ISM 3232 - Intermediate Programming. 

Review: Flow-of-Control - Repetition Structures

• Repetition structure: a control structure that repeats a statement or a sequence of statements.
• Many programming tasks require a repetition structure.

You are already familiar with the purpose and varieties of loops from your study of other programming languages such as Visual BASIC or C. This chapter will be largely review of numerous looping concepts. What will be new is how loops are implemented in Java.

For a simple example, what if we wanted to print "Hello" 100 times?

Solution: Write a method containing 100 println() statements,

or (much better)

write a method that contains 1 println(), then place it inside a loop structure:

What defines the loop structure?

• It begins with for and ends with a semi-colon;
• How many times? 100
• This is an example of a counting loop.

Counting Loop

• If number of iterations is known, use a counting loop.

For example, here is the pseudocode for:

• The for statement is used in counting loops.
• Each loop must have an initializer, a loop entry condition, and an updater, that ensures that the loop will eventually end

Loop Bounds

Not only must an updater exist within the body of the loop, but it must be updated such that eventually it will end the loop.

Infinite Loop Examples (what needs to be changed in each?)

In each case the updater fails to make progress toward the bound and the loop entry condition never becomes false.

Loop Indentation. Indenting properly is extremely important for readability and debugging purposes, even though the compiler doesn't recognize it. These examples all appear the same to the compiler, but the first most clearly illustrates to the reader what is intended.

Loop Variable Scope

If k is declared within the for statement, it cannot be used after the for statement:

If k is declared before the for statement, it can be used after the for statement:

Compound Loop Body

• Compound statement or block: a sequence of statements enclosed within braces, {...}.
• For loop body: Can be a simple or compound statement.

Nested Loops

• Suppose you wanted to print the following table:

Another Example: Printing a pattern

Another Example: Car Loan Table

• Design a program to print a table for the total cost of car financing options.

Car Loan Interest Rate

Implementation: CarLoan Class

Conditional Loops: While .. , Do .. While

• When the number of iterations is unknown, we can use a conditional loop. Examples in pseudocode:

The While Structure

Effective Design: Loop structure.

• A loop structure must include an initializer, a boundary condition, and an updater. The updater should guarantee that the boundary condition is reached, so the loop will eventually terminate.

Example while : Computing Average

Design Principles: Modularity and Localization

The Computing Average example above demonstrates design principles of Modularity and Localization, by using separate methods for the input and averaging tasks. We can then call getInput() when needed, rather than repeat the code in multiple places.

Computing Average: Main Program

The Do-While Structure

Example:

Data Validation: Example

Example: Modifications to CyberPet

• Problem: Modify CyberPetApplet to present an animation of the pet’s eating behavior.
• Class Design: No changes in CyberPet itself are necessary.
• Algorithm Design: Use a loop to iterate among a set of images that show CyberPet chewing its food:

Modified CyberPet Applet

Principles of Loop Design

• A counting loop can be used if you know in advance how many iterations are needed. The for statement is used for counting loops.
• A while structure should be used if the loop body may be skipped entirely. The while statement is used.
• A do-while structure should be used only if a loop requires one or more iterations. The do-while-statement should be used.
• The loop variable, which is used to specify the loop entry condition, must be initialized to an appropriate value and updated on each iteration.
• A loop's bound, which may be a count, a sentinel, or, more generally, a conditional bound, must be correctly specified in the loop-entry expression, and progress toward it must be made in the updater.
• An infinite loop may result if either the initializer, loop-entry expression, or updater expression is not correctly specified.

Structured Programming

• Structured programming: uses a small set of predefined control structures.
• Sequence --- The statements in a program are executed in sequential order unless their flow is interrupted by one of the following control structures.
• Selection--- The if, if/else, and switch statements are branching statements that allow choice by forking of the control path into two or more alternatives.
• Repetition --- The for, while, and do-while statements are looping statements that allow the program to repeat a sequence of statements.
• Method Call --- Invoking a method transfers control temporarily to a named method. Control returns to the point of invocation when the method is completed.

Structured Programming Constructs

• No matter how large or small a program is, its flow of control can be built as a combination of these four structures.
• Note that each structure has one entry and one exit.

Debugging Problem: Structured vs. Unstructured Code

Preconditions and Postconditions

• A precondition is a condition that must be true before some segment of code is executed.
• A postcondition is a condition that must be true after some segment of code is executed.

Using Preconditions and Postconditions

• Design stage: Using pre- and postconditions helps clarify the design and provides a precise measure of correctness.
• Implementation and testing stage: Test data can be designed to demonstrate that the preconditions and postconditions hold for any method or code segment.
• Documentation stage: Using pre- and postconditions to document the program makes the program more readable and easier to modify and maintain.
• Debugging stage: Using the pre- and postconditions provides precise criteria that can be used to isolate and locate bugs. A method is incorrect if its precondition is true and its postcondition is false. A method is improperly invoked if its precondition is false.

Design/Programming Tips

• Preconditions and postconditions are an effective way of analyzing the logic of your program's loops and methods. They should be identified during the design phase and used throughout development, testing and debugging , and included in the program's documentation.
• Develop your program's documentation at the same time that you develop its code.
• Acquire and use standard programming techniques for standard programming problems. For example, using a temporary variable to swap the values of two variables is an example of a standard technique.
• Use methods wherever appropriate in your own code to encapsulate important sections of code and thereby reduce complexity.

From the Java Library: TextArea

Key terms : 

Suggested Exercises :  2, 3, 4, 7, 8, 9