Programming

Regular Expressions : How Exactly they work internally ?

Regular Expressions : How Exactly they work internally ?

Regular expressions were playing major role in computers from several decades, they are not as simple as we think. They are need extensive computational power, a decade before regular expressions were a big headache for computer programmers and scientists because they were consuming lot of time to complete the task.

History behind Regular Expressions :

A neuroscientist  who tried to understand how the human brain could produce complex patterns using simple cells that are bound together. In 1943, Warren McCulloch and Walter Pitts published “A logical calculus of the ideas immanent in nervous activity”.  this paper had a great influence on computer science during that time. In 1956, mathematician Stephen Kleene developed this model one step further. In the paper “Representation of events in nerve nets and finite automata” he presents a simple algebra. The terms Regular Sets and Regular Expressions were born from then. As mentioned above, Kleene’s algebra had only two operations and one function. In 1968, the Unix pioneer Ken Thompson published the article “Regular Expression Search Algorithm” in Communications.

A Pattern in regular expression :

How to learn regular expressions ? How to understand regular expressions?. They look confusing to understand regular expression but regular expressions are an elegant way to do some string manipulations.

We have studied finite automata and the regular expressions all just works on this theory, but Most modern programming languages (Perl, Python, Ruby, Java (and JVM based languages), C#) do not use this approach rather they use recursive backtracking approach. But finite automata is really good to understand regular expressions with its state diagrams and not a bad idea to implement in this approach may be recursive backtracking is better. So do you need a help to understand regular expression? here it is…

Let us take one example, Validating an email address

If this is your regular expression

[a-z][a-z|0-9|]*([_][a-z|0-9]+)*([.][a-z|0-9]+([_][a-z|0-9]+)*)

The respective state diagram might look like below

Regular expression state diagram

Regular expression state diagram

 

Regular expression parser Source Code

If you want to try out with your own regular expression code you can better go through here 

http://www.codeproject.com/Articles/5412/Writing-own-regular-expression-parser

 

MIT’s Robot can jump over hurdles : Cheetah robot

MIT’s Robot can jump over hurdles : Cheetah robot

Human’s can achieve anything, human mind is the greatest gift of god. If you run in hurry or if you run on a moving floor your mind will get confused and some times you fell down. If you are not practiced then you can’t even cross two three hurdles continuously, you will fell down. Have you ever wondered a robot can do this ? it is so difficult to program even a simplest automation or even a simplest robot which can just move by itself. Now we have everything ready made so you may say it is not so difficult, but Cheetha which can run and jump over the hurdles ? really challenging.

A MIT’s Cheetah Robotic TEAM

Deborah Ajilo, Negin Abdolrahim Poorheravi,John Patrick Mayo,Justin Cheung, Sangbae Kim, Shinsuk Park, Kathryn L. Evans, and Matt Angle.  Will Bosworth, Joao Luiz Almeida Souza Ramos, Sehyuk Yim, Albert Wang, Meng Yee Chuah, and Hae Won Park.

Algorithm Behind MIT’s Cheetha robot :

The bounding algorithm which made this possible will program each of the robot’s legs to exert a certain amount of force in the split second during which it hits the ground, in order to maintain a given speed: In general, the faster the desired speed, the more force must be applied to propel the robot forward.

the cheetaha robot adopts the force as per the surface and hence it successfully passed the test in grassy area and treadmill.Robot can run 16 km/h, jump about 40 centimetres high, land safely and continue running for at least 15 minutes .

Technical Details of MIT’s Bounding Algorithm and Robotic Cheetah

It took more than 5 years to achieve this and Cheetah 2 contains the custom electric motor designed by Jeffrey Lang, the Vitesse Professor of Electrical Engineering at MIT and the amplifier designed by David Otten, a principal research engineer in MIT’s Research Laboratory of Electronics. 

“bounding algorithm” calculates the varying amounts of force required to propel an animal forward or to jump over obstacles. Algorithm models the running mechanics of world-class sprinters like Olympian Usain Bolt: the faster the desired speed, the greater the force the legs exert.

This robotic cheetah carries out its movements with the help of battery powered electric motors which do not make much noise even.  Though the US-military has been trying for several years to make a stealthy and fast robot, it hadn’t been successful until now so this DARPA funded research by the MIT researchers may just open a whole new vision regarding the making of a stealthier bot.

 

Android: ItemClickListner with CheckBox or RadioButton.

Android: ItemClickListner with CheckBox or RadioButton.

Lets take a scenario to understand the need of a listview which will have checkbox embedded in it.

Consider the flipkart mobile app, in that we want to compare 2 or more products, in order to do that I will go on selecting the items which I want to compare and then click on the compare button.

 

So we have multiple items in that and hence we cant use the android built in types for creating the list view. Hence we need to follow the below mentioned approach.

 

  1. Create an activity, which contains 2 listviews, one listview for displaying all available mobiles and another listview for displaying selected mobiles.
  2. Create an adapter for filling the mobile list for the above mentioned listviews.
  3. The layout which will be used for creating row which will make ItemClickListner work is as follows,

 

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:descendantFocusability="blocksDescendants"
    android:focusable="false"
    android:focusableInTouchMode="false"
    android:clickable="false">

    <CheckBox
        android:id="@+id/selectMob"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:focusable="false"
        android:focusableInTouchMode="false"
        android:clickable="false"
        />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:weightSum="2"
        android:layout_toRightOf="@id/selectMob"
        android:layout_alignTop="@id/selectMob">
    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Nokia"
        android:id="@+id/make"
        android:layout_gravity="center_horizontal"
        android:layout_weight="1"/>
        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Lumia"
            android:id="@+id/model"
            android:layout_weight="1"/>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:weightSum="3"
        android:layout_below="@id/selectMob">
        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="Android"
            android:id="@+id/os"
            android:layout_gravity="center_horizontal"
            android:layout_weight="2"/>
        <TextView
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:text="20000"
            android:id="@+id/price"
            android:layout_weight="1"/>
        </LinearLayout>

</RelativeLayout>

Observe the following attributes for Checkbox,

        android:focusable="false"
        android:focusableInTouchMode="false"
        android:clickable="false"

These make the checkbox non clickable and eventually the event will be transferred to its parent i.e the row and hence the ItemClickListner will be invoked.

You can download the files here

How To Make You Web Site Faster : HTML , Javascript Tricks

How To Make You Web Site Faster : HTML , Javascript Tricks

 

The performance of your web application is highly important factor to get more user visits, experts have given many advices to improve the performance of a web application and few are listed below.

1. Meta refreshMeta refresh is a method of instructing a web browser to automatically refresh the current web page or frame after a given time interval, using an HTML meta element with the http-equiv parameter set to “refresh” and a content parameter giving the time interval in seconds.

Avoid Meta refresh as much as possible. This will slow down your site.

2. @imports will synchronously load the style sheet, Avoiding @import will improve the performance of your site. Instead of @imports use LINK tag as follows

Use
<link rel=’stylesheet’ href=’knowledge-cess-style.css’>

Than   <style> @import url(‘knowledge-cess-style.css’); </style>

3. Avoid use of multiple framework – Some people use JQuery for some functionality and DOJO for some functionality . Loading of framework will take time and your app becomes heavy. Stick to one framework.

4. Placing scripts at the end

Most people will load all their scripts at the beginning of the page. Load scripts at the end, some times it is not possible, it gives an error if a function being used in the page. In that case make two js files and keep necessary stuff in one js and load it at the beginning.

5. Use ‘differ’ attribute to if some scripts must be loaded at the beginning.

<script src=“knowledge-cess.js” defer></script>

6. To construct dynamic pages it is better to use .innerHTML rather than always creating DOM elements dynamically.

7. Referencing more DOM elements using document.querySelectorAll()

var matches = document.querySelectorAll("div.note, div.alert"); Reference

8. Use Javascript timers carefully. Don't let timers run unnecessarily.
9. Javascript animation and Math

Time to play with Math now, animation is nothing but playing with frames, higher the frame rate smoother the animation and more frames means more processing. The time interval we set for an animation should depend on screen refresh rate.

Understand how Javascript Animation related to Screen refresh rate 

Most screens have refresh rate of 60Hz, so 60 frames per second (60fps) is best frame rate.
Javascript timer will be set in milliseconds. so considering 1000ms and 60FPS which will give
1000ms/60fps = 16.7ms. so it is best timer value, so set 16.7ms or 17ms for your javascript timer while playing with animations.

function setInterval(function(){
          animate();
 },17);

10. Performance factor for PNG and JPG – PNG will be of more size compared to jpg but jpg uses more complicated decoding algorithm compared to png.

Image format PnG vs JPG and performance
The reason png size will be more is because png format will use lossless compression algorithm and jpg will use lossy compression algorithm.
11. Using requestAnimationFrame : Mozilla team proposed requestAnimationFrame method to over come from performance issues in javascript animations. Have a look into this Link
There Many more performance tuning techniques.Now let us look into some best practices of javascript

 

Best Practices in Javascript

 1. Use Shortcut notations

var cities = ["Bangalore","Delhi","Mysore"]

than

var cities = new Array();
     cities[0]='Bangalore'; 
     cities[1]='Delhi';
     cities[2]='Mysore';

2. Keep related stuff together

Best Practice – 

var settings = function(){
     var userDefaults = {
         categories: {"defaultCategory" : "xyz", "lastCategory":"pqr"},
         modules : {}
        }

      function initUserDefaults(){ }


      function clearUserDefaults(){}
   } //End of settings

The above way is clean and better approach rather than separating all methods.

3. Do not use primitive types as objects

 var name = "Ram";  //Best practice
      var name = new String("Ram"); //Not recommended

4. Use === operator

   Always use === operator for comparison, It always converts (to matching types) before comparison.

   The === operator forces comparison of values and type:

 100 == "100" //true which causes problem most of the times
      100 === "100" //false Best Practice, strict type checking

5. Global Variables in Javascript

Global variables are not recommended, Avoiding global variables is best practice.

6. Using JSLint 

Identify most common problems in your javascript code using JSLint tool

 

Check : how Animation in javascript works and how CSS3 is better than jQuerylink