Aligning numbers in an array

Question

I need to typeset column vectors containing two parameters (variables) and an absolute value. All coefficients can be either integer or decimal numbers. I would like to achieve that all components are nicely aligned (numbers at decimal point if any, variables and operators).

The MWE

\documentclass{article}

\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{siunitx}

\begin{document}

\section{Not aligned}

\subsection{Integer coefficients}

\begin{math}
\begin{pmatrix}
2 - 2 \, s + t \\
28 - 3 \, t \\
-14 - 3 \, s + 4 \, t
\end{pmatrix}
\end{math}

\subsection{Decimal coefficients}

\begin{math}
\begin{pmatrix}
2 - \num{2,3} \, s + t \\
28 - \num{3,1} \, t \\
\num{-14,75} - 3 \, s + 4 \, t
\end{pmatrix}
\end{math}

\section{Manually aligned}

\subsection{Integer coefficients}

\begin{math}
\left(
\begin{array}{rcrcr}
2 & - & 2 \, s & + & t \\
28 & & & - & 3 \, t \\
-14 & - & 3 \, s & + & 4 \, t
\end{array}
\right)
\end{math}

\subsection{Decimal coefficients}

\begin{math}
\left(
\begin{array}{rcrcr}
2 & - & \num{2,3} \, s & + & t \\
28 & & & - & \num{3,1} \, t \\
\num{-14,75} & - & 3 \, s & + & 4 \, t
\end{array}
\right)
\end{math}

\end{document}

produces the following result.

In both examples in section 2, the spacing before and after the operator is far too much. In section 2.2, the numbers are not aligned at their decimal points.

(1) How can I fix these issues?

(2) Is there any package that does the job automatically without the need to manually place & characters?

Answer 1

The column specifications of environment array are not limited to c, l, or r:

\documentclass{article}

\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{siunitx}

\begin{document}

\begin{math}
\left(
\begin{array}{
  @{\,}
  S[table-format=-2.2]
  @{\>}c@{\>}
  S[table-format=1.1]
  @{\,}c@{\>}c@{\>}
  S[table-format=1.1]
  @{\,}c
  @{\,}
}
2 & - & 2,3 & s & + & & t \\
28 & & & & - & 3,1 & t \\
-14,75 & - & 3 & s & + & 4 & t
\end{array}
\right)
\end{math}

\end{document}

However, I have the suspicion, that the vertical alignments are a little overkill and a simple non-aligned version suffices.

Answer 2

If you don't mind decimal coefficients not being aligned on the decimal comma, the systeme package gives you an easy and short code:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{amsmath}
\usepackage{siunitx}
\usepackage{systeme}

\begin{document}

\subsection*{Integer coefficients}

\begin{math}
\sysdelim()\systeme{2-2s + t, 28-3t, -14-3s + 4t}
\end{math}

\subsection*{Decimal coefficients}

\begin{math}
\sisetup{ copy-decimal-marker}
\sysdelim()\systeme{\num{2} - \num{2,3}s + t,\num{28} - \num{3,1}t, -\num{14,75} - 3s + 4t}
\end{math}

\end{document} 

Answer 3

Two more array-based solutions: the first with simple right-alignment of the numbers, the second with decimal-alignment of the numbers.

\documentclass{article}
\usepackage{array}
\newcolumntype{C}{>{{}}c<{{}}}
\usepackage{siunitx}
\newcolumntype{T}[1]{S[table-format=#1]}
\begin{document}
\[
\setlength\arraycolsep{0pt}
\left(\begin{array}{r C r C r}
  2    & - & 2.3s & + &     t \\
 28    &   &      & - & 3.1 t \\
-14.75 & - &   3s & + & 4   t
\end{array}\right)
\]

\[
\setlength\arraycolsep{0pt}
\left(\begin{array}{T{-2.2} C T{1.1}r C T{1.1}r}
  2    & - & 2.3 & s & + &     & t \\
 28    &   &     &   & - & 3.1 & t \\
-14.75 & - & 3   & s & + & 4   & t
\end{array}\right)
\]
\end{document}