Cfrgtkky

I have the following construct (using package mathtools)

begin{drcases}

    Content....

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

I am using cases to display some long text and I found out that by putting the text into a parbox I can spread it across mutliple lines. But I ended up with manually adjusting the text width (.53textwidth)

And because I have this kind of construct multiple times in my document, I was wondering if it is not possible to automatically calculate the text width, so that I don't need to do this each time by hand.

An application of xparse (see edit history for versions of xparse prior to 2019-03-05)

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

   {

    $begin{drcases}#3end{drcases}$

   }

  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}

The optional argument (default 4em) sets the space to be left empty; as you see, in case of long equation numbers 4em can be too small, so one can act consequently.

You might want to examine the following variant, where the text to be put on the side comes after the sidecondition macro; if sidecondition doesn't appear, the environment is the same as 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}#2end{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}