2 left alignments under cases

Question

I would like to align something under cases as follows:

value1                    if condition
value2  line1ofvalue2...  otherwise
        line2ofvalue2..
        line3ofvalue2.

First, there are only 2 conditions (each one for one value), so I want if condition and otherwise are left adjusted. Then as my value2 is a long, I need to split it into 3 lines, so I want these 3 lines to be left adjusted.

Does anyone know how to do this? Thank you very much

Answer 1

The array environment provides a very similar layout and typesetting to the amsmath package cases environment. In fact, amsmath defines cases exactly as an array with some stretch (an \arraystretch of 1.2) and a specific column alignment (@{}ll@{}). Consider the following minimal example displaying the two different styles:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\noindent Original \texttt{cases} example:
\[
X=%
  \begin{cases}
    0& \text{if $r-j$ is odd},\\
    r!\,(-1)^{(r-j)/2}& \text{if $r-j$ is even}.
  \end{cases}
\]

\noindent Using the \texttt{array} environment:
\[
\renewcommand{\arraystretch}{1.2}%
X=\left\{%
  \begin{array}{@{}ll@{}}
    0& \text{if $r-j$ is odd},\\
    r!\,(-1)^{(r-j)/2}& \text{if $r-j$ is even}.
  \end{array}
  \right.
\]
\end{document}

Therefore, using the array environment, one could attempt:

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

\noindent Requested environment using \texttt{array}:
\[
\renewcommand{\arraystretch}{1.2}%
X=\left\{%
  \begin{array}{@{}lll@{}}
    0& & \text{if $r-j$ is odd},\\
    k& +7& \text{if $r-j$ is even}. \\
    & +1+2i& \\
    & \binom{3}{n}
  \end{array}
  \right.
\]
\end{document}​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

The gap between the first two left-aligned columns can also be adjusted (using @{}l@{}ll@{}, say) if needed. The advantage of this approach is flexibility in terms of the array environment, while still retaining the visual equivalence with the cases environment; that is, a user-defined cases environment.

Answer 2

Here's an extension of the amsmath cases environment. There's a new optional argument for customizing the cases columns. Without argument, it work like the original cases.

\makeatletter
\renewcommand\env@cases[1][@{}l@{\quad}l@{}]{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrace
  \def\arraystretch{1.2}%
  \array{#1}%
}
\makeatother

The original definition in amsmath.sty contains @{}l@{\quad}l@{}, which we replaced by #1. Now you can use all features of array column specifications.

Complete example with usage:

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\renewcommand\env@cases[1][@{}l@{\quad}l@{}]{%
  \let\@ifnextchar\new@ifnextchar
  \left\lbrace
  \def\arraystretch{1.2}%
  \array{#1}%
}
\makeatother
\begin{document}
\[
\begin{cases}
  value1 & if condition \\
  value2 & otherwise
\end{cases}
\]
\[
\begin{cases}[@{}lp{3cm}l@{}]
  value1 & & if condition \\
  value2 & line1ofvalue2 \newline
           line2ofvalue2 \newline
           line3ofvalue2          & otherwise
\end{cases}
\]
\end{document}

Answer 3

Is this what you are looking for?

\documentclass{article}
\usepackage[fleqn]{amsmath}
\begin{document}
\newcommand{\Phantom}{\phantom{value2}}%
\begin{align*}
f(x) = \begin{cases}
value1& condition1 \\
value2 \text{ line1ofvalue2}& condition2\\
\Phantom \text{ line2ofvalue2}\\
\Phantom \text{ line3ofvalue2}
\end{cases}
\end{align*}
\end{document}