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I am trying to compile a simple cases example, but I'm having trouble with the tabular separator &. Here's my code:

\begin{equation}
    f(x) = 
    \begin{cases}
        x + 1 & \mbox{blabla} \\
        x & \mbox{otherwise}
    \end{cases}
\end{equation}

I am getting the following errors:

Misplaced alignment tab character &. x + 1 &

Misplaced alignment tab character &. x &

What could be wrong?

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closed as too localized by cmhughes, Marco Daniel, percusse, cgnieder, Torbjørn T. Aug 27 '12 at 6:30

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8  
Welcome to tex exchange! Are you loading the amsmath or mathtools package? The following MWE works for me \documentclass{article} \usepackage{amsmath} \begin{document} \begin{equation} f(x) = \begin{cases} x + 1 & \mbox{blabla} \ x & \mbox{otherwise} \end{cases} \end{equation} \end{document} –  cmhughes Jul 23 '12 at 16:32
    
I'll also confirm that @cmhughes MWE works for me. When I remove \usepackage{amsmath}, I get the "Misplaced alignment tab character &." error that you mention. –  Henry DeYoung Jul 23 '12 at 16:34
8  
It's a known problem: for historical reasons, LaTeX has an undocumented \cases macro inherited from Plain TeX, which has a very different syntax than the cases environment provided by amsmath. –  egreg Jul 23 '12 at 16:55
    
@egreg: Interesting. –  Marco Daniel Jul 23 '12 at 17:15
    
Thank you for the answers! I had to load the amsmath package. I had just tried a couple of examples I found on the web which didn't mention the package. –  erickrf Jul 23 '12 at 18:57
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1 Answer 1

up vote 13 down vote accepted

Short answer

The cases environment requires the amsmath package:

\usepackage{amsmath}

Long answer

For historical reasons, LaTeX defines a \cases macro: on page 232 of Lamport's manual one reads

Most Plain TeX commands can be used in LaTeX, but only with care.

and \cases is indeed a Plain TeX command which has quite a different syntax than the cases environment. It would be possible to use it without loading amsmath, but the features of this package are much better than the Plain TeX commands: more flexible and with a uniform syntax.

By the way, here's how one could input the equation with this method:

\begin{equation}
f(x) = 
\cases{
  x + 1 & blabla \cr
  x & otherwise \cr
}
\end{equation}

It's evident that the syntax is foreign to LaTeX: the second column is typeset in text mode and not math mode and the lines must be terminated by \cr. Most important, the alignment must go between braces: this is the main cause of the error.

Don't use it and stick to \usepackage{amsmath}.

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