How to align equations cases environment

Question

The following code:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\begin{cases}
   Af &=g \quad \text{if} \ c,\\
   f&=0 \quad \text{if} \ d.
\end{cases}$
\end{document}

gives

How to align the two f's vertically (so that they are both in the same distance from the '=' sign)?

I don't see the reason for \quad there. In the second line, write \phantom{A}f to get the fs aligned. — Manuel, Mar 23 '16 at 09:02
@Manuel Yes, that is what I wanted. But is there not any other solution, like specifying the alignment requirements in the options of 'cases'? — wondering, Mar 23 '16 at 09:05
Ah, I see, in {cases}, the & is there to separate columns, not before the equal. — Manuel, Mar 23 '16 at 09:06
@Manuel Sorry, I don't get what you mean and how it can help me. — wondering, Mar 23 '16 at 09:11
You are not supposed to write &= inside a {cases} environment. — Manuel, Mar 23 '16 at 10:22

Henri Menke · Accepted Answer · 2016-03-23T09:14:56.747

7

Use aligned instead of cases. If you wonder, why there is \! after \left\{, see Why is there a \, space at the beginning of the “aligned” environment?.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$\left\{\!\begin{aligned}
   Af &=g && \text{if} \ c,\\
   f &=0 && \text{if} \ d.
\end{aligned}\right.$
\end{document}

edited Mar 23 '16 at 09:14

answered Mar 23 '16 at 09:09

Henri Menke

109,596

score 5 · Answer 2 · answered Mar 23 '16 at 09:08

\documentclass{scrartcl}
\usepackage{mathtools}
\begin{document}
\[
  \begin{cases}
    Af=g & \text{if} \ c, \\
    \phantom{A}f = 0 & \text{if} \ d.
  \end{cases}
\]
\end{document}

Altough I tend to prefer

\begin{cases*}
  Af=g & if $c$, \\
  \phantom{A}f = 0 & if $d$.
\end{cases*}

How to align equations cases environment

2 Answers2