How to align decimal points when using cases statement

Question

I have a statement like

\begin{equations*}
\begin{cases}
{\zeta}_{A} = -10.1234567 \cdots, \\
{\zeta}_{B} = 1{,}001.2345678 \cdots.
\end{cases}
\end{equations*}

How do I align the decimal points with the equations being left justified and the case spacing remaining unaffected.

Answer 1

A solution that uses an array environment; note that using a cases environment seems like overkill for the case (pun intended) at hand.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\left\{
\begin{array}{@{} r @{{}={}} r}
\zeta_A & -10.1234567\cdots,\\
\zeta_B & 1{,}001.2345678\cdots.\\
\end{array} \right.
\end{equation*}
\end{document}

---

Addendum: The preceding solution assumes that both numbers have the same number of digits following the decimal marker. If this assumption isn't valid, the following modified solution, which still uses an array environment but now employs three rather than just two columns, should work for you.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
\left\{
\begin{array}{@{} r @{{}={}} r @{.} l}
\zeta_A     &     -10 & 1234\cdots,\\
\zeta_{BCD} & 1{,}001 & 12345678901 \cdots.\\
\end{array} \right.
\end{equation*}
\end{document}

Answer 2

Some \phantoms can be used to push content into the appropriate location. Overlapping allows also for accommodating for the use of \phantoms.

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{align*}
  A &= \begin{cases}
    \zeta_A = -10.1234567 \cdots, \\
    \zeta_B = 1{,}001.2345678 \cdots.
  \end{cases} \\
  B &= \begin{cases}
    \rlap{$\zeta_{A}$}\phantom{\zeta_B} = \phantom{1{,}001}\llap{$-10$}.1234567 \cdots, \\
    \zeta_B = 1{,}001.2345678 \cdots.
  \end{cases}
\end{align*}

\end{document}

Answer 3

With siunitx and its powerful features for printing numbers.

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

\newenvironment{xcases}[2][]
 {\left\lbrace\def\arraystretch{1.2}% like cases does
  \begin{array}{
    @{} r @{}
    >{{}}c<{{}} @{}
    S[table-format=#2,table-space-text-post={#1}] @{}
  }}
 {\end{array}\right.}

\begin{document}

\begin{equation*}
\begin{xcases}[$\,\cdots$,]{4.7}
\zeta_A &=& -10.1234567$\,\cdots$, \\
\zeta_B &=& 1001.2345678$\,\cdots$.
\end{xcases}
\end{equation*}

\begin{equation*}
\begin{xcases}{-4.7}
\zeta_A &=& -1001.1234567 \\
\zeta_B &=&  1001.2345678
\end{xcases}
\end{equation*}

\end{document}

The mandatory argument is the number format; in the first case four digits for the integral part and seven for the fractional part. Also some text can be added, the optional argument is a literal text that specifies the allotted space. In the second case, four digits and the sign for the integral part.

Answer 4

Use of tabular with @{.} skill as an alternative. Further Mico's kind reminder: use of array.

Code

\documentclass[]{article} 

\usepackage{amsmath,array}


\begin{document}
Given

\[
\begin{cases}
{\zeta}_{A} = -10.1234567 \cdots, \\
{\zeta}_{B} = 1{,}001.2345678 \cdots.
\end{cases}
\]

Suggested
\[
\begin{cases}
\begin{tabular}{ >{$}r<{$} @{ = } >{$} r <{$} @{.} >{$}l<{$}}
{\zeta}_{A} &  -10    & 1234567 \cdots, \\
{\zeta}_{B} & 1{,}001 & 2345678 \cdots.
\end{tabular}
\end{cases}
\]

@Mico's credits

\[
\begin{cases}
\begin{array}{ r @{ = }  r  @{.} l}
{\zeta}_{A} &  -10     & 1234567 \cdots, \\
{\zeta}_{B} & 1{,}001&2345678 \cdots.
\end{array}
\end{cases}
\]

\end{document}

Answer 5

Here I use the tabstackengine package. The OP's example can be done with a \tabbedCenterstack, though I then show a slightly more difficult version that requires a \tabularCenterstack.

\documentclass{article}
\usepackage{amsmath,tabstackengine}
\setstackEOL{\#}% end-of-line character in stack
\setstackTAB{\&}% end-of-field character in stack
\setstackgap{L}{16pt}% Inter-baseline gap in stack
\setstacktabulargap{0pt}% Inter-field gap in tabularstack
\stackMath
\begin{document}
\begin{equation*}
\begin{cases}
\tabbedCenterstack[r]{\zeta_{A} =\& -10.\&1234567 \cdots,\#
                      \zeta_{B} =\& 1,001.\&2345678 \cdots.}%
\end{cases}
\end{equation*}
\begin{equation*}
\begin{cases}
\tabularCenterstack{rrl}{\zeta_{AC} =\& -10.\&1234 \cdots,\#
                      \zeta_{B} =\& 1,001.\&2345678 \cdots.}%
\end{cases}
\end{equation*}
\end{document}