Top-Down Design and Modular Development

The process of developing methods for objects is mostly a process of developing algorithms;

each method is an algorithm.

Divide and conquer

• It is easier to solve small problems than it is to solve big ones.

• Designers solve a problem by breaking an algorithm into parts that are more manageable.

• Parts are solved individually; the smaller solutions are assembled into a big solution.

• This process is called decomposition, divide and conquer, or more commonly…top-down design.

Example: Payroll

Payroll

Example: Payroll

Modular implementation

• The process of top-down design leads to modular implementation.

• The parts, or modules that make up a complete solution to a problem are implemented individually.

• Then combined to form a complete solution

Advantages of modular implementation

• More manageable projects– Smaller tasks are easier to understand– Less demanding of resources

• Faster: Can have different people work on different modules at the same time

• Easier to ensure correctness:– Easier to isolate the cause of an error– Error will be localized to a module…easier to fix.

• Reusability: Solutions to smaller problems are more likely to be useful elsewhere than solutions to bigger problems.

Reusable modules

Reusable code

• Computer programs contain many small tasks that must be performed in many other programs.

• Example: username/password• Reusable code makes programming

easier and more efficient; you need develop a solution to a problem only once.

• Over time you build libraries of software modules for different tasks.

Skating

• Michelle Kwan

• Sasha Cohen

• Kennedy

The program development cycle

Program Development Cycle

Software developers base many of their techniques on traditional approaches to mathematical problem solving.

One such approach is based on how engineers solve problems.

Program Development Cycle

Engineers must:

• Design a solution to a problem

• Implement the solution

• Test the solution

• Fix any errors in the solution

Program Development Cycle

This approach can be used in computer programming:

• Design a solution to a problem (design a program)

• implement the solution (code the program )

• Test the solution (test the program)

• Fix the solution (debug the program)

Program Development Cycle

Once the software is working, improvements can be designed into the system. This leads back to design, creating a four-phase cycle.

Design

Code

Test

Debug

Program Development Cycle

Design

Code

Test

Debug

Program Development Cycle

Design

Many different techniques are used to design methods

in new software. Top-down development and modular

design are most common.

Tools include flowcharts, pseudo-code, storyboards,

and Universal Modeling Language (UML).

Program Development Cycle

Design

In the design phase of object-oriented programming,

objects must be chosen from libraries, or the

properties and methods for new objects must be

described.

Program Development Cycle

Design

Code

Test

Debug

Program Development Cycle

Code

The program development

cycle’s code phase includes

translating a software design

into a particular language,

and then entering that

language on the computer.

Program Development Cycle

Design

Code

Test

Debug

Program Development Cycle

Test

Testing can be rather complicated:

Does the new method do what it is supposed to do?

This is known as a test for correctness. Tests for

correctness measure whether the program meets the

original specifications.

Program Development Cycle

Test

Testing can be rather complicated:

Is the method reasonably efficient? How much time

does it take for the method to complete its task, and

how much space does it use?

Program Development Cycle

Test

Testing can be rather complicated:

Does the method have any undesirable side effects?

How does one program affects the performance of

another program, or one method affects another

method?

Program Development Cycle

Test

Programmers perform unit tests and integration tests.

A unit test checks to see if a method works as expected all by itself.

An integration test checks to see if a method works in combination with other methods.

New Software Method

Testing Shell

Overall Program

Other methods

Other methods

New Software Method

New Software Method

Unit Test

Integration Test

Program Development Cycle

Design

Code

Test

Debug

Program Development Cycle

Debug

The causes of any problems discovered during testing need to be isolated. Here unit tests are most helpful.

Once you know the cause, you can develop a plan for fixing the problem, modify the necessary methods, and then test again.

Program Development Cycle

Design

Code

Test

Debug

An Alice Development Cycle

Design Describe the objects,

methods, and initial scene for the new Alice world.

ImplementAdd the necessary objects to the world, position them,

and code their methods.

Test Test each method to see

if it works by itself and with other methods.

DebugCorrect each method’s

design and coding, and the initial state of the world.

Working with primitive methods in Alice

Two types of methods

• Alice objects have two types of methods– User-defined methods– Primitive methods

• Primitive methods are built-in methods that are an inherent part of an Alice object.

• User-defined methods are written by people who use Alice. They can be edited.

Tutorial on primitive methods

• Start Alice

• Open a new world with the snow template

• Find the penguin in the animals folder of the objects gallery.