Chapter VI

 

Control Structures I

 

Chapter VI Topics

 

    6.1    Introduction

    6.2    Types of Control Structures

    6.3    Relational Operators

    6.4    Command Line Prompt

    6.5    One-Way Selection

    6.6    Two-Way Selection

    6.7    Multiple-Way Selection

    6.8    Fixed Repetition

    6.9    Conditional Repetition

    6.10      Worked Out Exercises

    6.11  Summary

   
6.1  Introduction

 

 

The previous chapter focused on using class methods and object methods, which is an integral part of Object Oriented Programming.  The intention in Exposure Java is to present OOP in stages.  A thorough understanding of Object Oriented Programming is vital in modern computer science.  At the same time, you also need to learn the syntax of a program language.  You cannot write proper Java OOP programs unless you know the Java sentence structure or syntax.  A prerequisite for a creative writing course is knowledge of grammar.

 

Frequently, it is mentioned that program language syntax is trivial.  The essence of programming is design, data structures and algorithms.  This is very true, and very lovely, but language syntax tends to be trivial only if you know syntax.  You will get very frustrated with your programs when they do not compile because of syntax problems. 

 

In an earlier chapter I mentioned that program execution follows the exact sequence of program statements.  That was true, but also a rather incomplete explanation.  There is a lot more to the program flow picture.  We can expand on the exact program sequence by stating that program flow follows the sequence of program statements, unless directed otherwise by a Java control structure.

 

 

Program Flow

 

Program Flow follows the exact sequence of listed program

statements, unless directed otherwise by a Java control

structure.

 

 

 

Programs in any computer language require control structures.  It may appear that all our program examples were written without the use of any control structures, but the control was subtle.  As mentioned earlier, control was provided by the sequence of program statements.  That type of control is called simple sequence.

 

Simple sequence alone is not very satisfactory for programs of any type of consequence.  Programs constantly make decisions.  A payroll program needs to change the amount paid, if the hours exceed 40 per week.  The same payroll program can have many variations of tax deductions based on the number of dependents claimed.  A payroll program also needs to have the ability to repeat the same actions for additional employees.  In other words, a program needs to have the ability to repeat itself, or repeat certain program segments.  The language features that allow that type of control will be introduced in this chapter.

 

You will note that the title of this chapter is Control Structures I.  You have guessed correctly. There will be a second control structure chapter.  The second chapter will explain a variety of advanced control structure features.  At this stage, all we want to do is get our feet wet and feel the temperature of the water.  You will be diving in the deep-end before you know it.

 

 

 

 

 

 

 

6.2  Types of Control Structures

 

 

Program-execution-flow is controlled by three general types of control structures.  They are simple sequence, selection, and repetition.  Java provides syntax, and special keywords for each one of these three control structures.  Before we look at actual Java source code required to implement control, let us first take a look at diagrams that explain each control structure.

 

 

Simple Sequence

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Simple sequence holds no surprises.  A series of program statements are executed in the exact sequence that they are written.  Altering the program sequence requires altering the sequence of the program statements.

 

 

 

Selection

 

Frequently, programs cannot follow a single, simple sequence, path.  Decisions need to be made.  Should the applicant be hired or not?  Does the employee get overtime pay?  Which tax bracket is the deduction to be computed from?

 

Selection is also called conditional branching or decision making.  The program flow encounters a special condition.  The value of the condition determines if the program flow will “branch off” from the main program sequence.  Three diagrams will be shown.  The first diagram shows one-way selection, the second diagram shows two-way selection, and the third one shows multiple-way selection.

 

 

One-Way Selection

 

 

 

 

 

 


Condition
 
                                                               True         

 


                               False

 

Program Statement
 
 

 

 

 

 

 

 

 


Selection control structures use a special conditional statement.  If the condition is true, some action is performed, such as branching off to another sequence of program statements.  In the case of one-way selection, the true condition branches off.  If the condition is false, the program flow continues without change in program sequence.

Consider the analogy of driving South from Dallas to Austin.  Along the way you check if your gas tank is low.  If the tank is low, you stop for gas, and then continue to Austin.  If the tank is not low you continue to drive south.  Keep in mind that regardless of the tank condition, you are heading to Austin.

 

 

Two-Way Selection

 

 

 

 

 

 


Condition
 
                                   True                                  False 

 

 

 

 

 

 

 

 

 

 

Program Statement
 
 

 

Program Statement
 
 

 


The two-way selection control structure also checks to see if some special condition is true.  But there is a significant difference in how the program flow is handled.  With one-way selection, a true condition means to do something, like get off the road and get gas, before continuing.  The two-way selection structure selects one direction, or the other direction, but not both.   

 

The one-way analogy describes a trip traveling South from Dallas to Austin.  Regardless of the gas tank situation, the trip travels to Austin in the same car.  Now consider an analogy for two-way selection.  You are in Las Vegas gambling in the casino.  You are placing all your money on one large bet.  If you win the bet, you will fly home first-class, otherwise you will hitchhike home.  Perhaps you argue that in both cases the final destination is home.  True, but there are two totally different travel modes getting there. 

 

 

 

 

 

Multiple-Way Selection

 

 

 

 

 

 

 


Condition
 
                                          True

 


                                False

Condition
 

Program Statement
 
                                          True    

                                          

                                False

Condition
 

Program Statement
 
                                          True    

                                           

                             False

Condition
 

Program Statement
 
                                          True    

                                          

                             False

 

 

 

 

 

 

 

 

 

 


Multiple selection is a commonly used control structure that simulates many situations in real life.  The program flow encounters a group of conditions, one after the other.  If the condition is true, the program flow branches off to execute some other statement.  The main program flow is rejoined below all the conditional statements.

 

Multiple selection in real life is encountered with any multiple choice test, an ATM machine or any other situation where you encounter more than two options that can be selected.

 

 

 

 

 

 

 

Repetition

 

Another common application occurs when repetition is required.  A grade book program needs to average grades for every student in a class of twenty-five students.  A payroll program needs to process paychecks for many employees.  Practically everything you can imagine is done multiple times.  Nobody is interested in repeating program source code 500 times for some task that is to be performed 500 times.  We want to create one program segment, and place this segment in some type of loop control structure that repeats 500 times.

 

 

 

 

 

 

 

 

 

 

 

 

 


             

Condition
 
              True         

    

                             False

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


6.3  Relational Operators

 

 

Both the selection control structure diagrams and the repetition diagram indicate a change of program flow occurring after some condition.  Understanding conditional statements is the essence of understanding, and using, control structures.  However, before we plunge into the syntax of the various conditional statements, you need to understand the relational operators that are used by Java in the conditional statements.

 

 

 

Conditional Statement Definition

 

A conditional statement is a program expression that

evaluates to true or false.

 

Most conditional statements require a relational operator.

 

All conditions must be placed inside parentheses.

 

 

 

Consider an expression, such as 5 + 4.  Does that expression evaluate to true or false?  Neither, it evaluates to 9.  A relational operator is required to make an expression evaluate to true or false.  Java has six relational operators:  equals, not equals, greater than, less than, greater than or equal, and less than or equal. 

 

 

The idea of conditional statements that are based on a relational operator can be considered in regular English statements:

 

If we save more than $200.00 a month, we can go on a vacation

 

If your SAT score is high enough you will be admitted to college, otherwise

you will not be able to go to college

 

Repeat calling established customers until you have 25 surveys

 

 

 

 

 

 

Name

Operator

Expression

Evaluates

 

Equals

 

==

 

5 == 5

K == 10

 

 

true

depends

 

Not equals

 

!=

 

50 != 25

100 != 100

 

 

true

false