Saturday, December 10, 2005

A Discussion on Stack ADT

"Stack" - is a very basic and neatly designed Abstract Data Type that is available to us. If we have to describe the structure of the STACK ADT, it can be simply said as a ADT which follows LIFO queue servicing model. It is very much similar to a Stack of Plates. When you want to add a plate to the Stack, you add the new plate to the top end of the Stack and if you want to remove a plate from the stack, you do it the same way i.e., remove the plate from the top end of the stack.

Advantage :
It is easy to implement.
The accessor methods ( PUSH & POP) are much simpler than the accessor methods of most other ADTs.
Easy to handle the data using Stack ADT.

Implementation Details :
Stack are usually implemented with Arrays. Arrays are the most primitive Data type that is usually available with any programming language. In Stack, we simply abstract the access to the array and restrict the access only to the last element ( top most element of the stack). By using array, we can implement the basic accessor methods i.e., PUSH & POP in O(1) time. We simply store the last element's index and access is using Array indexing. Hence it is accessible in Constant time. ie., O(1).

Assume Stack is implemented using an array Stack[] of size N and the index of the top most element in the Stack is stored in "top". The parameter "o" refers to the generic Object which can be anything from int to float.

Other supporting methods ( can be implemented easily for any array):
size() - returns the current size of the Stack.
isEmpty() - returns boolean value telling whether the Stack is empty or not.

PUSH(o) :
if Stack.size() == N then
throw stackFullError
top <- top + 1
Stack[top] <- o

if Stack.isEmpty() then
throw stackEmptyError
o <- Stack[top]
Stack[top] <- null
top <- top - 1
return o

The problem in the above implementation is the array size should be initialized at the beginning. Thus if the stack size turns out to be small, space is wasted and if stack size turns out to be large, then the stack can crash when array indexing exceeds the upper limit.

Thus it is better to use Linked List in place of Array when space efficiency is our main concern.
Traditionally, we have a link to the head of the Linked List and we update the elements at the end opposite to the head of the Linked list. But the problem in using Linked List is the accessor methods turns out to be O(t) where t refers to the current size of the stack. Here, a reference to the Head of the Linked List is not absolutely necessary at all. The main aspect of Stack is they allow access only at one end of the ADT. Hence holding a pointer to the top most end of the stack should improve the accessor method's running time. Thus, we can simply implement the linked list in reverse order and hold pointer to the head.

Linked List Structure:

topmost -> n-1 -> n-2 -> ........ -> bottommost

Here by holding a pointer to the topmost element, we can implement PUSH & POP methods in O(1) time. Thus Stack turns out to be an efficient ADT even while using Linked List. Hence using a reverse order of storage in Linked List, we get space efficient and time efficient Stack ADT.

Tuesday, November 22, 2005

SQL Server: minus operator

Lets see some Database stuff.

Minus operator in Oracle: Consider two tables T1 and T2. Hence T1-T2 or T1[minus]T2 means return all rows in T1 that are NOT in T2.

Consider the tables as,


As per Oracle,
Query: Select * from T1 minus Select * from T2
Result: 3.........C
Query:Select * from T2 minus Select * from T1
Result: 3.........X

Minus operator in SQL Server: But unfortunately SQL Server does not support this operator. But here is work around to make it simulate in SQL Server.
Select * from T1 where
IsNull(cast(Col1 as varchar, '')) +
IsNull(cast(Col2 as varchar, ''))
Not in
(Select IsNull(cast(Col1 as varchar, '')) +
IsNull(cast(Col2 as varchar, '')) from T2)

Result: 3.......C

Explanation: The "In" operator works as per syntax. But it could be applied only to single column. Hence the basic idea is to concatenate all the columns to a single column. Similarly with the other table columns are also concatenated to a single column. Now using the "In" operator they are filtered out. The "cast" is for converting column values to varchar, the "IsNull" to remove NULL values. This is one such idea of doing it.

Monday, November 21, 2005

Java: String & String Buffer usage.

While performing any string manipulations like searching for a character in a string and replacing by a given character, our natural choice would be to use String. But, this is not advisable.

This is because when we make any modification to the existing string, a new string object is created by the JVM. Hence for making four replacement in a string, four new string object would be created and hence a total of five string object would be present in place of one string object. So the better option is to use String Buffer. String buffer doesn't create new objects, instead makes the modification in the original string. Hence string buffer are more space efficient than string.

Strings in Java are stored in "String Buffer pool" maintained by JVM. This is the region each time a new string gets created. The string buffer is created and manuplated in the Heap like any other object.


String strFirstName, strLastName;
strFirstName = "Rakesh";
strFirstName = "Sharma";
strLastName = "Rakesh";

What happens in the String Buffer pool is, strFirstName will point to a location contaning the value "Rakesh". Then it points to a new location contaning "Sharma". when strLastName is assigned a value "Rakesh", no new memory location is created for storing instead it points to "Rakesh" which was created earlier.

regex package in java & handling wild card characters

Java offers you regex package to handle the regular expression. Suppose you take a string from the user and search for the string in the database using regular expression, you may have some problems in implementing wildcard characters handling. Regex package can generate regular expression for all the string that you pass. But wildcard character * in regular expression has a different meaning from our normal interpretation.

Take for an example,

The search string entered : a*b

In regular expression, a*b means any number of 'a's followed by a 'b'.
so in this case, valid match could be

ab, aab, aaab, (a)-n times followed by (b).

but what user wants could be a followed by any character, any number of times and finally a 'b'. To convert this search string in to a proper format to be passed to the regex method is to replace '*' with a '.*' . And similarly for '?', it should be replaced by '.'

So, a search string "a*b"( in our normal sense, meaning 'a' followed by any character any number of times, followed by 'b') should be modified as "a.*b" before passing on to the regex package. If this search string is passed to the regex and asked to generate a regular expression from it, then we can get this regular expression to do the normal wildcard operation.

Similarly, a search string "a?b" ( meaning 'a' followed by any one character, followed by 'b') should be converted into "a.b" before passing on to the regex for generating the regular expression.

File Organisation in Web Applications

In Web Applications, you may have different kind of files like Servlet files ( .java files which define all the servlet methods), JSP Pages ( used for presentation purpose i.e., displaying information. It can be used in place of HTML to provide dynamic content unlike HTML's static content by embedding java commands in the JSP page), .properties file which contain the configuration details as (key,value) pair and also other .java files (which contains the business logic implemented in them).

To make the file organisation to be clean, we can follow seperate out presentation files and the business logic files and make them to be independent of each other. We should not include any business logic like reading from a database, writing to database or any other action in JSP files. Though it is always possible to accomodate any kind of java command and operation in jsp file, it is not advisable to do so. In Industry practice, the JSP Page and the .java file which has the business logic may be developed by two different person. JSP professional needs no knowledge of java. So it is better to avoid embedding complex business logic in jsp files. Minimal amount of java commands in jsp page for the purpose of presentation is acceptable.

So we can have all the business operations in .java file as a separate package and call them in the servlet files as and when needed. All the constants used in the application can be added in a .java file and could be declared as interface. We can make all the files that needs to use these constants to implement that interface for efficient space management. Moreover it is better to handle all the exceptions in any one particular level. Best practice is to handle the exceptions in the .java files which includes the business logic, instead of handling them in the servlet files. This method provides a greater amount of independence between the servlet files and the .java files.

All the configuration information could be stored in the .properties file and could be read from that file. It is easy to change any configuration in this method. Only the key,value pair in the .properties file needs to be changed to implement any change in the configuration.


Problem Description :
You are asked to create a simple web application which fetches the data from the Backend database and display all the entries on the Jsp page.

Possible file organisation:

Now, your file organisation can be like,

rootdir/src/"package1"/"any servlet/portlet file".java
rootdir/src/"package2"/"any business logic implementation file".java

Here you "jsps" directory holds all your jsp files. You can have
rootdir/web/jsps/help/ directory to hold your help.jsp files.

Then you can organise all your servlet/portlet files under one package.

And all your business logic implementation files i.e., here it could be the .java file that implements the jdbc connectivity methods to connect to the database, retrieve the data and all other database operations, could be placed in another package.

This organisation makes the package2 to be used independently in any other application when needed. You can import this package where ever needed.

Moreover, all the constants used in the web application can be added to an interface, may be, called "applicationname" and this file can be implemented by which ever file, which needs to use these constants.

If you want to create a package out of this application, you can add the following files and directories.

Using "ant" command, you can "build" the application using the build.xml file. Then snippet file contains all the files that has to be added into the package and also the file access permission for each of them. You can write script to read the snippet file and create package accordingly!

This is one possible method, but in Industry practice this method is widely used. If you feel this orientation is wrong, please correct me!

Null Pointer Exception

Right now, I'm doing my project in SUN Microsystems. An interesting thing that i learned here with respect to null pointer exception in Java language is as follows:

When you encounter a NULL pointer exception ( Null pointer exception is the exception that is thrown by the JVM when you perform some operation on an object which is null or calling some method on the object that is null), you should not try to handle that exception i.e., DONT try ...catch the null pointer exception. It is nothing but patching the problem and it isin't the right way. You should try to avoid performing any operation on null object.



Assume this statement generates a null pointer exception.

Wrong method :

try {
} catch ( Exception Ex) {
// handle the exception

Correct Method :

if ( object != null) {

C++: What is Slicing problem?

This makes a clear explanation!

CPlusPlus Gems

Friday, November 18, 2005


To Err is human! Unintentionally, we might make some mistakes in the informations given here. We are taking all the precautions possible to avoid any such mistakes. We regret anything of that sort, if at all it exists. You are free to correct our mistakes and provide us with valuable feedback. Thanks!