Looking for a more appropriate version of \cases

Question

Often-times I find myself wanting to write something like,

That is, a vertical brace followed by a table consisting of three columns, the first and the last are math, aligned where-ever I like, and the middle one is text, right aligned.

I have now found the solution that I want using tabularx:

\left\{
\begin{tabular}{>{$}r<{{}$}@{}>{${}}l<{$}r>{$}r<{{}$}@{}>{${}}l<{$}}
&\frac{n(x)}{1+j^2F^2} & for &  j&=0\\
2&\frac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))& for & j&>0\\
2&\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & for& j&<0 
\end{tabular}
\right.

(I put the extra braces inside the column definition so that that operators near the alignment character remain relative, which is the usual intention)

My question is two fold really:

1. Is there a package or a command that does this (I think that cases is supposed to but I still need to use \text{} which is annoying)?

2. What is the best way to turn this into an environment so that I can simply write

   \begin{mycases}
   &\frac{n(x)}{1+j^2F^2} & for &  j&=0\\
   2&\frac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))& for & j&>0\\
   2&\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & for& j&<0 
   \end{mycases}
   

Answer 1

Here is a "solution" with the package IEEEtranstool :

\documentclass{minimal}
\usepackage{mathtools}
\usepackage[retainorgcmds]{IEEEtrantools}
\def\FD{\text{FD}}
\begin{document}
\begin{IEEEeqnarray*}{rl"t"rC}
 &\frac{n(x)}{1+j^2F^2} & for &  j&=0\\
 2&\frac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))& for & j&>0\\
 2&\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & for& j&<0
\end{IEEEeqnarray*}
\end{document}

The syntax is yours, but these solution lacks the vertical brace. Maybe a TeX wizard may helps.

For the explanation : rcl for right center left column in math mode, RCL is for math operator (extra-space), stu for text column and " for extra-space

The `retainorgcmds option prevents IEEEtrantools from overriding existing LaTeX commands (itemize, enumerate and description)

See also the documentation : p.17 and more

Answer 2

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}
\def\FD{\text{FD}}
\def\for{\text{for }}
\begin{document}

\[
\begin{cases}
\phantom{2}\dfrac{n(x)}{1+j^2F^2}                           & \for j=0\\[15pt]
         2 \dfrac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))        & \for j>0\\[15pt]
         2 \dfrac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & \for j<0 
\end{cases}
\]

\[\left\{
\begin{matrix*}[l]
   & \dfrac{n(x)}{1+j^2F^2}                           & \for j=0\\[15pt]
 2 & \dfrac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))        & \for j>0\\[15pt]
 2 & \dfrac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & \for j<0 
\end{matrix*}
\right.\]

\end{document}

Answer 3

The Plain version of \cases works pretty much the way you are hoping for:

\documentclass{article}
\begin{document}
$ \cases{
  \enspace{n(x)\over1+j^2F^2a}&
    for\quad$j=0$\cr\noalign{\vskip1\jot}
  2{\cos(jk)\over1+j^2F^2}f_j^{\rm FD}\left(n(x)\right)&
    for\quad$j>0$\cr\noalign{\vskip1\jot}
  2{\sin(jk) jF/\tau\over1+j^2F^2}f_j^{\rm FD}\left(n(x)\right)&
    for\quad$j<0$
}$
\end{document}

In case you'd like to just & away, you could:

\catcode`@=11
\def\cases#1{\left\{\,\vcenter{\normalbaselines\m@th 
\ialign{$##\hfil$&&\quad##\hfil&\quad$##\hfil$\crcr#1\crcr}}\right.}
\catcode`@=12

in the document preamble, which is just the Plain \cases with a repeating horizontal alignment preamble. Like so:

$$ \cases{ math & text & math & text & etc \cr
  1234\alpha & actual numbers, too & \sum435 & there as well \cr
}$$

Of course, you could add another math-alignment tab for the digit if you don't fancy the \enspace/\phantom:

\catcode`@=11
\def\cases#1{\left\{\,\vcenter{\normalbaselines\m@th 
\ialign{$\hfil##$&$##\hfil$&&\quad##\hfil&\quad$##\hfil$\crcr#1\crcr}}\right.}
\catcode`@=12
$\def\rep{1+j^2F^2}\cases{
  &{n(x)\over1+\rep a}&for&j=0\cr
  2&{\cos(jk)\over\rep}f_j^{\rm FD}(n(x))&for&j>0\cr
  2&{\sin(jk)jF/\tau\over\rep}f_j^{\rm FD}(n(x))&for&j<0\cr
}$
\bye

Answer 4

Here is a more elegant syntax, which you can even use in your \newenvironment command:

 \documentclass{article}
 \usepackage{amsmath}
 \def\FD{\text{FD}}
 \newcommand\when[1]{&\text{for #1,}\\}
 \newcommand\otherwise{&\text{otherwise.}}
 \begin{document}
 \[ \begin{cases}
 \phantom{2}\dfrac{n(x)}{1+j^2F^2}                           \when{$j=0$}
     2 \dfrac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))        \when{$j>0$}
     2 \dfrac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) \when{$j<0$} 
    e^{i\pi} +1 \otherwise
 \end{cases} \]
 \end{document}

Answer 5

No magic here:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\newcommand\FD{\mathrm{FD}}
\begin{document}
\[
\begin{dcases}
\phantom{2}\frac{n(x)}{1+j^2F^2} & \text{for } j=0\\
2\frac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))& \text{for } j>0\\
2\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & \text{for } j<0 
\end{dcases}
\]
\end{document}

Answer 6

Another solution, which I believe obeys your specifications, is below. I prefer Herbert's and Yossi Gil's answers.

\documentclass{article}
\usepackage{mathtools}
\begin{document}

\makeatletter
% The main environment takes one argument (column specification, which
% looks like {mttmtM{...}T{...}mM{...}}). It parses the column specification
% to create the preamble of a dcases-like environment, then uses this preamble.
% 
% Uncomment the "\newlinechar..." line to see what the preamble looks like.
%
\newenvironment{customdcases}[1]{%
  \edef\dcases@custom@preamble{%
    \dcases@custom@build{} #1\dcases@custom@qstop}%
  %{\newlinechar`&\show\dcases@custom@preamble}%
  \expandafter\dcases@custom@start\expandafter{\dcases@custom@preamble}%
}{\enddcases}

% To build the preamble, we take letters one by one, and transform them 
% to the corresponding macro \dcases@custom@<letter> defined below. I still
% need to learn how to do this better. \dcases@custom@qstop is just a "quark"
% which expands to itself, useful for \ifx comparison purposes. 
\newcommand*{\dcases@custom@build}[2]{%
  \ifx \dcases@custom@qstop #2%
  \else\expandafter\@secondoftwo%
  \fi
  \@gobble{#1\csname dcases@custom@#2\endcsname%
  \dcases@custom@build{&\quad}}%
}
\newcommand{\dcases@custom@qstop}{\dcases@custom@qstop}

% The t and m variants produce text- and math- mode. The T and M variants take
% one argument and tell \dcases@custom@build to use it rather than the default
% "&\quad". Because of how badly \dcases@custom@build, the actual commands
% take three arguments: \dcases@custom@build itself, {&\quad} (default 
% argument), and the new custom argument.
\newcommand{\dcases@custom@t}{\unexpanded{\strut@{##}\hfil}}
\newcommand{\dcases@custom@m}{\unexpanded{\strut@$\m@th\displaystyle{##}$\hfil}}
\newcommand{\dcases@custom@T}[3]{\dcases@custom@t#1{&#3}}
\newcommand{\dcases@custom@M}[3]{\dcases@custom@m#1{&#3}}

% For the actual typesetting, we copy \MT_start_cases:nnn from mathtools and
% change it a little bit.
\newcommand{\dcases@custom@start}[1]{%
  \relax \ifmmode \else \nonmatherr@ {\begin {\@currenvir }}\fi
  \iffalse {\fi \ifnum 0=`}\fi
  \left\lbrace
    \vcenter\bgroup
    \Let@  \chardef\dspbrk@context\@ne  \restore@math@cr 
    \let\math@cr@@\AMS@math@cr@@  \spread@equation \ialign 
    \bgroup #1\crcr
}

% Finally, our specific custom environment
\newenvironment{mycases}{\customdcases{M{}mT{$\;$}m}}{\endcustomdcases}
\makeatother

We can then call our environment either using \texttt{customdcases} with the relevant column specifier, or \texttt{mycases}. In general, spacing next to relation symbols (in e.g., \verb|&=|) will be wrong, but in this specific case, it works.
\begin{align*}
\begin{customdcases}{M{}mT{$\;$}m}
&\frac{n(x)}{1+j^2F^2} & for &  j=0\\[15pt]
2&\frac{\cos(jk)}{1+j^2F^2}f_j^{FD}(n(x))& for & j>0\\[15pt]
2&\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{FD}(n(x)) & for& j<0 
\end{customdcases}
\\[30pt]
\begin{mycases}
&\frac{n(x)}{1+j^2F^2} & for &  j=0\\[15pt]
2&\frac{\cos(jk)}{1+j^2F^2}f_j^{FD}(n(x))& for & j>0\\[15pt]
2&\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{FD}(n(x)) & for& j<0 
\end{mycases}
\end{align*}

\end{document}

Answer 7

cloning the amsmath cases environment, increasing the \arraystretch from the default setting of 1.2:

\makeatletter
\newenvironment{mycases}{%
  \matrix@check\mycases\env@mycases
}{%
  \endarray\right.%
}
\def\env@mycases{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrace
  \def\arraystretch{1.5}%
  \array{@{}l@{\quad}l@{}}%
}
\makeatother

you can adjust the 1.5 as necessary to get a pleasing result.

granted, this won't adjust for the missing "2" in the first line.

Answer 8

The mathtools package also provides the dcases environment.

\documentclass[11pt]{article}
\usepackage[T1]{fontenc}
\usepackage{mathtools}

\begin{document}
  \begin{equation}
    \begin{dcases}
      \phantom{2}\frac{n(x)}{1+j^2F^2} & \text{for } j=0 \\
      2\frac{\cos(jk)}{1+j^2F^2}f_j^\text{FD}(n(x))& \text{for } j>0\\
      2\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^\text{FD}(n(x)) & \text{for } j<0 
    \end{dcases}
  \end{equation}
\end{document}

Answer 9

This solution use the delarray package. The following:

\documentclass{article}

\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage{delarray}

\newenvironment{mycases}[1][@{}@l>{$}l<{$}c@{}]{%
    \renewcommand{\arraystretch}{1.3}%
    \begin{array}\{{#1}.%
}{%
    \end{array}%
}

\newcommand{\FD}{\text{FD}}

\begin{document}
This is your example:
\[
\begin{mycases}
\frac{n(x)}{1+j^2F^2}                            & for & j=0 \\
2\frac{\cos(jk)}{1+j^2F^2}f_j^{\FD}(n(x))        & for & j>0 \\
2\frac{\sin(jk)jF/\tau}{1+j^2F^2}f_j^{\FD}(n(x)) & for & j<0 
\end{mycases}
\]
\end{document}

produces:

However sometimes it's useful to have more or less columns, you can control that using the optional argument of the mycases environment, for example:

\[
|x|=\begin{mycases}[rc]
 x, & x\geq 0 \\
-x, & x< 0
\end{mycases}
\]

gives: