11

I'm trying to use the cases environment to show an arbitrary equation. But I noticed that it only allows me to use 2 columns. However, I would require a third column, or perhaps a fourth as well.

An example is shown below:

\begin{equation*}
  \Md_1 \Cup \Md_2=
  \begin{cases}
        \Md_2            & when \ \Md_1 = \nothing \\
        \Md_1            & when \ \Md_2 = \nothing \\
        \Md_1 \cup \Md_2 & otherwise
  \end{cases}
\end{equation*}

What I would like to write is something like this (this does not actually work):

\begin{equation*}
  \Md_1 \Cup \Md_2=
  \begin{cases}
        \Md_2            & when \ \Md_1 = \nothing & where & x = foo    \\
                         &                         &       & y = bar    \\
        \Md_1            & when \ \Md_2 = \nothing & where & z = notfoo \\
        \Md_1 \cup \Md_2 & otherwise               &       &            \\
  \end{cases}
\end{equation*}
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3
  • Make your own either using a left right construction plus array or left right plus the aligned env
    – daleif
    Mar 25, 2015 at 20:48
  • For text inside math mode, you should use \text.
    – Werner ♦
    Mar 25, 2015 at 20:48
  • daleif, could you please elaborate?
    – Christophe De Troyer
    Mar 25, 2015 at 20:52

3 Answers 3

Reset to default
6

I would just use a \phantom to yield a similar alignment. Alternatively, make up your own array with the alignment you require:

enter image description here

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
  M_1 \cup M_2 =
  \begin{cases}
        M_2            & \text{when $M_1 = foo$} \\
                       & \text{\phantom{when }$M_2 = bar$} \\
        M_1            & \text{when $M_2 = baz$} \\
        M_1 \cup M_2   & \text{otherwise}
  \end{cases}
\end{equation*}

\begin{equation*}
  M_1 \cup M_2 =
  \setlength{\arraycolsep}{0pt}
  \renewcommand{\arraystretch}{1.2}
  \left\{\begin{array}{l @{\quad} l r l}
        M_2            & \text{when } & M_1 &{}= foo \\
                       &              & M_2 &{}= bar \\
        M_1            & \text{when } & M_2 &{}= baz \\
        M_1 \cup M_2   & \text{otherwise}
  \end{array}\right.
\end{equation*}

\end{document}

The former may not work or be easy in general, but it works in your case. The latter should work in a more general setting.

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3
  • How would I add the two extra columns (where x = foo)?
    – Christophe De Troyer
    Mar 25, 2015 at 21:00
  • 1
    @ChristopheDeTroyer: You would add more elements in the array column specification. For example, l @{\quad} l r l r l would add another two columns - first is right-aligned and the second is left-aligned.
    – Werner ♦
    Mar 25, 2015 at 21:01
  • Thanks. I want to mark both answer as they are both correct but this one seems a little less messy to me (the latter). Thank you!
    – Christophe De Troyer
    Mar 25, 2015 at 21:14
7

Using only amsmath package, the environement aligned allows you to use as many columns as you want :

\documentclass{article}
\usepackage{amsmath}

\begin{document}
    \begin{equation*}
        M_1 \cup M_2 =
            \begin{cases}
            \begin{aligned}
                M_2 & \text{ when } M_1 = 0 &\text{where } & x = f\\
                    &                       &              & y = b\\
                M_1 & \text{ when } M_2 = 0 &\text{where } & z = n\\
                M_1 \cup M_2 & \text{ otherwise }&         &      \\
            \end{aligned}
        \end{cases}
    \end{equation*}
\end{document}

result using amsmath aligned environment

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1
  • You could use aligned for just the "where" clauses
    – Andrew Swann
    Nov 21, 2017 at 11:37
6

I probably would find a different way to express my ideas. Anyway, here's a possibility:

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

\makeatletter
\newenvironment{mcases}[1][l]
 {\let\@ifnextchar\new@ifnextchar
  \left\lbrace
  \def\arraystretch{1.2}%
  \array{@{}l@{\quad}#1@{}}}
 {\endarray\right.}
\makeatother

\begin{document}

\begin{equation*}
M_1 \Cup M_2=
\begin{mcases}[ll@{\ }l]
  M_2          & \text{when $M_1 = \emptyset$} & \text{where} & x = f \\
               &                               &              & y = b \\
  M_1          & \text{when $M_2 = \emptyset$} & \text{where} & z = n \\
  M_1 \cup M_2 & \multicolumn{3}{@{}l}{\text{otherwise}}
\end{mcases}
\end{equation*}

\end{document}

Without an optional argument, mcases is the same as cases. The optional argument should specify the column types after the first, which is fixed.

enter image description here

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