# Piecewise cases ; align the x of each condition

I need to notate this function:

\begin{align*}
f(x)=\begin{cases}
-1,-\pi<x<0 \\
1,0<x<\pi
\end{cases}
\end{align*}


How can I get the two conditions to align by the x? Simply putting an ampersand before the x doesn't work (that causes strange space from the lower bound to the x.)

Use alignedat; I also add another form, where adding “for” seems to make the alignment unnecessary.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{equation*}
\begin{cases}
\begin{alignedat}{2}
-1, & \quad & -\pi & < x < 0   \\
1, & \quad &    0 & < x < \pi
\end{alignedat}
\end{cases}
\end{equation*}

\begin{equation*}
\begin{cases}
-1, & \text{for $-\pi < x < 0$} \\
1, & \text{for $0 < x < \pi$}
\end{cases}
\end{equation*}

\end{document}


• Why are you using the equation environment instead of align? Please can you have a look at my follow-up question – theonlygusti Sep 21 '17 at 13:08
• @theonlygusti For a single equation it's better to use equation; align is for multiline displays (cases isn't). – egreg Sep 21 '17 at 13:12
• what about the equation environment vs $$? – theonlygusti Sep 21 '17 at 13:22 • @theonlygusti $$ should never be used in LaTeX. See Why is $…$ preferable to ? – egreg Sep 21 '17 at 13:41

I propose this, with empheq and alignat*:

\documentclass{article}
\usepackage{empheq}

\begin{document}

\begin{empheq}[left={f(x)=\empheqlbrace}]{alignat*=3}
-1, &\quad & \text{if} &~ & -\pi & < x < 0 \\
1, & & \text{if}& & 0 & < x < \pi
\end{empheq}

\end{document}


You can use some \phantom overlapping to achieve the same result:

\documentclass{article}

\usepackage{amsmath}

\begin{document}

$f(x) = \begin{cases} -1, & -\pi < x < 0 \\ \phantom{-}1, & \phantom{-\pi}\llap{0} < x < \pi \end{cases}$

\end{document}