latex() etc.: How to nicely format a matrix for a LaTeX document?

From: Marius Hofert <m_hofert at web.de>
Subject: [R] latex() etc.: How to nicely format a matrix for a LaTeX document?
To: "Help R" <r-help at stat.math.ethz.ch>
Received: Thursday, December 30, 2010, 11:50 AM
Dear (T)eXpeRts,

I try to create a LaTeX table from an R matrix for the
first time. I am not sure what the "best" approach is, I
just read about latex() from Hmisc (toLatex() didn't work).

Consider the following minimal example:

library(Hmisc)
mat <- matrix(c(1,NA,3,100,10000,4), ncol = 3, byrow =
TRUE)
latex(mat, file = "", booktabs = TRUE, numeric.dollar =
FALSE, table.env = FALSE)

I am only interested in the part between \midrule and
\bottomrule [I couldn't figure out how to remove the tabular
environment]. It looks like this:

? 1&&3\tabularnewline
100&10000&4\tabularnewline

My questions/problems are:

(1) if there are NA's, you can see that the output is not
aligned according to the &-symbols. That makes it hard
to read in a LaTeX *source* file. How can I get something
like:
? 1&?
???&3\tabularnewline
100&10000&4\tabularnewline

(2) it would even be nicer to read if the output was like
this:
? 1 &? ? ???& 3
\tabularnewline
100 & 10000 & 4 \tabularnewline
How can I achieve this?

(3) is there another package/function to get output like
this more easily? I wrote a one-liner which formats the
lines separately, but it would be nice to have the columns
aligned as given in (2). I believe it is most helpful to
have the output in a form which is readable in a *source*
file (i.e., .tex), since the headers/footers from tabular
[or tabularx etc.] are often easy to put in the document. 

Cheers,

Marius