bojan.blog » Tips & Tricks

Create a symmetric matrix in Mathematica

Bojan — Wed, 18 Nov 2009 19:03:20 +0000

Here is a quick way to create a symmetric matrix in Mathematica. The code below creates a generic 6×6 matrix (line 1), then mirrors the above-diagonal elements to their below-diagonal counterparts (substitution in lines 3-5).

am = Array[Subscript[a,##] &, {6,6}];
am = am /.
  Flatten[Table[
    Subscript[a,i,j] -> Subscript[a,j,i],
    {i,2,6}, {j,1,i-1}]]

ezproxy redirect bookmarklet

Bojan — Tue, 10 Nov 2009 15:17:02 +0000

A while ago I wrote a Google Toolbar button for redirecting the current page to the same page accessed through an EZproxy server. As I was missing this functionality in browsers without Google Toolbar (it’s still not available in Google Chrome), I realized that it’s a very simple thing to do with a bookmarklet. Here is the code:

javascript:window.location=location.protocol+'//'+location.hostname+'.ezproxy1.library.arizona.edu'+location.pathname+location.search+location.hash;

To make it work, copy the above code, and paste it as the URL in a bookmark. In all modern browsers, alternatively, you can just drag this link to the bookmarks bar:

In either case, make sure you edit the javascript code and replace ezproxy1.library.arizona.edu by your EZproxy server.

Once "ez" is in your bookmarks, clicking on the bookmark will have the same effect as clicking the ezproxy Google Toolbar button.

XML parsing with Perl using XPath

Bojan — Mon, 21 Aug 2006 22:01:43 +0000

use strict;
use XML::LibXML;

# data source
my $xml_filename = "xml/data.xml";

my $parser = XML::LibXML->new;
my $doc = $parser->parse_file($xml_filename);

my $xpath = "//word/entry[\@src=\"pr\"]/..";
my @words = $doc->findnodes($xpath);

foreach my $word (@words) {
	print $word->findvalue("\@id");
	print "\n";
}

File modified timestamp with Perl

Bojan — Mon, 21 Aug 2006 21:43:29 +0000

This is how to quickly get the timestamp of when the file was last modified in Perl:

$lastmod = (stat($filename))[9];

XSLT transform with Perl

Bojan — Mon, 21 Aug 2006 21:27:11 +0000

Here is a Perl snippet that transforms an XML file with XSLT:

use strict;
use XML::XSLT::Wrapper;

my $xml_input_filename = "xml/data.xml";
my $xml_output_filename = "xml/out.xml";
my $xsl_filename = "xml/transform.xslt";

my $xslt = XML::XSLT::Wrapper->new(
		ProcessorList => [
			'libxslt',
			'sablotron',
			'xslt'
		],
	);
my $result = $xslt->transform(
		OutFile => $xml_output_filename,
		xml => $xml_input_filename,
		xsl => $xsl_filename,
	);
$xslt->dispose();

Values given to [code]]czo2OlwieG1sID0+XCI7e1smKiZdfQ==[[/code] and [code]]czo2OlwieHNsID0+XCI7e1smKiZdfQ==[[/code] can be either file names, or xml/xslt code passed as a string.

If [code]]czo3OlwiT3V0RmlsZVwiO3tbJiomXX0=[[/code] is ommited, then the result of the transformation is returned to the variable [code]]czo3OlwiJHJlc3VsdFwiO3tbJiomXX0=[[/code].

Sitemap generator configuration for Blogger

Bojan — Tue, 08 Aug 2006 20:28:00 +0000

If you are using Google’s sitemap generator [code]]czoxNDpcInNpdGVtYXBfZ2VuLnB5XCI7e1smKiZdfQ==[[/code] to generate a sitemap of your Blogger blog , here are the configuration lines to add to the config.xml file:

In this way, void URLs will not be included in the sitemap.xml output.

If you're wondering "Why would I be running the sitemap generator on my blog?", it's because Googlebot does not handle well the posts where the time & date has been altered.

If you don't know what the sitemap generator is, see Google Sitemaps Help.

Function call counting in Matlab

Bojan — Thu, 23 Mar 2006 13:38:15 +0000

Here’s a neat way to keep track of the cost, i.e. number of function calls in Matlab: rather than using a counter variable and increment it each time a function is called, use the internal Matlab counters, accessible through the FunctionTable struct array.

The loop below is necessary only to get the index of the desired function in the struct array (If you know a better way of looking up a function in FunctionTable, let me know!).

Replace the function name ftest if needed.

profile on

% your code comes here
% (cost is counted from "profile on" to "profile off")

p = profile('info');
for i = 1:size(p.FunctionTable,1)
    if strcmp(p.FunctionTable(i).FunctionName,'ftest')
        break
    end
end
cost = p.FunctionTable(i).NumCalls;
profile off

Images in LaTeX

Bojan — Thu, 08 Dec 2005 19:16:49 +0000

Problem:: you can’t get those images to appear in a typeset document.
Cause:: [code]]czo4OlwicGRmbGF0ZXhcIjt7WyYqJl19[[/code] and [code]]czo1OlwibGF0ZXhcIjt7WyYqJl19[[/code] have virtually no graphic drivers in common. For instance, [code]]czo4OlwicGRmbGF0ZXhcIjt7WyYqJl19[[/code] can't handle eps files.
Solution:: I converted my eps files to pdf using a thing called [code]]czo4OlwiZXBzdG9wZGZcIjt7WyYqJl19[[/code], which is included in latex distributions, and [code]]czo4OlwicGRmbGF0ZXhcIjt7WyYqJl19[[/code] handled those pdf graphs perfectly, so that the output was identical to that of eps with [code]]czo1OlwibGF0ZXhcIjt7WyYqJl19[[/code].