Problem with defining shortcuts for TikZ matrices

Question

For answering a question yesterday I wanted to define a shortcut for creating a matrix inside a tikzpicture. As a minimal example, consider

\newcommand\mymatrix[1]{
    \begin{tikzpicture}
        \matrix[matrix of math nodes] {#1};
    \end{tikzpicture}%
}

which would then be used as

\mymatrix{
    a & b \\
    e & f \\
}

But LaTeX complains about Package pgfbasematrix Error: Single ampersand used with wrong catcode at the line with the closing brace of \amatrix{}. Why does it do that and how can I avoid it?

Answer 1

TikZ cannot make the ampersand given in the argument to an active character. You could

use \pgfmatrixnextcell instead of & and perhaps define a shortcut for it,

or use the ampersand replacement option:

\newcommand\mymatrix[1]{
    \begin{tikzpicture}
        \matrix[ampersand replacement=\&,matrix of math nodes] {#1};
    \end{tikzpicture}%
}
\mymatrix{
    a \& b \\
    e \& f \\
}

Answer 2

From p179 of the manual (emphasis added, one minor but significant error corrected):

Even though TikZ seems to use & to separate cells, pgf actually uses a different command to separate cells, namely the command \pgfmatrixnextcell and using a normal & character will normally fail. What happens is that, TikZ makes & an active character and then defines this character to be equal to \pgfmatrixnextcell. In most situations this will work nicely, but sometimes & cannot be made active; for instance because the matrix is used in an argument of some macro or the matrix contains nodes that contain normal {tabular} environments. In this case you can use the following option to avoid having to type \pgfmatrixnextcell each time:

/tikz/ampersand replacement= macro name or empty          (no default)

If a macro name is provided, this macro will be defined to be equal to \pgfmatrixnextcell inside matrices and & will not be made active. For instance, you could say ampersand replacement=\& and then use \& to separate columns as in the following example:

\tikz
\matrix [ampersand replacement=\&]
{
\draw (0,0) circle (4mm); \& \node[rotate=10] {Hello}; \\
\draw (0.2,0) circle (2mm); \& \fill[red] (0,0) circle (3mm); \\
};

There is presumably some horrendously complicated way to turn on the active nature of the ampersand again inside the macro argument, but the above would seem the cleanest way.