如何对齐分段函数

Question

I thought a properly aligned piecewise function should be something like this:

Commas and character x s are perfectly aligned in two lines.

I tried

\vert x \vert =
\begin{cases} 
  x, &x \geq 0 \\
 -x, &x < 0  \\
\end{cases}

which only aligned the domain parts like this:

In that way, the commas come after x and -x is not aligned, which looks strange.

I tried to solve that problem by adding spaces preceding the x in first line, which is:

\vert x \vert =
\begin{cases} 
 \enspace \; x, &x \geq 0 \\
 -x, &x < 0  \\
\end{cases}

It gives the correct result (as above) but this solution seems rather strange, if the second line changes, then the first line have also to be modified to keep the spaces fit. I reckon there must be some better way to do it.

Answer 1

The default alignment behaviour of cases is l eft- l eft for the value and domain components. In this specific case (for aesthetic reasons), you can insert a \phantom negation to align the values:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\[
  \lvert x \rvert = \begin{cases}
     x, & x \geq 0 \\
    -x, & x < 0
  \end{cases}
\]

\[
  \lvert x \rvert = \begin{cases}
    \phantom{-}x, & x \geq 0 \\
    -x, & x < 0
  \end{cases}
\]

\end{document}