Extra alignment tab error trying to justify within cases environment

Question

I am trying to align brackets inside of cases. This is my equation:

\begin{align}
y(x)=
\begin{cases}
  \bigg[ & 1+2 & \\
         & +5+6 \bigg] & \text{if } x < 2, \\
  \bigg[ & 11+12 & \\
        & +15+16 \bigg] & \text{if }x > 2,\\
        0 & & \text{otherwise.}
\end{cases} 
\end{align}

When I compile this, I get the following error

! Extra alignment tab has been changed to \cr.

The same happens when I remove the align environment change it to equation instead.

\begin{equation}
y(x)=
\begin{cases}
  \bigg[ & 1+2 & \\
        & +5+6 \bigg] & \text{if } x < 2, \\
  \bigg[ & 11+12 & \\
        & +15+16 \bigg] & \text{if }x > 2,\\
        0  & & \text{otherwise.}
\end{cases} 
\end{equation}

I searched for this problem, but all the questions seem to be about table environments.

I think you can only use a single alignment tab (&) per case. When I delete the second & in each case, the error goes away. — NickD, Mar 11 '17 at 03:14
Right, it works that way. I was wondering if its possible to align brackets inside my multiline cases to make it easier to understand which brackets pair up. — highBandWidth, Mar 11 '17 at 03:21
Please complete your code so it produces the error you report. Right now it obviously will not get that far. — cfr, Mar 11 '17 at 04:39

score 1 · Answer 1 · edited Apr 13 '17 at 12:35

Nick has explained by this doesn't work. Two possible workarounds are shown below, one using array instead of cases, the other using a multlined (requires the mathtools package) inside cases.

And note that for single equations you should be using equation, not align (align vs equation).

\documentclass{article}
\usepackage{mathtools}
\begin{document}
\begin{equation}
y(x)= \left\{
\begin{array}{@{\,}r@{}ll}
  \biggl[ & 1+2 & \\
        & +5+6 \biggr] & \text{if } x < 2, \\
  \biggl[ & 11+12 & \\
        & +15+16 \biggr] & \text{if }x > 2,\\
        0  & & \text{otherwise.}
\end{array}\right.
\end{equation}

\begin{equation}
y(x)=
\begin{cases}
  \begin{multlined}[b]
   \bigg[1+2  \\
        +5+6 \bigg]
\end{multlined} & \text{if } x < 2, \\
  \begin{multlined}[b]
\bigg[11+12  \\
        +15+16 \bigg]
\end{multlined} & \text{if }x > 2,\\
        0  &  \text{otherwise.}
\end{cases} 
\end{equation}
\end{document}

Extra alignment tab error trying to justify within cases environment

1 Answers1