Grouping Rows of a Matrix

I spent a large chunk of yesterday afternoon doing something I thought would be simple (relatively speaking) in LaTeX. I wanted to group rows of a matrix (actually, in my case, a vector) with right braces, and label the groups. An example of what I wanted is in the image below.

This seems to me to be a fairly common thing to do, and LaTeX has been around over 35 years (TeX even longer), so by now it must be pretty easy. Right? Um, not so much. I wore out Google looking for packages that would do this. Curiously, it's easy to put braces over and under things:

• $\overbrace{[x_1,\dots,x_n]}$ [\overbrace{[x_1,\dots,x_n]}];
• $\underbrace{[x_1,\dots,x_n]}$ [\underbrace{[x_1,\dots,x_n]}].

There are packages to let you surround matrices, arrays etc. with a variety of delimiters (not just parentheses or square brackets). Nowhere, though, could I find a command or package to do the above.

Fortunately, something pointed me in the direction of the PGF/TiKZ package, which I've used in the past for doing drawings. It's an incredible tool in terms of both what it can do and the outstanding quality of its manual. Because it does so many things, I've never really gotten to know all its capabilities, and in particular its ability to do matrices in a picture environment.

Here is the code to do my illustration. You need to load the TiKZ package and two of its libraries in your document preamble, as follows:

\usepackage{tikz}
\usetikzlibrary{matrix, decorations.pathreplacing}

The code for the drawing is:

\begin{tikzpicture}
 \matrix (vec) [matrix of math nodes, left delimiter = {[}, right delimiter = {]}] {
f_1 \\
\vdots \\
f_{a} \\
f_{a + 1} \\
\vdots \\
f_{b} \\
f_{b + 1} \\
\vdots \\
f_{c} \\
};
\node (a) at (vec-1-1.north) [right=20pt]{};
\node (b) at (vec-3-1.south) [right=20pt]{};
\node (c) at (vec-4-1.north) [right=20pt]{};
\node (d) at (vec-6-1.south) [right=20pt]{};
\node (e) at (vec-7-1.north) [right=20pt]{};
\node (f) at (vec-9-1.south) [right=20pt]{};
\draw [decorate, decoration={brace, amplitude=10pt}] (a) -- (b) node[midway, right=10pt] {\footnotesize something};
\draw [decorate, decoration={brace, amplitude=10pt}] (c) -- (d) node[midway, right=10pt] {\footnotesize something else};
\draw [decorate, decoration={brace, amplitude=10pt}] (e) -- (f) node[midway, right=10pt] {\footnotesize something silly};
\end{tikzpicture}

The name of the matrix ("vec") is arbitrary. The amplitude for the brace (10pt) and the offsets (10pt and 20pt) are matters of taste.

If you happen to know a faster way of doing this, please do share in a comment.

Arrowheads in TiKZ

For most publication graphics (including drawings used in blog posts), I favor the TiKZ package for LaTeX. Many of those drawings have included lines with arrowheads, and the arrowheads never posed any problems ... until recently. Consider the following small chunk of TiKZ code:

\begin{tikzpicture}
   \draw[->] (0,0) -- (1,1);
\end{tikzpicture}

and the similar chunk

\begin{tikzpicture}[>=latex]
   \draw[->] (0,0) -- (1,1);
\end{tikzpicture}

Both should draw a diagonal arrow, with the arrowheads differing only in terms of their style. (The optional argument in the first line of the second version specifies the use of LaTeX-style arrowheads.) Indeed, when compiling the document with latex (producing DVI output), ps2pdf (producing PDF output by way of an intermediate Postscript file) or XeTeX, I get the expected output:

On the other hand, when I compile the document with pdflatex (the program I normally use) or LuaTeX, the arrow head in the first version goes missing:

(As a sidebar, compiling with dvipdfm, which I never use, produces a PDF file with a single blank page.)

This occurs on a Linux Mint 14 (Nadia) system with the TeXLive 2012 LaTeX distribution. As I said, the problem is new to me. I've previously drawn arrows in TiKZ using TeXLive (perhaps the 2011 or 2010 version) without specifying LaTeX-style arrowheads and compiled successfully using pdflatex. The workaround is at least straightforward. I just have to remember to specify the arrowhead style.

On an unrelated note, if anyone is curious how I converted the PDF output to the PNG images above, I took a somewhat circular route. For whatever reason, the ImageMagick convert utility, which on a good day requires specification of various command line options to produce  decent looking PNG, insisted on producing empty image files from my PDF files. So I opened each PDF file in a viewer, took a screen capture by pressing Control-PrtScn, and pasted the image into IrfanView (running under WINE). In IrfanView, which has excellent image manipulation tools, I just cropped the image and saved it to a PNG file.