How to make \left\{ and \right. work in an new environment?

Question

I have tried to define a new environment numcases, which just like the cases except tagging the equations as a whole, and

\newenvironment{numcases}{\begin{equation}\left\{\begin{array}{rll}}%
{\end{array}\right.\end{equation}}

also I know the cases package, but it will tag all lines of equations (not as a whole). I guess that the problem in my definition is that the \left\{ and \right. not in a block (i.e., a {}), so any solution?

Is there really a need for it? Why not \begin{equation}\begin{cases} ... \end{cases}\end{equation} which would be clearer? (Needs \usepackage{amsmath}) — egreg, Nov 08 '12 at 09:30
As David pointed out in his answer, the code works just ok. This means that in the present form, there's nothing to answer. — yo', Nov 08 '12 at 10:30

David Carlisle · Accepted Answer · 2017-06-02T21:38:57.120

6

Please always post complete documents that show the problem. There is nothing wrong with your fragment so presumably your problem is in a part you have not shown.

enter image description here

\documentclass{article}
\newenvironment{numcases}{\begin{equation}\left\{\begin{array}{rll}}%
{\end{array}\right.\end{equation}}

\begin{document}

\begin{numcases}
a&1&x\\
b&2&y\\
c&3&z
\end{numcases}

\end{document}

edited Jun 02 '17 at 21:38

answered Nov 08 '12 at 09:20

David Carlisle

757,742

Thanks, you were definitely right, the problem is in other part. My definition works perfectly. – user19832 Nov 08 '12 at 13:19

score 1 · Answer 2 · edited Apr 13 '17 at 12:35

1

The environ package allows you to define such a environment, that first reads the whole body and then puts it into some command. (compare also https://tex.stackexchange.com/a/67849/15616)

In your case this should look something like:

\usepackage{environ}
\NewEnviron{numcases}{%
    \begin{equation}\left\{\begin{array}{rll}%
        \BODY%
    \end{array}\right.\end{equation}}

edited Apr 13 '17 at 12:35

Community

1

answered Nov 08 '12 at 09:19

bodo

6,228

1

It does. but there is nothing wrong with there definition as posted either. – David Carlisle Nov 08 '12 at 09:21
@DavidCarlisle: Ok, just saw your answer. Good to know, I just did not try it myself ;-) – bodo Nov 08 '12 at 09:22
Yes, it quite helpful in some other circumstances. – user19832 Nov 08 '12 at 13:20

How to make \left\{ and \right. work in an new environment?

2 Answers2