Lab 2.1 Good Programming Practices

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Oracle® PL/SQL® Interactive Workbook, Second Edition
By Benjamin Rosenzweig, Elena Silvestrova
Table of Contents
Chapter 1.  Programming Concepts


Lab Objectives

After this Lab, you will be able to:

  • Understand the Nature of Good Programming Practices

  • Understand Formatting Guidelines

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. Sequence Structure

graphics/01fig01.gif

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. Selection Structure

graphics/01fig02.gif

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.

Figure 1.3. Iteration Structure

graphics/01fig03.gif

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.

graphics/intfig03.gif 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.

graphics/intfig03.gif 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.

graphics/intfig03.gif 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.

graphics/intfig03.gif 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.

Lab 1.2 Exercises

1.2.1 Understand the Nature of Good Programming Practices

a)

What is a top-down approach?

b)

What is structured programming?

c)

Create the following selection structure: Determine which season each month of the year belongs to.

d)

Create the following iteration structure: For every day of the week display its name.

e)

Create the following structure: For every day that falls within the business week, display its name. For every day that falls on the weekend, display "The weekend is here, and it is here to stay!!!" Hint: You will need to use iteration and selection structures. The selection structure must be placed inside the iteration structure.

1.2.2 Understand Formatting Guidelines

a)

What is the reason for formatting your code?

b)

What are two main guidelines of good formatting?

Lab 1.2 Exercise Answers

This section gives you some suggested answers to the questions in Lab 1.2, with discussion related to how those answers resulted. The most important thing to realize is whether your answer works. You should figure out the implications of the answers here and what the effects are from any different answers you may come up with.

1.2.1 Answers

a)

What is a top-down approach?

A1:

Answer: The technique of breaking a problem into parts and solving each part is called a top-down approach to problem solving. By breaking down the problem, it is easier to focus on possible solutions and manage each part. Once each part is fully understood, the solution to the overall problem can be readily developed.

b)

What is structured programming?

A2:

Answer: 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.

c)

Create the following selection structure: Determine which season each month of the year belongs to.

A3:

Answer: Your selection structure should look similar to the following:

 IF MONTH IN ('DECEMBER', 'JANUARY', 'FEBRUARY')     IT IS WINTER  IF MONTH IN ('MARCH', 'APRIL', 'MAY')     IT IS SPRING  IF MONTH IN ('JUNE', 'JULY', 'AUGUST')     IT IS SUMMER  IF MONTH IN ('SEPTEMBER', 'OCTOBER', 'NOVEMBER')     IT IS FALL 

The test conditions of this selection structure use the operator IN. This operator allows you to construct the list of valid months for every season. It is important to understand the use of the parentheses. In this case, it is not done for the sake of a syntax rule. This use of parentheses allows us to define clearly the list of values for a specific month, hence helping us to outline the logic of the structure.

Now, consider the following fragment of the selection structure:

 IF MONTH IS 'DECEMBER'     IT IS WINTER  IF MONTH IS 'JANUARY'     IT IS WINTER  IF MONTH IS 'FEBRUARY'     IT IS WINTER   

This selection structure results in the same outcome, yet it is much longer. As a result it does not look well structured, even though it has been formatted properly.

d)

Create the following iteration structure: For every day of the week display its name.

A4:

Answer: Your selection structure should look similar to the following:

 WHILE THERE ARE MORE DAYS IN THE WEEK     DISPLAY THE NAME OF THE CURRENT DAY     GO TO THE NEXT DAY 

Assume that you are starting your week on Mondaythere are six days left. Next, you will display the name of the current day of the week, which is Monday for the first iteration. Then, you move to the next day. The next day is Tuesday, and there are five more days in the week. So, you will display the name of the current dayTuesdayand move to the next day, and so forth. Once the name of the seventh day (Sunday) has been displayed, the iteration structure has completed.

e)

Create the following structure: For every day that falls within the business week, display its name. For every day that falls on the weekend, display "The weekend is here, and it is here to stay!!!" Hint: You will need to use iteration and selection structures. The selection structure must be placed inside the iteration structure.

A5:

Answer: Your structure should look similar to the following:

 WHILE THERE ARE MORE DAYS IN THE WEEK     IF DAY BETWEEN 'MONDAY' AND 'FRIDAY'        DISPLAY THE NAME OF THE CURRENT DAY     IF DAY IN ('SATURDAY', 'SUNDAY')        DISPLAY 'THE WEEKEND IS HERE, AND IT IS HERE TO STAY!!!'     GO TO THE NEXT DAY 

This structure is a combination of two structures: iteration and selection. The iteration structure will repeat its steps for each day of the week. The selection structure will display the name of the current day or the message "The weekend is. . . ."

Assume that you are starting your week on Monday again. There are six days left. Next, control of the flow is passed to the selection structure. Because the current day happens to be Monday, and it falls within the business week, its name is displayed. Then, control of the flow is passed back to the iteration structure, and you are ready to move to the next day.

The next day is Tuesday, and there are five more days in the week. So, control is passed to the iteration structure again. Tuesday also falls within the business week, so its name is displayed as well. Next, control is passed back to the iteration structure, and you go to the next day, and so forth. Once the day falls on the weekend, the message "The weekend is . . ." is displayed.

1.2.2 Answers

a)

What is the reason for formatting your code?

A1:

Answer: A well-formatted program is easier to understand and maintain because format can reveal the logical structure of the program.

b)

What are two main guidelines of good formatting?

A2:

Answer: First, the code of the program should be indented so that the logical structure of the program is clear. Second, the program should contain comments describing what is being accomplished.

Lab 1.2 Self-Review Questions

In order to test your progress, you should be able to answer the following questions.

Answers appear in Appendix A, Section 1.2.

1)

Which one is not a feature of the structured programming?

  1. _____ Iteration

  2. _____ Sequence

  3. _____ GOTO

  4. _____ Modularity

2)

Structured programming allows control of the program to jump all over the code.

  1. _____ True

  2. _____ False

3)

Which of the following is true about sequence structure?

  1. _____ It refers to the decision-making process.

  2. _____ It refers to the linear execution of code.

  3. _____ It refers to the repetition of code.

4)

A test condition must evaluate to which of the following in order for the selection to execute?

  1. _____ TRUE

  2. _____ FALSE

  3. _____ None of the above

5)

A poorly formatted SELECT statement produces output different from a well formatted SELECT statement.

  1. _____ True

  2. _____ False

  3. _____ None of the above


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    Oracle PL. SQL Interactive Workbook
    Oracle PL/SQL Interactive Workbook (2nd Edition)
    ISBN: 0130473200
    EAN: 2147483647
    Year: 2002
    Pages: 146

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