <{$}}
\newcommand{\eff}{\textrm{eff}}

\begin{document}

\begin{equation}
R = \sqrt{Q \frac{t}{\pi b n_{\eff}}}
\end{equation}
Where:\\*[\bigskipamount]
\begin{tabular}{M l s}
R & Radius for BNBO & \meter\\
Q & Extraction rate & \cubic\meter\per\second\\
t & Time frame for BNBO & \second\\
b & Aquifer thickness & \meter\\
n_{\eff} & Effective porosity & \multicolumn{1}{c}{--}\\
\end{tabular}
\end{document}

Answer 4

To my test is more appropriate for explanation of variables of some equation to use list environment than table. For example something as follows:

\documentclass{article}
    \usepackage{amsmath,amsfonts,amssymb}
    \usepackage{siunitx}

\newcommand{\mathentrylabel}[1]{%
     \makebox[\labelwidth][l]{\parbox[t]{\labelwidth}{%
                                         \raggedleft$#1$:}
                               }}
\newenvironment{mathentry}[1]%
    {\begin{list}{}{%
        \renewcommand{\makelabel}{\mathentrylabel}%
        \settowidth{\labelwidth}{\textsf{\quad#1}}%
        \setlength{\leftmargin}{\labelwidth + \labelsep}
        \setlength{\parsep}{0\baselineskip}
    }}%
    {\end{list}}

        \begin{document}
\begin{table}
    \begin{equation}
R = \sqrt{Q \frac{t}{\pi b n_{\text{eff}}} }
    \end{equation}
Where:
    \begin{mathentry}{$n_{\text{eff}}$}
\item[R]                Radius for BNBO     [\si{\meter}]
\item[Q]                Extraction rate     [\si{\cubic\meter\per\second}]
\item[t]                Time frame for BNBO [\si{\second}]
\item[b]                Aquifer thickness   [\si{\meter}]
\item[n_{\text{eff}}]   Effective porosity
    \end{mathentry}
        \end{document}

With this approach the units given in brackets probably seems more natural as in table.