Octopress — adding category tags to the blog RSS feed

Right from the beginning, I’ve assigned broad categories to every post I’ve written here. (For example, this is my—very lacking—Health Monitoring series of posts.) However, Octopress does not include these category tags by default into the RSS feed. So if a reader is using an RSS feed-reader app or website, they cannot make use of the assigned categories even if the app or website was capable of doing so.

I’ve now added some code necessary to add the categories to the RSS feed, and this is what I did.

cellArray = {'Alpha','Beta','Gamma','Delta','GammaSquared'};
refString = 'Gamma';

At the outset, here is the code that I added:

{% for post in site.posts limit: 20 %}
<entry>
<!-- Other items that are included in the feed -->

{% capture catnum %}{{ post.categories | category_links | size }}{% endcapture %}
{% unless catnum == '0' %}
    <categories>
    {% for cct in post.categories %}
        {% assign idx=forloop.index0 %}<category>{{ post.categories[idx] }}</category>
    {% endfor %}
    </categories>
{% endunless %}

<!-- Other items that are included in the feed -->
<content type="html"><![CDATA[{{ post.content | expand_urls: site.url | cdata_escape }}]]></content>
</entry>

{% endfor %}

This code works great, but allow me to confess that I am not sure that this is the optimum implementation. To me this seems inelegant, but until I have a better solution, this performs the function appropriately and perfectly adequately.

I’ve only included the relevant portion and the context in which it must be inserted. (See the comment tags <!-- Other items that are included in the feed -->.)

The meat of the algorithm is from lines 7 through 11.

  • A <categories> tag is defined, and a for loop is executed over post.categories, which contains the list of categories for the post.
  • Within the for loop, each post category is enclosed in a <category></category> tag.

Now I had initially thought that the loop variable (cct here) would inherit sequentially the value of each category in post.categories, but apparently that does not work properly. Therefore, the workaround is to

  • identify the loop index (assign idx=forloop.index0) and
  • use individual values of the categories (post.categories[idx]).

We must use forloop.index0 and NOT forloop.index (both are valid commands; the index key starts numbering from 1) because the array numbering starts from 0, not 1.

OK, now that the meat of the algorithm is done, we must put in some code to handle the “unusual” cases—what happens if a post does not have any categories assigned? Such a scenario is handled by the capture command (line 5) and the unless segment that encloses our actual algorithm. The capture command simply captures a value, in our case the number of categories that exist. We only want to include the categories when they exist, therefore our algorithm is run only unless catnum=='0' i.e. when the number of categories is not 0.

Well, that’s it! I have added the code segment before the actual content of each post, but I don’t think it makes any difference if the segment appears after the <content> tag. It should work fine anywhere within the <entry> environment.


Using MathJax with Octopress

I’ve been meaning to try and implement MathJax on this website for a while now. For including math equations on a website, MathJax is probably one of the more elegant ways to do it. I can write equations in TeX format, and MathJax renders the equations properly for you!

Finally, in the last couple of days I’ve been forced to get around to it, thanks to a new post that I’m writing that includes a little bit of math. So anyway, I just wanted to jot down that process.

The thing with MathJax is that it’s meant to, and does, work with HTML. But since I’m working with Octopress and Markdown, I have to ensure that the conversion from Markdown to HTML produces no unwanted syntactical errors for MathJax. To get around this problem, Zac Harmany (I hope I got the name right) suggests tweaking the Markdown rendering engine to Pandoc, so that the conversion works as desired. I’m sure that works great, but I had no intention of tweaking my Markdown conversion engine. Instead, I discovered a nifty ruby bundle (here) that serves a great purpose.

Next, the typical MathJax “installation” involves adding a line in the <head> section of your Octopress theme (Add it to %octopress_root%/source/_includes/custom/head.html), so that when every page loads, the necessary Javascript files are also loaded, ready to render your equations. I did not want the Javascript to execute to load along with all my pages, given that I don’t expect to have equations in all my posts. Instead, I’ve included the call to the script in the <body> of the post, i.e. in the meat of the post itself. Remember that the declaration needs to be before your first equation.

I also contemplated downloading the MathJax distribution and hosting it locally on my own server, but that did not work at all. There are way too many files to be uploaded (each file is small; the total package is ~50MB; there are too many files, though) and it just took forever to upload to my server until I just gave up. I’ll revisit that option if I think using MathJax’s own servers is not working well—which I doubt will happen.

With those details, here’s how I set things up:

  • Install the verbatim.rb plugin from this Github repository. To do this, simply download the file (‘Gist’ in Github parlance) and place it in %octopress_root%/plugins. When inserting equations, there’s a syntax to using this plugin; I’ll demo it below.

  • Create a MathJaxLocal.js file at %octopress_root%/source/javascripts/ to add local configurations for MathJax. Note that the last line of the code must point to the full path of the local file, in my case http://arnabocean/javascripts/MathJaxLocal.js

    Here’s what my MathJaxLocal.js looks like (I started with Zac Harmany’s file and modified to suit my needs.):

    MathJax.Hub.Config({
        jax: ["input/TeX","output/HTML-CSS"],
        extensions: ["tex2jax.js","MathMenu.js","MathZoom.js"],
        tex2jax: 
            {
                inlineMath: [ ['$','$'], ['\\(','\\)'] ],
                displayMath: [ ['$$','$$'], ['\\[','\\]'] ],
                skipTags: ["script","noscript","style","textarea","pre","code"],
                processEscapes: true
            },
        TeX:
            { 
                equationNumbers: { autoNumber: "AMS" },
                TagSide: "left",
    
            },
        "HTML-CSS": { availableFonts: ["TeX"] }
        });
        MathJax.Ajax.loadComplete("http://arnabocean.com/javascripts/MathJaxLocal.js");
    
  • Declare the location of the MathJax files. The easiest thing to do is to use MathJax’s own servers. However, in addition to just their servers, you’ll have to link to your own local config file as well, so we’ll add both of these at the same time. In the main body of your markdown post (preferably after the “Read More” fold), add the following:

    <script type="text/javascript"
    src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML,http://arnabocean.com/javascripts/MathJaxLocal.js">
    </script>
    

In the above, the first link points to MathJax servers, the second points to my own config file.

And that’s basically it! Now you’re all set to write beautiful equations. Here’s a demo:

<!-- MathJax configuration -->
<script type="text/javascript" src="http://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML,http://arnabocean.com/javascripts/MathJaxLocal.js">
</script>
<!-- End MathJax Configuration -->

{% raw %}{% verbatim tag:p %}{% endraw %}
\[ 
f(x)= a_0 + a_1\sin(x) + a_2\sin(2x) + ...
\]

\[  
+b_1\cos(x) + b_2\cos(2x) + ...
\]

\[
f(x)=a_0+\sum_{k=1}^\infty\big( a_k\cos(kx)+b_k\sin(kx) \big)
\]
{% raw %}{% endverbatim %}{% endraw %}

And here’s what that would look like:

\[ f(x)= a_0 + a_1\sin(x) + a_2\sin(2x) + … \] \[ +b_1\cos(x) + b_2\cos(2x) + … \] \[ f(x)=a_0+\sum_{k=1}^\infty\big( a_k\cos(kx)+b_k\sin(kx) \big) \]

There’s still a lot of more that I need to find out, and from the looks of it, Zac’s website is a great resource. I’ll add more posts if I find anything useful that I end up using.


Creating Bandpass Bessel Filter with MATLAB

Bessel filters are incredibly useful in numerical analysis, especially for acoustic-type waveforms. This is because analog Bessel filters are characterized by almost constant group delay over any frequency band, and this means that the shape of waves does not change when passed through such a filter.

Well, MATLAB provides some of the building blocks required to create a bandpass analog filter, but does not actually combine the pieces to make a usable filter function.

I created a function for my own research (sourced from pieces I found elsewhere, but it’s been too long—I don’t remember where I found each piece, sorry!), and can be found at my MATLAB repository, specifically, here.

Here’s the documentation that I included with the function:

besselfilter. Function to implement a bandpass Bessel Filter.

[filtData, b, a] = besselfilter(order,low,high,sampling,data)

Inputs:

    - order:      Number of poles in the filter. Scalar numeric value.
                    Eg.: 4  
    - low:        Lower frequency bound (Hz). Scalar numeric value.
                    Eg.: 50000 (= 50kHz)
    - high:       Upper frequency bound (Hz). Scalar numeric value.
                    Eg.: 1000000 (= 1MHz)
    - sampling:   Sampling frequency (Hz). Scalar numeric value.
                    Eg.: 25000000 (= 25MHz)
    - data:       Input data. Numeric vector.
                    Eg.: data vector of size (n x 1)

Output:

    - filtData:   Output filtered data. Numeric vector. 
                    Eg.: data vector of size (n x 1)
    - b, a:       Transfer function values for the filter. Scalar numeric.

Matlab: find a string within a cell array of strings

I just wanted to jot down a few points about Matlab programming. Specifically, this is about finding a string within another cell array of strings, where the thing I’m really interested in is the index of the cell array where the reference string occurs. For example, if my reference string is 'Gamma', and my cell array is {'Alpha','Beta','Gamma','Delta'}, then the result of the code should be 3.

Say,

cellArray = {'Alpha','Beta','Gamma','Delta','GammaSquared'};
refString = 'Gamma';

Method 1

This method uses the Matlab function strfind (link).

index = strfind(cellArray,refString);
index = find(~cellfun(@isempty,index));

Result:

index = 
    3   5

This method works great if the idea is to find a substring, i.e. in the case where we are looking for all possible matches. It doesn’t work too well, however, if we’re looking for a specific match.

Method 2

This uses the Matlab function ismember (link).

index = find(ismember(cellArray,refString));

Result:

index = 
    3

Works great if the idea is to find a perfect match. However, let’s also keep tabs on the computation time.

tic; index = find(ismember(cellArray,refString)); toc;

Result:

Elapsed time is 0.001047 seconds.

Method 3

This uses the Matlab function strcmp (link).

index = find(strcmp(cellArray,refString));

Result:

index = 
    3

Same result as in Method 2, but what about computation time?

tic; index = find(strcmp(cellArray,refString)); toc;

Result:

Elapsed time is 0.000025 seconds.

Turns out Method 3 is more than 41 times faster to execute. So we have a winner!

Reference

Stack Overflow: How to search for a string in cell array in MATLAB?