Programming - IT - Software Design

CHAPTER 3

Modules

Chapter Topics 3.1 Introduction 3.2 Defining and Calling a Module 3.3 Local Variables 3.4 Passing Arguments to Modules 3.5 Global Variables and Global Constants

3.1 Introduction • A module is a group of statements that exists

for the purpose of performing a specific task within a program.

• Most programs are large enough to be broken down into several subtasks.

• Divide and conquer: It’s easier to tackle smaller tasks individually.

3.1 Introduction Benefits of using modules

– Simpler code • Small modules easier to read than one large one

– Code reuse • Can call modules many times

– Better testing • Test separate and isolate then fix errors

– Faster development • Reuse common tasks

– Easier facilitation of teamwork • Share the workload

– Easier Maintenance • Smaller, simpler code is easier to maintain

3.2 Defining and Calling a Module

• The code for a module is known as a module definition.

Module showMessage() Display “Hello world.” End Module

• To execute the module, you write a statement that calls it.

Call showMessage()

3.2 Defining and Calling a Module

• A module’s name should be descriptive enough so that anyone reading the code can guess what the module does.

• No spaces in a module name. • No punctuation. • Cannot begin with a number.

3.2 Defining and Calling a Module

• Definition contains two parts – A header

• The starting point of the module – A body

• The statements within a module

Module name( ) Statement Statement Etc.

End Module

3.2 Defining and Calling a Module

• A call must be made to the module in order for the statements in the body to execute.

Figure 3-2 The main module

• When flowcharting a program with modules, each module is drawn separately.

3.2 Defining and Calling a Module

Figure 3-6 Flowchart for 
 Program 3-1

• A “top-down-design” is used to break down an algorithm into modules by the following steps:

– The overall task is broken down into a series of subtasks.

– Each of the subtasks is repeatedly examined to determine if it can be further broken down.

– Each subtask is coded.

3.2 Defining and Calling a Module

• A hierarchy chart gives a visual representation of the relationship between modules.

• The details of the program are excluded.

3.2 Defining and Calling a Module

Figure 3-7 A hierarchy chart

3.3 Local Variables

• A local variable is declared inside a module and cannot be accessed by statements that are outside the module. 


• A variable's scope is the part of the program in which the variable can be accessed.

3.3 Local Variables

• Duplicate Variable Names: Variables within the same scope must have different names.

3.3 Local Variables • Duplicate Variable Names: Variables in

different scopes can have the same name.

3.4 Passing Arguments to Modules

• Sometimes, one or more pieces of data need to be sent to a module.

• An argument is any piece of data that is passed into a module when the module is called.

• A parameter is a variable that receives an argument that is passed into a module.

• The argument and the receiving parameter variable must be of the same data type.

• Multiple arguments can be passed sequentially into a parameter list.

Figure 3-15 Two arguments passed into two parameters

3.4 Passing Arguments to Modules

Pass by Value vs. Pass by Reference • Pass by Value means that only a copy of the

argument’s value is passed into the module. – One-directional communication: Calling module can

only communicate with the called module. • Pass by Reference means that the argument is

passed into a reference variable. – Two-way communication: Calling module can

communicate with called module; and called module can modify the value of the argument.

3.4 Passing Arguments to Modules

3.5 Global Variables & Global Constants

• A global variable is accessible to all modules. • Should be avoided because:

– They make debugging difficult – Making the module dependent on global variables

makes it hard to reuse module in other programs – They make a program hard to understand

• A global constant is a named constant that is available to every module in the program.

• Since a program cannot modify the value of a constant, these are safer than global variables.

3.5 Global Variables & Global Constants