Typesetting a large matrix in LaTeX

Question

I have a 3x12 matrix I'd like to input into my LaTeX (with amsmath) document but LaTeX seems to choke when the matrix gets larger then 3x10:

\begin{equation}
\textbf{e} = 
\begin{bmatrix} 
1&1&1&1&0&0&0&0&-1&-1&-1&-1\\
1&-1&0&0&1&1&-1&-1&0&0&1&-1\\
0&0&1&-1&1&-1&1&-1&1&-1&0&0
\end{bmatrix}
\end{equation}

The error: Extra alignment tab has been changed to \cr. tells me that I have more & then the bmatrix environment can handle. Is there a proper way to handle this? Also it seems that the alignment for 1's and the -1's are different, is that also expected of the bmatrix?

Answer 1

From the amsmath documentation (texdoc amsmath):

The amsmath package provides some environments for matrices beyond the basic array environment of LATEX. The pmatrix, bmatrix, Bmatrix, vmatrix and Vmatrix have (respectively) ( ), [ ], { }, | |, and ∥ ∥ delimiters built in. For naming consistency there is a matrix environment sans delimiters. This is not entirely redundant with the array environment; the matrix environments all use more economical horizontal spacing than the rather prodigal spacing of the array environment. Also, unlike the array environment, you don’t have to give column specifications for any of the matrix environments; by default you can have up to 10 centered columns. (If you need left or right alignment in a column or other special formats you must resort to array.)

i.e. bmatrix defaults to a 10 column maximum.

A footnote adds

More precisely: The maximum number of columns in a matrix is determined by the counter MaxMatrixCols (normal value = 10), which you can change if necessary using LATEX’s \setcounter or \addtocounter commands.

Answer 2

The answer by Scott is correct, but I've since learned you can override the alignment. Taken from http://texblog.net/latex-archive/maths/matrix-align-left-right/

\makeatletter
\renewcommand*\env@matrix[1][c]{\hskip -\arraycolsep
  \let\@ifnextchar\new@ifnextchar
  \array{*\c@MaxMatrixCols #1}}
\makeatother

Now allows the command:

\begin{bmatrix}[r] ....

to have right-alignment!