Why Constraint Programming

The N-Queens puzzle is very popular and we were able to find several working examples in C and in Prolog.  We analyzed, compiled and tested all these programs and weighed all the pros and cons these two types of languages.

Prolog, being a higher level language, is intuitively easier to understand and easier to program. You can think about the problem with more abstraction. The challenge in actually programming in prolog is that you need to be able to correctly “define” and thus “constrain” the program to do what needs to be done. You don’t have to worry about data types, pointers, data structures and other lower level issues. All the prolog solutions we came across were between 40 and 50 lines of code. On the downside, it was inconvenient when trying to test any n size greater than 12. Trying to solve the puzzle for n=13 takes about 148 seconds.

We found one good example of the n queens problem written in C. We compiled it under Visual Studio 2008, and ran it. The obvious upside in the C program was the speed. It could still solve the puzzle in under a second for n=13. That’s a huge difference compared with the prolog program.  The downside is trying to understand the code and trying to write the code. It’s not as obvious to see what’s happening. The level of abstraction is not as narrow as it is in prolog. On top having to worry about correctly implementing a solution, you have to worry about correctly defining a data structure, defining the correct data types, properly handling arrays and pointers, and other lower level issues not in any way related to the puzzle’s algorithm. The C program that we were testing was 400 lines of code. 236 lines if you remove the comments. That’s almost 5 times the size of the prolog program.

Time Comparisons between Prolog and C

Next, we wanted to discuss the simplicity in solving the n-queens problem in prolog. Let’s define our solution for N=4.

We define our solution by stating that each of the 4 queens must occupy it’s very own row, column, downward diagonal, and upward diagonal on a 4×4 board.

And that’s our definition! Now we need to tell prolog to do this and write our constraints.

%% DEFINE OUR BOARD     
%% sq(y-coord, x-coord, downward-diag, upward-diag) 
 sq(1,1,4,1).
 sq(1,2,5,2).
 sq(1,3,6,3).
 sq(1,4,7,4).
 sq(2,1,3,2).
 sq(2,2,4,3).
 sq(2,3,5,4).
 sq(2,4,6,5).
 sq(3,1,2,3).
 sq(3,2,3,4).
 sq(3,3,4,5).
 sq(3,4,5,6).
 sq(4,1,1,4).
 sq(4,2,2,5).
 sq(4,3,3,6).
 sq(4,4,4,7).
 
%% CONSTRAINT
%% We define our solution by stating that
%% each of the 4 queens must occupy it’s
%% very own row, column, downward diagonal,
%% and upward diagonal on a 4x4 board.
 chess(A,B, E,F, I,J, M,N) :-
 sq(A,B,C,D), sq(E,F,G,H), sq(I,J,K,L), sq(M,N,O,P),
 A=1, E=2, I=3, M=4,
 A\=E, A\=I, A\=M,
 B\=F, B\=J, B\=N,
 C\=G, C\=K, C\=O,
 D\=H, D\=L, D\=P,
 E\=I, E\=M,
 F\=J, F\=N,
 G\=K, G\=O,
 H\=L, H\=P,
 I\=M,
 J\=N,
 K\=O,
 L\=P.

Here is our output once we run our program:

?- chess(A,B,E,F,I,J,M,N).
 
A = 1,
B = 2,
E = 2,
F = 4,
I = 3,
J = 1,
M = 4,
N = 3 ;

Which means:  { Q1(1, 2) , Q2(2, 4) , Q3(3, 1) , Q4(4, 3) }

Which is of course, the right solution. Here was the other solution.

A = 1,
B = 3,
E = 2,
F = 1,
I = 3,
J = 4,
M = 4,
N = 2 ;

Which means:  { Q1(1, 3) , Q2(2, 1) , Q3(3, 4) , Q4(4, 2) }

I can tell you now that the CSS hacks for a sticky footer doesn’t cooperate well when you have margins declared in other div classes. I spent about an hour trying to get my footer to work right. It would work well in some pages but in others, would display a vertical scroll bar with 20 pixels or so left to scroll.

I got fed up and decided to try and use JQuery to solve my problems. It worked brilliantly! And it’s as simple as this. If my window height is greater than my content plus footer height, add the appropriate top padding to the footer that will keep it always on the bottom. If the content height is greater than the windows height, leave the padding as it was previously declared in your css. And just make sure you calculate the top padding on page load and when the window is resized.

Here is the code. Just put it at the end of your page somewhere. Make sure you change the class name ‘.footer’ with whatever class or id name you are using for your footer.

$(function(){
	positionFooter(); 
	function positionFooter(){
		var padding_top = $(".footer").css("padding-top").replace("px", "");
		var page_height = $(document.body).height() - padding_top;
		var window_height = $(window).height();
		var difference = window_height - page_height;
		if (difference < 0) 
			difference = 0;
 
		$(".footer").css({
			padding: difference + "px 0 0 0"
		})
	}
 
	$(window)
		.resize(positionFooter)
});

Window->Preferences->Web->HTML Files->Editor->Typing

And then uncheck everything. You should be good to go.

Personally, I cater towards a screen size of 1024 x 768. The web page you are viewing now has a fixed width of 900 pixels. Here is a list of widths of what other websites are using:

Here are some statistics on screen size usage from w3schools.com

More specific screen resolution statistics from w3schools.com.

If Employee table has three rows id,name and address. How do we fetch those names (id,name,address) using sql query in cakephp?

First of all, you can’t actually use mysql_field_name() in cakephp. But there are workarounds. Say you want to find the field names of the the table ‘Post’. Here is what you can do.

$array = $this->Post->find('all');
 
$array = Set::extract('/0/Post', $array);
pr($array);
 
$field_names = array_keys($array[0]['Post']);
pr($field_names);

The result of the print statements will display the following

Array
(
    [0] => Array
        (
            [Post] => Array
                (
                    [id] => 2
                    [title] => A title once again
                    [body] => And the post body follows.
                    [created] => 2010-02-08 07:48:03
                    [modified] => 
                )
 
        )
 
)
 
Array
(
    [0] => id
    [1] => title
    [2] => body
    [3] => created
    [4] => modified
)

I hope this helps. Feel free to comment on this page. I’ll get notified immediately.

1. Dramatically increase the rate at which Google crawls my site. Before, Google crawled my site once a month. But Web Krunk provided me with great tips and now Google crawls my site immediately after I post new articles.
2. I was also able to achieve a higher ranking in Google’s organic search for certain keywords vital to the success of my business.
3. I have a better understand now on how to use twitter and certain wordpress plugins to increase my traffic.

Blogging is one of the best ways for anyone to get ahead of the curve. I never ever pictured myself blogging. But there is a lot power in it if you can utilize it correctly. If you are new to blogging, check out webkrunk.com. It is really helping me to bring more traffic to my site.

Also, if you know of any other good blogging sites out there, leave a comment. I would love to check it out.

<title>
    <?php wp_title('&laquo;', true, 'right'); ?> 
    <?php bloginfo('name'); ?>
</title>
 
/*
* This Generates:
* <title>
*     Your Blog Name
* </title>
*
* when you are at you home page. And:
*
* <title>
*     Your Post Title « Your Blog Name
* </title>
*
* when you are on a post page
*/

The problem is that wp_title() doesn’t have any title to return because your blog page isn’t an editable page in your dashboard. Here is a small hack to get it to work. You will need to make sure to fill out your tagline in Administration->Settings->General->Tagline. This input will act as your title.

<title>
    <?php if (is_home()) : 
        bloginfo('description'); 
        echo " &laquo; "; 
    else : 
        wp_title('&laquo;', true, 'right'); 
    endif;?> 
    <?php bloginfo('name'); ?>
</title>
 
/*
* This Generates:
* <title>
*     Your Tagline Description « Your Blog Name
* </title>
*
* when you are at you home page. And:
*
* <title>
*     Your Post Title « Your Blog Name
* </title>
*
* when you are on a post page
*/

Dreamweaver was the first IDE I ever used for developing websites. It has a nice color scheme and the ftp support is very solid. The only draw back is that it can be a little slow at times to upload and download files. Dreamweaver supports auto-upload on save command. It also nicely synchronizes all your server files with your local files. It feels good to know that I always have the latest back up of my website. What I don’t like about it is that it’s a very heavy program. It’s designed not just for coders, but also for designers. There are more configuration options than I care to think about. It’s also a very expensive piece of software.

Next comes Notepad++. This text editor is so close to being the perfect text editor. If it had a better ftp plugin, I would give this editor a grade of a 100. Sadly though, the ftp plugin is quite annoying. First, if you are editing a file for more than 15 minutes without using the ftp, the plugin will automatically disconnect and collapse your tree structure. I get quite a few errors when trying to run general operations like save or download. There have been a few times where the ftp plugin lost some of my files. When downloading, it would download 80% of the file without displaying an error. So if I was editing code somewhere along the top, and a uploaded the file back to the server, I would lose the bottom 20% percent. If you have WAMP server and you are coding locally, Notepad++ is perfect. But if you have to edit files through ftp, It can be a little frustrating.

Next comes Aptana Stuido. I was very excited when I heard and read about Aptana. First, their ftp support is very good. I can actually open up multiple ftp sites, and drag and drop files between them. It also has nice CSS support like Dreamweaver. The downside is that it’s written in JAVA. Not to say there is anything wrong with JAVA applications, but it can be a little slow at times. Sometimes, I feel like there is a small delay between my keystrokes and the text display. It’s not the most stable program and Aptana has crashed enough for me to make that comment. Sometimes, the FTP can be slow and I just end up using Filezilla to transfer images and videos. It’s a bit of a hassel to set up at first because you have to install the PHP and JQuery packages in order to start editing files. But, I’m just being really picky. Aptana is still a good Web Development IDE and it’s the application I use when I develop websites on Windows. Although, it’s definitely not my default text editor. It’s still a little too heavy to get that kind of treatment. Notepad++ gets that award.

Finally we have Coda! It’s just a beautiful text editor. It’s lightweight, but at the same time, you can do so much with it. It’s FTP support is fast and stable. It almost feels like I’m browsing a local directory. I just get things done faster when working in Coda, and I also feel happier. This is the difference between free open source software and paying for software. Coda cost a $100 to use. But it’s true that you get what you pay for. All the other text editors listed above are free. Also take note that Coda is only for Mac OS.

Which now brings up another question. Should you develop websites on a Mac or a Windows PC?

Honorable Mentions

Gedit – Gedit is for linux. I feel like I spend more time trying to customize it and make it my perfect text editor than actually programming on it. But it’s very good.

UltraEdit – This text editor is not free. But it’s pretty good. I didn’t actually purchase it because Aptana could do everything I needed to do while still being free. UltraEdit is like Notepad++, but with better support.