rcases 配有 parbox 以适应剩余空间

Question

我有以下构造（使用包mathtools）

\begin{drcases}
    Content....
\end{drcases} \parbox{.53\textwidth}{Long Multiline Text Blablabla}

我用cases它来显示一些长文本，我发现通过将文本放入其中，\parbox我可以将其分布在多行中。但我最终手动调整了文本宽度（.53\textwidth）

而且因为我的文档中多次出现这种构造，所以我想知道是否可以自动计算文本宽度，这样我就不需要每次手动执行此操作。

Answer 1

的一个应用程序（查看2019-03-05 之前xparse版本的编辑历史）xparse

\documentclass{article}
\usepackage{xparse,mathtools}
\usepackage{lipsum}

\ExplSyntaxOn
\NewDocumentEnvironment{xdrcases}{O{4em}mb}
 {
  \axmx_xdrcases:nnn { #1 } { #2 } { #3 }
 }
 {}

\box_new:N \l_axmx_xdrcases_box
\dim_new:N \l_axmx_xdrcases_dim

\cs_new_protected:Npn \axmx_xdrcases:nnn #1 #2 #3
 {
  \hbox_set:Nn \l_axmx_xdrcases_box
   {
    $\begin{drcases}#3\end{drcases}$
   }
  \dim_set:Nn \l_axmx_xdrcases_dim
   {
    \displaywidth - \box_wd:N \l_axmx_xdrcases_box - #1
   }
  \box_use:N \l_axmx_xdrcases_box
  \quad
  \parbox{\l_axmx_xdrcases_dim}{#2}
 }
\ExplSyntaxOff

\begin{document}
\renewcommand{\theequation}{1.1.\arabic{equation}} % just to show the problem

\lipsum*[2]
\begin{equation*}
\begin{xdrcases}{
  Some long descriptive text, very boring indeed, but
  that should be long enough to require splitting it
  across lines
}
\frac{1}{2}x  & \text{if $x$ is rational} \\
-x & \text{if $x$ is irrational}
\end{xdrcases}
\end{equation*}
\lipsum*[2]
\begin{equation}
\begin{xdrcases}[6em]{
  Some long descriptive text, very boring indeed, but
  that should be long enough to require splitting it
  across lines
}
\frac{1}{2}x  & \text{if $x$ is rational} \\
-x & \text{if $x$ is irrational}
\end{xdrcases}
\end{equation}
\lipsum[4]

\end{document}

可选参数（默认 4em）设置留空的空间；如您所见，对于较长的方程式数字，4em 可能太小，因此可以采取相应的措施。

您可能需要检查以下变体，其中要放在侧面的文本位于宏之后\sidecondition；如果\sidecondition没有出现，则环境与相同drcases。

\documentclass{article}
\usepackage{xparse,mathtools}
\usepackage{lipsum}

\ExplSyntaxOn
\NewDocumentEnvironment{xdrcases}{O{4em}b}
 {
  \axmx_xdrcases:nn { #1 } { #2 }
 }
 {}

\box_new:N \l_axmx_xdrcases_box
\dim_new:N \l_axmx_xdrcases_dim
\seq_new:N \l_axmx_xdrcases_seq

\cs_new_protected:Npn \axmx_xdrcases:nn #1 #2
 {
  \seq_set_split:Nnn \l_axmx_xdrcases_seq { \sidecondition } { #2 }
  \int_compare:nTF { \seq_count:N \l_axmx_xdrcases_seq > 1 }
   {% there is a side condition
    \hbox_set:Nn \l_axmx_xdrcases_box
     {
      $\begin{drcases}\seq_item:Nn \l_axmx_xdrcases_seq { 1 }\end{drcases}$
     }
    \dim_set:Nn \l_axmx_xdrcases_dim
     {
      \displaywidth - \box_wd:N \l_axmx_xdrcases_box - #1
     }
    \box_use:N \l_axmx_xdrcases_box
    \quad
    \parbox{\l_axmx_xdrcases_dim}{\seq_item:Nn \l_axmx_xdrcases_seq { 2 }}
   }
   {% no side condition
    \begin{drcases}#2\end{drcases}
   }
 }
\ExplSyntaxOff

\begin{document}
\renewcommand{\theequation}{1.1.\arabic{equation}} % just to show the problem

\lipsum*[2]
\begin{equation*}
\begin{xdrcases}
  \frac{1}{2}x  & \text{if $x$ is rational} \\
  -x & \text{if $x$ is irrational}
\sidecondition
  Some long descriptive text, very boring indeed, but
  that should be long enough to require splitting it
  across lines
\end{xdrcases}
\end{equation*}
\lipsum*[2]
\begin{equation}
\begin{xdrcases}[6em]
  \frac{1}{2}x  & \text{if $x$ is rational} \\
  -x & \text{if $x$ is irrational}
\sidecondition
  Some long descriptive text, very boring indeed, but
  that should be long enough to require splitting it
  across lines
\end{xdrcases}
\end{equation}
\lipsum*[4]
\begin{equation*}
\begin{xdrcases}
  \frac{1}{2}x  & \text{if $x$ is rational} \\
  -x & \text{if $x$ is irrational}
\end{xdrcases}
\end{equation*}
\lipsum[3]

\end{document}