I have a lot of experience with jQuery, but less with Prototype. Recently, I needed to add some event handlers to some elements in a Ruby on Rails app I’m building. I searched for how to do the equivalent of jQuery’s $(document).ready() function in Prototype so that I could add the handlers after the document loaded, but most of the guides I found were out of date (I’m running Prototype 1.6.0.3, and I don’t know which version these guides were for, but they all made my Javascript console angry).

Eventually, I was able to piece it together after digging through the depths of the Prototype API documentation. It’s actually very simple:

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document.observe('dom:loaded', function(){
  // do yo thang...
});

Wrap whatever you’re doing with that, and it won’t be run until the document is loaded.

I’ve been banging my head against this wall for quite awhile now, and I just finally figured out the answer. Like I’ve done in other posts, I’ll just post what worked for me, and hopefully it’ll help other people.

I’m running Ruby 1.9 and Ruby on Rails 2.3.3 on Snow Leopard. I’ve been trying to install plugins (specifically, Authlogic and Carmen) for a couple days now using the following two commands (as taken from the main github pages):

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script/plugin install git://github.com/binarylogic/authlogic.git
script/plugin install git://github.com/jim/carmen.git

In return, I received the following errors:

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Plugin not found: ["git://github.com/binarylogic/authlogic.git"]
Plugin not found: ["git://github.com/jim/carmen.git"]

After a lot of poking around, it turns out you need to make two changes in order for this to work on Rails 2.3 or higher: change the git:// at the beginning of each URL to http://, and add a trailing slash to the end of each URL. So instead, run these:

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script/plugin install http://github.com/binarylogic/authlogic.git/
script/plugin install http://github.com/jim/carmen.git/

They both worked perfectly for me, so hopefully they’ll work for you. If not, leave a comment and I’ll try to help.

Here’s another in my series of “Installing X on Snow Leopard”. These aren’t official, well-tested guides; they’re just documentations of my attempts to compile and install various things on my personal computer. My last one (Installing MySQL on Snow Leopard) is my most popular post to date (aside from a couple that have been on Reddit). Erlang is less popular than MySQL, but hopefully this will still help a few people.

Downloading and unpacking

Go to http://erlang.org/download.html and download the Source for the newest version (when I was writing this, that was R13B03. After downloading, extract it to somewhere that’s convenient to get to with the Terminal.

Configure

Open the Terminal and cd into the directory you extracted Erlang to (mine was /Users/jake/src/otp_src_R13B03. Then run the following command:

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./configure \
    --prefix=/usr/local/ \
    --enable-smp-support \
    --enable-threads \
    --enable-darwin-64bit

Note: You will probably get three errors. Read about them in the Configuration Errors section coming up.

The first three configure options are the defaults according to the README. However, I’ve had experiences where supposed defaults aren’t really the defaults when compiled on OS X, so I don’t like to take chances. --enable-darwin-64bit enables experimental support for the 64bit x86 Darwin binaries. This may not be necessary, but in general, 64-bit stuff has fewer problems on Snow Leopard, so I figured this was a good idea.

Configuration Errors

I got the following configuration errors:

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jinterface    : No Java compiler found
wx            : Can not combine 64bits erlang with wxWidgets on
                MacOSX, wx will not be useable
documentation : fop is missing. The documentation can not be built.

These aren’t a problem. If you get any errors besides these, you’re in trouble. Leave a comment, and I’ll see if I can help.

Making and installing

These two commands shouldn’t give you any trouble:

make

And then, after make is done:

sudo make install

If you get any errors at either of these stages, leave a comment and I’ll try to help.

Making sure it works

Note: This canonical test is gratefully borrowed from erlang.org.

Put the following into a text file:

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-module(test).
-export([fac/1]).

fac(0) -> 1;
fac(N) -> N * fac(N-1).

Save it as test.erl in a directory that’s easy to get to with the Terminal. Then, from the Terminal, cd into that directory and type erl (which, if everything worked right, should start the Erlang command-line interpreter). From the interpreter, run the following commands:

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1> c(test).
{ok,test}
2> test:fac(20).
2432902008176640000
3> test:fac(40).
815915283247897734345611269596115894272000000000

Note: Lines starting with N> (where N is a number) are lines you should type (but just type the stuff coming after N>). The other lines represent output.

c(test). compiles test.erl (assuming it’s in the directory you cd‘ed into). test:fac(20). and test:fac(40). runs your factorial function.

So, that’s what worked for me. If anyone has any problems along the way, leave a comment and I’ll try to help.

It’s this culture of attacking women that has especially got to stop. Whenever I post a video of a female technologist there invariably are snide remarks about body parts and other things that simply wouldn’t happen if the interviewee were a man.

Blogger Robert Scoble, responding to threats against tech author Kathy Sierra

For the past few days, I’ve been reading various explanations of the Knuth-Morris-Pratt string searching algorithms. For some reason, none of the explanations were doing it for me. I kept banging my head against a brick wall once I started reading “the prefix of the suffix of the prefix of the…”.

Finally, after reading the same paragraph of CLRS over and over for about 30 minutes, I decided to sit down, do a bunch of examples, and diagram them out. I now understand the algorithm, and can explain it. For those who think like me, here it is in my own words. As a side note, I’m not going to explain why it’s more efficient than na”ive string matching; that’s explained perfectly well in a multitude of places. I’m going to explain exactly how it works, as my brain understands it.

The Partial Match Table

The key to KMP, of course, is the partial match table. The main obstacle between me and understanding KMP was the fact that I didn’t quite fully grasp what the values in the partial match table really meant. I will now try to explain them in the simplest words possible.

Here’s the partial match table for the pattern “abababca”:

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char:  | a | b | a | b | a | b | c | a |
index: | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 
value: | 0 | 0 | 1 | 2 | 3 | 4 | 0 | 1 |

If I have an eight-character pattern (let’s say “abababca” for the duration of this example), my partial match table will have eight cells. If I’m looking at the eighth and last cell in the table, I’m interested in the entire pattern (“abababca”). If I’m looking at the seventh cell in the table, I’m only interested in the first seven characters in the pattern (“abababc”); the eighth one (“a”) is irrelevant, and can go fall off a building or something. If I’m looking at the sixth cell of the in the table… you get the idea. Notice that I haven’t talked about what each cell means yet, but just what it’s referring to.

Now, in order to talk about the meaning, we need to know about proper prefixes and proper suffixes.

Proper prefix: All the characters in a string, with one or more cut off the end. “S”, “Sn”, “Sna”, and “Snap” are all the proper prefixes of “Snape”.

Proper suffix: All the characters in a string, with one or more cut off the beginning. “agrid”, “grid”, “rid”, “id”, and “d” are all proper suffixes of “Hagrid”.

With this in mind, I can now give the one-sentence meaning of the values in the partial match table:

The length of the longest proper prefix in the (sub)pattern that matches a proper suffix in the same (sub)pattern.

Let’s examine what I mean by that. Say we’re looking in the third cell. As you’ll remember from above, this means we’re only interested in the first three characters (“aba”). In “aba”, there are two proper prefixes (“a” and “ab”) and two proper suffixes (“a” and “ba”). The proper prefix “ab” does not match either of the two proper suffixes. However, the proper prefix “a” matches the proper suffix “a”. Thus, the length of the longest proper prefix that matches a proper suffix, in this case, is 1.

Let’s try it for cell four. Here, we’re interested in the first four characters (“abab”). We have three proper prefixes (“a”, “ab”, and “aba”) and three proper suffixes (“b”, “ab”, and “bab”). This time, “ab” is in both, and is two characters long, so cell four gets value 2.

Just because it’s an interesting example, let’s also try it for cell five, which concerns “ababa”. We have four proper prefixes (“a”, “ab”, “aba”, and “abab”) and four proper suffixes (“a”, “ba”, “aba”, and “baba”). Now, we have two matches: “a” and “aba” are both proper prefixes and proper suffixes. Since “aba” is longer than “a”, it wins, and cell five gets value 3.

Let’s skip ahead to cell seven (the second-to-last cell), which is concerned with the pattern “abababc”. Even without enumerating all the proper prefixes and suffixes, it should be obvious that there aren’t going to be any matches; all the suffixes will end with the letter “c”, and none of the prefixes will. Since there are no matches, cell seven gets 0.

Finally, let’s look at cell eight, which is concerned with the entire pattern (“abababca”). Since they both start and end with “a”, we know the value will be at least 1. However, that’s where it ends; at lengths two and up, all the suffixes contain a c, while only the last prefix (“abababc”) does. This seven-character prefix does not match the seven-character suffix (“bababca”), so cell eight gets 1.

How to use the Partial Match Table

We can use the values in the partial match table to skip ahead (rather than redoing unnecessary old comparisons) when we find partial matches. The formula works like this:

If a partial match of length partial_match_length is found and table[partial_match_length] > 1, we may skip ahead partial_match_length - table[partial_match_length - 1] characters.

Let’s say we’re matching the pattern “abababca” against the text “bacbababaabcbab”. Here’s our partial match table again for easy reference:

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char:  | a | b | a | b | a | b | c | a |
index: | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 
value: | 0 | 0 | 1 | 2 | 3 | 4 | 0 | 1 |

The first time we get a partial match is here:

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bacbababaabcbab
 |
 abababca

This is a partial_match_length of 1. The value at table[partial_match_length - 1] (or table[0]) is 0, so we don’t get to skip ahead any. The next partial match we get is here:

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bacbababaabcbab
    |||||
    abababca

This is a partial_match_length of 5. The value at table[partial_match_length - 1] (or table[4]) is 3. That means we get to skip ahead partial_match_length - table[partial_match_length - 1] (or 5 - table[4] or 5 - 3 or 2) characters:

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// x denotes a skip

bacbababaabcbab
    xx|||
      abababca

This is a partial_match_length of 3. The value at table[partial_match_length - 1] (or table[2]) is 1. That means we get to skip ahead partial_match_length - table[partial_match_length - 1] (or 3 - table[2] or 3 - 1 or 2) characters:

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// x denotes a skip

bacbababaabcbab
      xx|
        abababca

At this point, our pattern is longer than the remaining characters in the text, so we know there’s no match.

Conclusion

So there you have it. Like I promised before, it’s no exhaustive explanation or formal proof of KMP; it’s a walk through my brain, with the parts I found confusing spelled out in extreme detail. If you have any questions or notice something I messed up, please leave a comment; maybe we’ll all learn something.

If you write software, you need to write unit tests. If you’ve written a method/function, and you haven’t written a unit test for it, it’s safe to assume that it’s broken (even if it compiles and your other tests pass).

I’m not necessarily advocating full-fledged test-driven development. I’m just saying, if you release code into “the wild,” and there are methods you haven’t unit tested, your customers will almost certainly run into multiple bugs in each one of them.

That’s an atomic point. Separate from that, I’d like to mention that this isn’t always a bad thing. For a startup that wants to iterate as quickly as possible (and is writing non-life-critical software), writing the code with no unit tests, releasing it, and reproducing each customer-discovered bug with a unit test before fixing it is a totally reasonable model. These startups just shouldn’t operate under the illusion that their software “works”. In the hours after they make one of these releases, they should feel blessed if a single customer is able to register or log in.

I’m much less experienced in C than I am in higher-level languages. At Cisco, we use C, and I sometimes run into something that would be easy to do in Java or Python, but very difficult to do in C. Now is one of those times.

I have a dynamically-allocated array of unsigned integers which I need to convert to a comma-separated string for logging. While the integers are not likely to be very large, they could conceptually be anywhere from 0 to 4,294,967,295. In Python, that’s one short line.

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my_str = ','.join([str(num) for num in my_list])

How elegantly can people do this in C? I came up with a way, but it’s gross. If anyone knows a nice way to do it, please enlighten me.

I’ve spent the last month or so teaching myself the basics of Ruby on Rails. By far my favorite thing about it is that, at every turn, I find a little feature or helper that works really well and does exactly what I want without feeling messy or “magical”.

Unfortunately, I have no one to share these little joys with, as pretty much none of my programmer friends know Ruby or Ruby on Rails. So, I figured I’d start sharing them on here. Hopefully, Rails newbies (like myself) will learn something, and Rails veterans will be able to chuckle knowingly as they fondly look back on the glory days of learning Rails.

Renaming Associations

If you have one model that belongs_to another, the association (in both directions) is named after the models. For example, if I have Users and Courses, and I want a course to belong_to its teacher (who is a User), I would start with the following:

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class User < ActiveRecord::Base
  has_many :courses
end

class Course < ActiveRecord::Base
  belongs_to :user
end

This is nice and simple, but if I want to access a course’s teacher, I have to do it like this:

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my_course = Course.first
puts my_course.user # Print out the course's teacher

That’s a little confusing. What if my course also has_many students (also Users)?

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my_course = Course.first
puts my_course.users.first # Print out the course's first student

So, my_course.user is the teacher, and my_course.users are the students. There’s nothing to indicate that, I just have to know it. Wouldn’t it be nicer if the associations could be named for what they represent?

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class Course < ActiveRecord::Base
  belongs_to :teacher, :class_name => 'User'
end

To make this work properly, the foreign key needs to be teacher_id instead of user_id (that can be overridden too, but I won’t go into that right now). Then, you get to do this:

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my_course = Course.first
puts my_course.teacher # Print out the course's teacher

Much better!

Now, I recognize that this is far from a revolutionary feature. I’m pretty sure it’s possible in Django and most other MVC frameworks. I was just very happy to bump into it.

Early on at Cisco, I realized that it’s really beneficial to write down (step by step) what I’ve done when I install something new. It forces me to think about what I’m doing, it provides me with a guide in case I mess up and have to start over, and other people can benefit from it later.

With that in mind, I’ve decided to document my installation of MySQL on Snow Leopard (OS X 10.6). Hopefully, someone will get some use out of it, but if not, at least I’ll have documentation of what I did.

The PATH Variable

MySQL has a bunch of useful executables that aren’t in PATH by default. I could symlink to them, but I think that’s less maintainable than just appending to PATH.

1. Open the Terminal

2. Open up the .bash_profile file in your home directory. This is a bash script that runs every time you start the Terminal, so the PATH variable will be extended properly every time. If you have TextMate and its UNIX command line tools installed, do it like this:

   bash mate ~/.bash_profile

   Otherwise, do it like this:

   bash /Applications/TextEdit.app/Contents/MacOS/TextEdit ~/.bash_profile &

   If you use the second one, make sure to add the ampersand at the end.

3. We want to add MySQL’s bin folder to PATH. MySQL’s going to be installed at /usr/local/mysql, so let’s add /usr/local/mysql/bin to PATH. Add the following line to .bash_profile:

   bash export PATH="/usr/local/mysql/bin:/usr/local/sbin:$PATH"

   This replaces PATH with two new locations and the old value of path (so essentially, appending the two new locations to the beginning of PATH). You’ll notice that, in addition to /usr/local/mysql/bin (which I mentioned earlier), I also added /usr/local/sbin. OS X doesn’t include this in PATH by default, but I think it should, so I added it. I have no defense for that position, but this is as much a guide for myself as it is a guide for other people, so I don’t need to defend it :)

4. Save the updated .bash_profile and close it.

5. Quit and reopen Terminal so that .bash_profile will be rerun (you could also run it explicitly, but I prefer quitting and reopening).

6. Run the following command:

   bash echo $PATH

   You should see /usr/local/mysql/bin and /usr/local/sbin in there now.

7. Edit: Restart your computer. My installation worked without doing this, but Jon (in the comments) said he needed to do this before it worked, so you might as well.

Downloading + Installing

I might try compiling and installing from source at some point, but I don’t see any reason to when there are official OS X binaries available. Worst case scenario, it messes with a bunch of my settings, and I’ll uninstall and then do it from source.

1. Go to http://dev.mysql.com/downloads/mysql/5.1.html#downloads
2. Scroll down to the Mac OS X section (near the bottom). Download the version labeled “Mac OS X 10.5 (x86_64)”. It’s very important that you download the 64-bit version. The 32-bit one will give you preference pane problems (I call them PPPs whenever I talk about them, though this is the first time I ever have). Eventually, there’ll probably be a Mac OS X 10.6 version, but for now, this works perfectly.
3. Open the .dmg, then run mysql-x.x.xx-osx10.5-x86_64.pkg. Continue through all the screens without changing anything, until it finishes.
4. Optional: install the preference pane by running MySQL.prefPane.
5. Optional: make MySQL start up along with OS X by running MySQLStartupItem.pkg. I didn’t do this (I often use my computer for things other than development, and I don’t want to bog it down unnecessarily), so I can’t provide any instruction or vouch for how well it works on Snow Leopard.

You should now be able to start MySQL by going into the MySQL preference pane and clicking “Start MySQL Server”. If it doesn’t work, leave me a comment, and I’ll try to help you.

This may be old news to many people, but it’s something I just recently learned (after doing it wrong for about nine months), so I figured someone else may be able to benefit from my mistakes.

For a long time, when I needed to access the currently logged-in user in one of my Django views, they would follow this pattern:

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from django.template import RequestContext

# some other view code

def my_view (request, obj_id):
  context = RequestContext(request)
  obj     = get_object_or_404(SomeModel, pk=int(obj_id))

  # do some stuff, including:
  some_function(context["user"])

  return render_to_response("some_url_name", {"some_var": some_val},
    context_instance=context)

Now, seasoned Django vets (why are you reading this post, by the way?) are probably laughing, but this seemed perfectly fine to me. Luckily, my ignorance was revealed to me while asking on IRC about a problem which, while didn’t seem so at the time, was completely tied to my misuse of RequestContext.

To put it simply, context processors are made to be used in templates. The only time they should ever be instantiated is at the very end of a view, like so:

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return render_to_response("some_url_name", {"some_var": some_val},
  context_instance=RequestContext(request))

I was using them all over views, and even in a few of my decorators. What’s wrong with this? The main thing is, certain mutation functions are sometimes performed when a RequestContext is instantiated (such as user.get_and_delete_messages(), which gets all of a user’s messages from the database and then deletes them), and performing them multiple times before loading the template can cause unexpected results. In my example, I was instantiating a RequestContext in a bunch of decorators, which meant that by the time my template was loaded, all of the user’s messages were deleted (and stored in an earlier instance of RequestContext), making it look to me as if my auth messages were being thrown out.

What’s the solution? Middleware. Middleware allows the programmer to attach variables to the request object before it reaches the view. With this (and the provided django.contrib.auth.middleware.AuthenticationMiddleware), I can still achieve my original goal (accessing the logged-in user from a view), but now I can do it without creating a RequestContext and potentially running mutation functions multiple times:

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from django.template import RequestContext

# some other view code

def my_view (request, obj_id):
  obj = get_object_or_404(SomeModel, pk=int(obj_id))

  # do some stuff, including:
  some_function(request.user)

  return render_to_response("some_url_name", {"some_var": some_val},
    context_instance=RequestContext(request))

I’ve removed all the references to RequestContext from my decorators, and made sure to only instantiate it at the very end of views. Now, messages work perfectly. I’ve even written my own middleware (which you can learn how to do here) to load instances of a few of my own models into the request.

To reiterate, I’m aware that this is common knowledge for many people, but it took me 9 months of moderate Django use and embarrassment on IRC to discover it for myself. Hopefully, this will help someone else in a similar position.