[OS X TeX] "Runaway argument?" error in figure caption

[Manfred Braun]

On 17.07.2012, at 16:38, John B. Thoo wrote:

> Hi, everyone.  I apologize that this is not a Mac-specific question, but rather a general LaTeX question; however, I don't know whom else to ask.
> 
> This code
> 
> \begin{figure}
> \begin{tikzpicture}
> \pgftransformscale{0.75}
> %% axes
>  \draw[help lines,lightgray] (-1,-1) grid (5,5);
>  \draw[thin] (-1,0) -- (5,0) node[anchor=north]{$x$};
>  \draw[thin] (0,-1) -- (0,5) node[anchor=west]{$y$};
> %% u and (u1,u2)
>  \draw[thick,-latex] (0,0) -- (3,4);
>    \draw (1.6,2) node[anchor=west]{$\vec{u} = \left[\begin{array}{c} 3 \\ 4 \end{array}\right]$};
>  \fill (3,4) circle (0.08) node[anchor=west]{$(3,4)$};
> \end{tikzpicture}
>  \caption{It is useful to picture vectors as arrows, although this may not be a literal representation of vectors.  Here we identify the vector $\vec{u} = \left[\begin{smallmatrix} 3 \\ 4 \end{smallmatrix}\right]$ in $\R{2}$ represented as an arrow with the point $(3,4)$ in the $xy$ plane.}
>  \label{fig.syslineqns:vectorasarrow}
> \end{figure}
> 
> produces this error
> 
> [5]
> ! Argument of \@caption has an extra }.
> <inserted text> 
>                \par 
> l.177 ...ith the point $(3,4)$ in the $xy$ plane.}

Within a caption the use of commands and environments is restricted.  Commands must be "protected".  A possible solution is as follows:  Define, for instance,

\newcommand\mymatrix{ \left[ \begin{smallmatrix} 3 \\ 4 \end{smallmatrix} \right] }

in the preamble.  Then replace your caption by

 \caption{It is useful to picture vectors as arrows, although this may not be a literal representation of vectors.  Here we identify the vector $\vec{u} = \protect\mymatrix$ in $\R{2}$ represented as an arrow with the point $(3,4)$ in the $xy$ plane.}

This should work.  Important is to include the \protect command.  It prevents the immediate expansion of the subsequent \mymatrix command, the expansion is postponed until the caption is typeset.

Manfred