Thursday, 19 April 2007

Ignoring folders with TODO Bundle in TextMate

TextMate's TODO bundle lets you keep track of the TODOs in your projects. You can mark a line to show up in the TODO list with FIXME, TODO or CHANGED. The following two images show a text file with these keywords and the resulting TODO list. The only problem is that if you keep a copy of Rails in vendor/, then the TODO plugin will also show the items in Rails (or any other external libraries and plugins you have in your project. This blog entry on describes how to get rid of the external items in your project with a sledgehammer: Add the vendor/rails folder to your global TextMate ignore list. However, there is a cleaner way: Open TextMate's Preferences Pane, go to Advanced » Shell Variables and the shell variable TM_TODO_IGNORE. You can set this variable to a regular expression as you would use it in Ruby (since the TODO bundle is written in Ruby). Et voilá: Rails-less TODO lists.

Wednesday, 11 April 2007

The Joys Of Floating Point Numbers

Sometimes, floating point numbers are the thing that futile hours of programming are made of. Whether they come as 32, 64 or any other precision you want, they can truly give you unnecessary and unexpected work if you do not use them correctly. The problem I stumbled over yesterday was the problem of rounding errors. Consider the following C snippet
double pi = 3.1415926535897932384626433832795;
double val = sqrt(pi) * sqrt(pi);

printf("pi  = %f\n", pi);
printf("val = %f\n", val);
which yields
pi  = 3.141593
val = 3.141593
Which is what we expected! val is the same as pi (the square is the inverse of the square root as we all know). However, the following C snippet shows the problem we run into:
if (val == pi) {
 printf ("pi == val\n");
} else {
 printf ("pi != val\n");
which yields
pi != val
So two apparently equal values are not equal any more! Where is the problem? It is in the precision of printf(), of course. We modify the first snippet so the printf() calls print the floating point values to more significant places
printf("pi  = %.16f\n", pi);
printf("val = %.16f\n", val);
and now we get
pi  = 3.1415926535897931
val = 3.1415926535897927
So how do we resolve this? Instead of comparing floating point values directly, we calculate their difference and make sure it is below a small value, for example 1e-9:
if (abs(val - pi) < 1e-9) {
 printf ("pi == val\n");
} else {
 printf ("pi != val\n");
which now gives us
pi == val
I guess most programmers are aware about the problems with floating point numbers and I have also known about the problem for a long time. What made me stumble, however, was that I trusted the printed value - which was inaccurate. As far as I know, there is no generic solution to the problem with floating point inaccuracies. You will get away with comparing whether the difference between two floating point values is smaller than a given value for comparing. In Ruby, you could hack the built in Float class with the following:
class Float
  def ==(value)
    self - value < 0.01
This allows us to compare floating point values “fuzzily”:
>> Math.sqrt(2) ** 2 == 2.0
=> false
>> require 'floathack'
=> true
>> Math.sqrt(2) ** 2 == 2.0
=> true
However, I do not think that this is a good idea! Overwriting the == operator of the Float as above breaks its transitivity, i.e. we get the following (in-)equalities 1.99 == 2.00 == 2.01 but 1.99 != 2.01 Another idea would be only to compare the first n numbers after the point, e.g. consider 3.1415926535897931 to be 3.141 for n = 2. However, this does not help us if a floating point error gives us two values just next to a “boundary”, i.e. a result of 1.999 would not be considered correct if 2.001 was expected and we only cared about the result being “sufficiently close” to the expected value. So be careful when using floating point values are checking two of them for equality. Trouble will strike and most probably find some way to trick you into wasting time. An extensive, academic - and arguably dry - discussion of the implications of floating point arithmetics can be found in What Every Computer Scientist Should Know About Floating-Point Arithmetic, by David Goldberg.

Saturday, 7 April 2007

SQLite gotcha

SQLite is great. No, really, I do mean it. If you ever need to work on structured data and editing it from the outside is a must, then SQLite is worth checking out and might be a superior choice to XML and will be a superior choice to your own proprietary format.

I cannot speak about performance - there are some outdated claims that SQLite is faster than postgres and mysql for the most common operations; but those benchmarks don't feature a single join, and that's where database implementation gets interesting and hard. I just cannot believe that SQLite stands up to any full scale DBMS.

I spent a few hours chasing down a bug in my own code after realising that I've been utterly stupid and put my parameters in the wrong order into my parameter list.

To boil it down, this is what happens in SQLite if I compare an integer field with a string in a WHERE part of a statement:

sqlite> CREATE TABLE my_favourite_numbers (number integer);
sqlite> SELECT * FROM my_favourite_numbers WHERE number = "foo";

You get no result. No error is thrown, like in Postgres:

# CREATE TABLE my_favourite_numbers (number integer);
# SELECT * FROM my_favourite_numbers WHERE number = 'bla';
ERROR: invalid input syntax for integer: "bla"

I know now what to look out for in the future.

Friday, 6 April 2007

First Post

Obviously, this is the first entry of the shining new Bitwiese Weblog. Feel warmly welcomed by these few bytes which propably travelled kilometre over kilometre from here to Blogger's servers and back again to you. This weblog is brought to you by Justin and me, Manuel. We hope to publish information related to the following
  • programming
  • computers in the widest sense
  • computer science (prepare for some math, hehe).
So have fun and stay tuned.

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