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How can I subdivide in cases with square brackets?

I mean I want to write:

              ⌈ 2_i        if ...
rank (q, l) = | 2_i + 1    if ...
              ⌊ ....

with square brackets, not braces.

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1  
I tried to rewrite your question so that it is more easily understood. I hope I understood it correctly. If not, please change it back. –  Caramdir Feb 26 '11 at 22:43
    
thanks for the correction, the only thing is that I do not know if the tags are corrent –  Antony Feb 26 '11 at 22:49

3 Answers 3

up vote 11 down vote accepted

You could patch the cases environment, the way Thorsten showed or using the etoolbox package and \patchcmd. However, it could be better to preserve the original cases environment and to define a new one for that purpose. Here is a way, very similar to the original definition, but also using \lbrack instead of \lbrace, I calles the new environment sqcases:

\documentclass{article}
\usepackage{amsmath}
\DeclareMathOperator{\rank}{rank}
\makeatletter
\newenvironment{sqcases}{%
  \matrix@check\sqcases\env@sqcases
}{%
  \endarray\right.%
}
\def\env@sqcases{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrack
  \def\arraystretch{1.2}%
  \array{@{}l@{\quad}l@{}}%
}
\makeatother
\begin{document}
\[
  \rank(q, l) = \begin{sqcases}
      2_i & \text{if \ldots} \\
      2_i+1 & \text{if \ldots} 
    \end{sqcases}
\]
\end{document}

enter image description here

Though it looks a bit complicated, it's very straightforward: I took the original cases definition of amsmath.sty, wrote sqcases instead and replaced \lbrace by \lbrack. I had to use \makeatletter and \makeatother because of the @ symbol in the original amsmath commands.

For completeness, heres the way using patching:

\usepackage{etoolbox}
\makeatletter
\patchcmd{\env@cases}{\lbrace}{\lbrack}
\makeatother

After loading etoolbox, the respective internal amsmath macro is changed to use \lbrack instead of \lbrace. It's a bit hazardous to patch internal commands, but it's a quick way and may even work after changes in amsmath while our new definition could become different to cases then. At least it's good to know such methods.

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It only requires a slight modification.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\def\env@cases{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrack
  \def\arraystretch{1.2}%
  \array{@{}l@{\quad}l@{}}%
}
\makeatother

\begin{document}
  \[
    \begin{cases}
      0 \\ 1 \\ 2
    \end{cases}
  \]
\end{document}

This is the original code from amsmath where only \lbrace has been replaced by \lbrack.

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Are you sure? doing so does not work –  Antony Feb 26 '11 at 22:54
    
Works fine for me as is. –  Thorsten Donig Feb 26 '11 at 22:57
1  
@Thorsten: Maybe you could add an explanation so that people who are not well versed with the internals of LaTeX can understand what that does. –  Caramdir Feb 26 '11 at 22:57
    
Did you copy the code from Thorsten's example? It works as advertised. Are you getting an error? –  Alan Munn Feb 26 '11 at 22:58
3  
@Caramdir: I added a short (but hopefully sufficient) explanation. –  Thorsten Donig Feb 26 '11 at 23:02

A Plain "extension" to support such constructs:

\catcode`@=11
\def\caseswithdelim#1#2{\left#1\,\vcenter{\normalbaselines\m@th
  \ialign{\strut$##\hfil$&\quad##\hfil\crcr#2\crcr}}\right.}% you might like it without the \strut
\catcode`@=12
%
\def\bcases#1{\caseswithdelim[{#1}}
\def\vcases#1{\caseswithdelim|{#1}}
%
$$\displaylines{
rank (q,l) = \cases{2_i& if $\ldots$\cr 2_i + 1&if $\ldots$\cr\ldots} \cr
rank (q,l) = \bcases{2_i& if $\ldots$\cr 2_i + 1&if $\ldots$\cr\ldots} \cr
rank (q,l) = \vcases{2_i& if $\ldots$\cr 2_i + 1&if $\ldots$\cr\ldots}
}$$\bye

casesext

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