Count distinct values in all columns of a table

The following rake task will show the number of different values of each column of a given table in the database. I wrote it to see statistics on which columns are really used.

namespace :db do
 desc "Count distinct values for each non empty column of a table (TABLE=xxx) "+
      "optionally using a condition (WHERE=\"yyy\")"
 task :count_distincts => :environment do
   raise "Specify option TABLE=<table_name>" unless ENV["TABLE"]
   puts "Distinct values in table #{ENV['TABLE']}:"
   puts "(condition: where #{ENV['WHERE']})" if ENV['WHERE']
   c = ActiveRecord::Base.connection
   columns = c.select_all("describe #{ENV['TABLE']};").map{|a|a["Field"]}
   columns.each do |col|
     sql = "select count(distinct #{col}) as c from #{ENV['TABLE']}"
     sql << " where #{ENV['WHERE']}" if ENV['WHERE']
     n = c.select_one(sql)['c']
     puts "#{col}: #{n}" unless n == "0"
     STDOUT.flush
   end
 end
end

assert.h

Assertions: you express your expectations in a certain position of the code, and this way you avoid bugs that would otherwise very difficult to trace.

In C use the standard library macro assert. You must #include <assert.h>, then you assert(expression); in the code, and if the expression is evaluating to 0 / false, the program aborts with an error msg printed on sterr that tells you which assertion fails and where it is (file/line).

It is also possible to turn assertions off, adding a #define NDEBUG before the #include <assert.h>.

Why main is int and not void.

The standard says so. And it actually makes a difference: here is a detailed discussion about this topic: http://users.aber.ac.uk/auj/voidmain.shtml

Why do you have to pass scanf the address of the variable to write in?

As I was learning C the first time, I remember I was asking this myself... I would have preferred it to return the scanned values, something like:

myVar = scanf("%s") /* don't do this :) */

but of course, in this case (1) you could assign only 1 variable, (2) you would have not had the return value (number of read items)...

Here is some discussion of it: ...

Turn colors on in vi/vim

If the color syntax highlighting is off, you can turn it on by editing (or creating) ~/.vimrc, adding the following line:

:syn on

square root

When I have some time, I will have a look to this algorithm to calculate the square root of a number:

    int sqrt(int num) {
     int op = num;
     int res = 0;
     int one = 1 << 14; // The second-to-top bit is set: 1L<<30 for long


     // "one" starts at the highest power of four <= the argument.
     while (one > op)
         one >>= 2;

     while (one != 0) {

         if (op >= res + one) {
             op -= res + one;
             res += one << 1;
         }

         res >>= 1;
         one >>= 2;
     }
     return res;
 }

(from Wikipedia)

XSLT

XSLT: take an XML doc and, through a sort of "stylesheet", make something with the data (another XML doc, or perhaps a XHTML doc). I don't know this language but I guess it's rather useful (although I have no use for it at the moment).

related stuff: libxml, libxml2, libxslt, xsltproc

Links:
- specifications@w3: http://www.w3.org/TR/xslt

- tutorial@w3: http://www.w3schools.com/xsl/
- http://en.wikipedia.org/wiki/XSLT
- http://en.wikipedia.org/wiki/LibXML

PragProg media: Expression Engine Techniques

I often have a look at the list of media of "PragProg" (http://www.pragprog.com/categories/all?sort=pubdate). The newest title is the screencast serie: "Expression Engine Techniques"

Well I definitely don't need it, anyway I'm always curious, so here is the info I collected (mainly from Wiki) about the topic:

ExpressionEngine http://expressionengine.com/ is a CMS, developer: EllisLab, there is a free version, and two paid ones. A "2.0" is expected in 2009, will be based on CodeIgniter, which is a PHP framework.  

capistrano using git

The cap deploy:update task when you are using git runs the following git command: 

git checkout -q -b deploy 3fe75somehash.....

meaning : 

- creates a new branch named "deploy" (what happens if there is already one called like that?)

- the source of the branch is the commit identified by the given hash

- -q option is "quiet mode"

rsync and symbolic links

some possible behaviours of rsync (from the man page):

(1) SKIP:

default case => symlinks are simply not followed

(2) COPY THE LINK:

"symlinks are recreated with the same target on the destination"

rsync --links

also: rsync --archive implies --links

(3) FOLLOW THE LINK

rsync --copy-links

safe/unsafe:

relevant for case 3 is the "safe" vs. "unsafe" difference which I did not understand good.

This is what is written: "An example where this might be used is a web site mirror that wishes ensure the rsync module they copy does not include symbolic links to /etc/passwd in the public section of the site. Using --copy-unsafe-links will cause any links to be copied as the file they point to on the destination. Using --safe-links will cause unsafe links to be ommitted altogether."

Always back up on a dedicated partition or disk!

My home dir on a certain server was backed up by another company. I set under my home a soft link to another location (several Gb of data). What I didn't know: their backup script was rync-based *with* the option --copy-unsafe-links. Rsync followed the link and backed up tons of stuff clugging the backup hard disk. They used the same hard disk and the same partition also for another function: email server. So that softlink disrupted the email server! At first I though I was to be blamed, but after a second thought, I understood, that it's not 100% true: they should have *never* run the mail server on the same partition where they backed up that stuff!

Ruby and GUIs

Is there a good ruby GUI toolkit?

I want to write a simple app, with a simple DB backend, just to keep some notes. As I want it to run on my laptop, I don't want any webserver running, so no rails app (and also because I want to do some gui programming, it's years I am only working on console or web apps). 

I started earlier this morning my Google quest to answer this question, I am not so far yet. I am no expert in GUIs. 

Some disorganized thoughts:

- Java of course is a good choice, isn't it? Yes I want to learn Java again (I learned it 10 years ago, and not using it a long time, so I guess my knowledge is totally out-of-date). But now I want just to write a little small app in that simple little lovely ruby language. 

- I guess most Win apps are based on "native" widgets. For that maybe you need VS or the like and I am in this moment in no mood to be a MS fan. And I want something cross-platform. 

- There a libraries, and like always in IT a lot of names and acronymes, just to confuse stupid newbies like me. QT is one, I think, then there are others (I think some GUI library-flame is also the reason why Gnome and KDE are 2 different desktops, isn't it?). So I guess you use one of that with some good ruby bindings and you are on it. Right?

- I read of Shoes: very simple, maybe good idea, but probably just for beginners, I don't know if I want to waste my time with it. Looks like is just something for kids learning programming?