Prentice Hall Oracle Plsql By Example 3Rd Edition [Electronic resources] نسخه متنی

Lab 1.2 Good Programming Practices

In the previous section of this chapter you encountered the terms

computer program and

programming language . You will recall that a program is a set of instructions, and a programming language is a tool that allows programmers to provide computers with these instructions. However, the process of creating a computer program is not as simple as just writing down instructions. Sometimes it can become a tedious and complicated task. Before a computer can be provided with these instructions, a programmer needs to know what instructions must be specified. In essence, the process of creating a program is akin to the process of applied problem solving.

Consider this mathematical word problem:

The 1980s speed record for human-powered vehicles was set on a measured 200-meter run by a sleek machine called Vector. Pedaling back-to-back, its two drivers averaged 69.92 miles per hour.

This awkward mix of units is the way data appeared in an article reporting the event. Determine the speed of the vehicle in meters per second.[1]

[1] From

Physics (with InfoTrac and Revised CD-ROM) Algebra/Trig , 2nd edition, by E. Hecht. © 1998. Reprinted with permission of Brooks/Cole, a division of Thomson Learning. Fax 800-730-2215.

This word problem involves conversion from miles per hour into meters per second. However, it contains information that has nothing to do with its solution, such as the name of the vehicle and the number of people needed to operate it. In order to achieve correct results, you must be able to filter out needed information and discard the rest. Next, you need to know what formulas must be used for actual conversion.

This is a relatively straightforward example of a problem-solving process that can be used for academic purposes. However, in the business world, problem descriptions are often incomplete or ambiguous. They are also harder to solve. These problems require the ability to ask questions that help clarify the problem and an ability to organize the problem into logical parts. By breaking down the problem, you will be able to focus better on possible solutions and more easily manage each part. Once each part is fully understood, the solution to the overall problem will readily develop.

This technique of breaking the problem into smaller parts and solving each part is called a

top-down approach to problem solving. When writing a program, you can also approach your task in a top-down manner. However, to solve the problem efficiently, you need to approach it in a structured manner.

Structured Programming

Structured programming embodies a disciplined approach to writing clear code that is easy to understand, test, maintain, and modify. A program can be organized into modules called

subroutines . These subroutines focus on a particular part of the overall problem that the program addresses. Subroutines are easier to understand and manage because they are only components of the overall program. Together, all of the subroutines compose the overall program.

Structured programming also embodies the following three attributes: sequence, selection, and iteration. These attributes, or structures, describe how statements in the program are executed. Furthermore, a program can contain any of these structures or a combination of them.

Sequence

Sequence refers to the linear execution of code. In other words, control is passed from one statement to the next statement in consecutive order. Consider Figure 1.1.

Figure 1.1 contains rectangular symbols. The rectangular symbol in the diagram can represent not only a single statement, but a subroutine as well. The arrows represent the flow of control between statements. Control is passed from statement 1 to statement 2 and then to statement 3. Thus, these statements are executed in the sequential order.

Selection

Selection refers to the decision-making process. For example, when I am trying to choose between different activities for this weekend, I start with the knowledge that on Friday night I want to go to the movies, Saturday night I want to go dancing, and Sunday I want to spend a quiet evening at home. In order for me to choose one of the activities, I need to know what day of the week it is. The logic for my decision of the weekend activities can be illustrated as follows:

IF TODAY IS 'FRIDAY'
I AM GOING TO SEE A MOVIE
IF TODAY IS 'SATURDAY'
I AM GOING DANCING
IF TODAY IS 'SUNDAY'
I AM SPENDING A QUIET EVENING AT HOME

The test conditions "TODAY IS . . ." can evaluate either to TRUE or FALSE based on the day of the week. If today happens to be Friday, the first test condition "TODAY is 'FRIDAY'" becomes TRUE, and the other test conditions become FALSE. In this case, I am going to see a movie, and the other activities can be discarded.

Figure 1.2 illustrates the general flow of control of the selection structure.

Figure 1.2 contains a diamond shape called the decision symbol. This indicates that a decision must be made or a certain test condition must be evaluated. This test condition evaluates to TRUE (Yes) or FALSE (No). If the test condition yields TRUE, statement 1 is executed. If the test condition yields FALSE, statement 2 is executed. It is important for you to remember that a rectangle can represent a set of statements or a subroutine.

Iteration

Iteration refers to an action that needs to be repeated a finite number of times. The number of times this action is repeated is based on some terminating factor. Consider the following example. You are reading a chapter from this book. Each chapter has a finite number of pages. In order to finish the chapter, you need to read through all of the pages. This is indicated as follows:

WHILE THERE ARE MORE PAGES IN THE CHAPTER TO READ
READ THE CURRENT PAGE
GO TO THE NEXT PAGE

The terminating factor in this example is the number of pages in the chapter. As soon as the last page in the chapter is read, the iteration is complete.

Figure 1.3 illustrates the general flow of control of the iteration structure.

As long as the condition evaluates to TRUE, the statements inside the iteration structure are repeated. As soon as the condition evaluates to FALSE, the flow of control is passed to the exit point of the iteration structure.

Differences Between Structured and Nonstructured Programming

Before structured programming became widely used, programs were simply sequential lines of code. This code was not organized into modules and did not employ many of the structures you encountered earlier in this chapter. The result was a meandering set of statements that was difficult to maintain and understand. In addition, these programs used multiple GOTO statements that allow program control to jump all over the code. Almost all programs that use GOTO statements can be rewritten using structures such as selection and iteration.

Formatting Guidelines

It was mentioned earlier that structured programming allows us to write clear code that is easy to understand, test, maintain, and modify. However, structured programming alone is not enough to create readable and manageable code. Formatting is a very important aspect of writing a program. Moreover, your formatting style should stay consistent throughout your programs.

Consider this example of a SELECT statement that has not been formatted.

FOR EXAMPLE

SELECT s.first_name, s.last_name, e.final_grade FROM student s, enrollment e WHERE s.student_id = e.student_id AND e.final_grade IS NOT NULL;

Even though this example contains only a very simple SELECT statement, you can see that the logic is hard to follow.

Consider the same SELECT statement with a few formatting changes.

FOR EXAMPLE

SELECT s.first_name, s.last_name, e.final_grade
FROM student s, enrollment e
WHERE s.student_id = e.student_id
AND e.final_grade IS NOT NULL;

You have probably noticed that the second version of the SELECT statement is much easier to read and understand. It is important to realize that both SELECT statements are syntactically correct. They produce the same output when run.

Usually, the logic depicted in the program is more complex than that of the SELECT statement. Therefore, proper formatting of the code is extremely important for two major reasons. First, a well-formatted program will facilitate any changes made later by the program's author. In other words, even the author will understand the logic of the program more easily if he or she needs to modify the program later. Second, any person who has to maintain the program can more easily follow the logical structure of the program.

In order for the program to be readable and understandable, there are two main guidelines to follow. First, the format of the program must illustrate the logical structure of the program. You can reveal the logical structure of the program by using indentation in your code. Consider the example of the selection structure used earlier in this chapter.

FOR EXAMPLE

IF TODAY IS 'FRIDAY'
I AM GOING TO SEE A MOVIE
IF TODAY IS 'SATURDAY'
I AM GOING DANCING
IF TODAY IS 'SUNDAY'
I AM SPENDING A QUIET EVENING AT HOME

You have probably noticed that each statement following the IF clause is indented. As a result, it is easier to understand what activity is taken based on the day of the week. You could take this example and format it differently.

FOR EXAMPLE

IF TODAY IS 'FRIDAY' I AM GOING TO SEE A MOVIE
IF TODAY IS 'SATURDAY' I AM GOING DANCING
IF TODAY IS 'SUNDAY' I AM SPENDING A QUIET EVENING AT HOME

This example also shows a formatted version of the selection structure. However, this formatting style does not reveal the logical structure of the selection as well as the previous example. As a matter of fact, this example looks like an extremely short story rather than a program.

Second,

your program should contain comments. Comments will help you explain what you are trying to accomplish. However, you should be careful because too many comments can make your code confusing.

You can use the code format used in this book's examples as you write your programs. It is not the only good format available, but it will be a good example of formatting technique, which will help you to develop your own style. However, regardless of your style, you should follow these guidelines when creating a program.