4.8 Describing Data


4.8 Describing Data

Data consists of one or more data items. For every computation, there is one or more associated data items (or entities) that are to be manipulated or transformed by the computations (computer operations). The input data is the set of data items that are transformed in order to produce the desired results.

Data descriptions are necessary together with algorithm descriptions. The algorithm is decomposed into the operations that manipulate the data and produce the results for the problem. For every data item, its description is given by:

  • The data item type

  • A unique name to identify the data item

  • An optional initial value

The name of a data item is an identifier and is given by the programmer; it must be different from any keyword in KJP (or in Java). The type defines:

  • The set of possible values that the data item may have

  • The set of possible operations that can be applied to the data item

4.8.1 Names of Data Items

Text symbols are used in all algorithm descriptions and in the source program. The special symbols that indicate essential parts of an algorithm are called keywords. These are reserved words and cannot be used for any other purpose. The other symbols used in an algorithm are the ones for identifying the data items and are called identifiers. The identifiers are defined by the programmer.

A unique name or label is assigned to every data item; this name is an identifier. The problem for calculating the area of a triangle used five data items, x, y, z, s, and area.

The data items usually change their values when they are manipulated by the various operations. For example, the following sequence of instructions first gets the value of x then adds the value x to y:

       read x         // read value of x from keyboard       add x to y 

The data items named x and y are called variables because their values change when operations are applied on them. Those data items that do not change their values are called constants, for example, Max_period, PI, and so on. These data items are given an initial value that will never change.

When a program executes, all the data items used by the various operations are stored in memory, each data item occupying a different memory location. The names of these data items represent symbolic memory locations.

4.8.2 Data Types

There are two broad groups of data types:

  • Elementary (or primitive) data types

  • Classes

Elementary types are classified into the three following categories:

  • Numeric

  • Text

  • Boolean

The numeric types are further divided into three types, integer, float, and double. The noninteger types are also known as fractional, which means that the numerical values have a fractional part.

Values of integer type are those that are countable to a finite value, for example, age, number of automobiles, number of pages in a book, and so on. Values of type float have a decimal point; for example, cost of an item, the height of a building, current temperature in a room, a time interval (period). These values cannot be expressed as integers. Values of type double provide more precision than type float, for example, the value of the total assets of a corporation.

Text data items are of two basic types: character and type string. Data items of type string consist of a sequence of characters. The values for these two types of data items are textual values.

A third type of variables is the one in which the values of the variables can take a truth-value (true or false); these variables are of type boolean.

Classes are more complex types that appear as types of object variables in all object-oriented programs. Data entities declared (and created) with classes are called objects.

4.8.3 Data Declarations

The data descriptions are the data declarations. Each data description includes the name of every variable or constant with its type. The initial values, if any, for the data items are also included in the data declaration. There are two general categories of variables:

  • Elementary

  • Object variables (references)

Note

Object-oriented programming is mainly about defining classes as types for object variables (references), and then declaring and creating objects of these classes. The type of an object reference is a class.

4.8.3.1 Variables of Elementary Types

In KJP, the declaration of variables of elementary types has the following basic syntactic structure:

        elementary type   variable-name  

The following are examples of data declarations of elementary variables of type integer, float, and boolean:

            variables               integer count               real salary               boolean active 

4.8.3.2 Object References

As mentioned previously, an object reference is a variable that can refer to (point to) an object. The KJP statement for declaration of object references has the following structure:

       object  object_ref_name  of class  class_name  

For example, consider a program that includes two class definitions, Employee and Ball. The declarations of an object reference called emp_obj of class Employee, and an object reference ball1 of class Ball are:

       objects           object emp_obj of class Employee           object ball1 of class Ball 

4.8.4 Scope and Persistence

When identifying data items in software development, there are two important concepts to consider:

  • The scope of a data item is that portion of a program in which statements can reference that data item

  • The persistence of a data item is the interval of time that the data item exists—the lifetime of the data item




Object-Oriented Programming(c) From Problem Solving to Java
Object-Oriented Programming (From Problem Solving to JAVA) (Charles River Media Programming)
ISBN: 1584502878
EAN: 2147483647
Year: 2005
Pages: 184

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