# [texhax] Matrices/alignment -- Repeated template at beginning?

Matthew Leingang leingang at math.harvard.edu
Sat Apr 7 16:55:31 CEST 2007

Hi,

I'm trying to typeset matrices where the last column is separated from the
rest by a vertical bar.  Normally I can use

\left[\begin{array}{rrr|r}
1 & 2 & 4 & 0 \\
...
0 & 0 & 1 & 2
\end{array}\right]

This is a suitable workaround.  But I also like to use the gauss package to
typeset row operations.  To have *both* the vertical bar and the rowops, I
need a matrix-like environment that does the vertical bar automatically.
Then I can pass that environment to the gauss package.

Looking in the TeX by Topic Book, I've found that the matrix-like
environments use \ialign, and a feature of that that allows for an arbitrary
number of columns:  If the preamble has a && rather than a &, the rest of
the preamble can be repeated as much as needed.  So the matrix environment
uses a preamble something like {\hfil$#$\hfil && \quad\hfil$#$\hfil\crcr}

This preamble allows for the first column, or first group of columns, to be
different from the rest.  Is there a way to make the *last* column different
from the rest?

Any clues would be appreciated.  TIA.

--Matt

--
Matthew Leingang
Preceptor in Mathematics
Harvard University

http://www.math.harvard.edu/~leingang/vCard.vcf