Program Modules in WMLScript

Program Modules in WMLScript
Modules in WMLScript are called functions. WMLScript programs are written by combining
new functions that the programmer writes with “prepackaged” functions and objects
available in WMLScript. The “prepackaged” functions that belong to WMLScript objects
(such as Lang.parseInt and WMLBrowser.go, introduced previously) are called
methods. The term method implies that the function belongs to a particular object; however,
the terms function and method can be used interchangeably. We will refer to functions that
belong to a particular WMLScript object as methods; all others are referred to as functions.
WMLScript provides several objects that have a rich collection of methods for performing
common mathematical calculations, string manipulations, browser manipulations
and other functions. These provide many of the capabilities programmers need. Some
common predefined objects of WMLScript and their methods are discussed in Chapter 19,
WMLScript: Objects.
Good Programming Practice 16.1
Familiarize yourself with the rich collection of objects and methods provided by WMLScript.
16.1
Software Engineering Observation 16.1
Avoid reinventing the wheel. If possible, use WMLScript objects, methods and functions instead
of writing new functions. This reduces program development time and avoids introducing
new errors. 16.1
Outline
16.1 Introduction
16.2 Program Modules in WMLScript
16.3 Programmer-Defined Functions
16.4 Function Definitions
16.5 Random Number Generation
16.8 Example: A Game of Chance
16.6 Duration of Identifiers
16.7 Scope Rules
Summary • Terminology • Self-Review Exercises • Answers to Self-Review Exercises • Exercises
Chapter 16 WMLScript: Functions 283
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Portability Tip 16.1
Using the methods built into WMLScript objects helps make programs more portable. 16.1
Performance Tip 16.1
Do not try to rewrite existing WMLScript methods to make them more efficient. You usually
will not be able to increase the performance of these methods. 16.1
In addition to using existing functionality, a programmer can write functions to define
specific tasks that may be used at various points in a script. These are sometimes referred
to as programmer-defined functions.
A function is invoked (i.e., made to perform its designated task) by a function call. The
function call specifies the function name and provides information (as arguments) that the
called function needs to do its task. A common analogy for this structure is the hierarchical
form of management. A boss (the calling function or caller) asks a worker (the called function)
to perform a task and return (report back) the results when the task is done. The boss
function does not know how the worker function performs its designated tasks. The worker
may call other worker functions, and the boss will be unaware of this. We will soon see how
this “hiding” of implementation details promotes good software engineering. Figure 16.1
shows the boss function communicating with several worker functions in a hierarchical
manner. Note that worker1 acts as a “boss” function to worker4 and worker5. Relationships
among functions may be other than the hierarchical structure shown in this figure.
.
Fig. 16.1 Hierarchical boss function/worker function relationship.
Functions (and methods) are called (invoked) by writing the name of the function (or
method), followed by a left parenthesis, followed by the argument (or a comma-separated
list of arguments) followed by a right parenthesis. For example, a programmer desiring to
convert a string stored in variable inputValue to an integer number and add it to variable
total, might write
total += Lang.parseInt( inputValue );
boss
worker1 worker2 worker3
worker4 worker5
284 WMLScript: Functions Chapter 16
© Copyright 2001 by Deitel & Associates, Inc. All Rights Reserved. review Packet 5/7/01.
When this statement executes, WMLScript’s Lang object method parseInt converts
the string contained in the parentheses (stored in variable inputValue in this case) to an
integer value and that value is added to total. The variable inputValue is the argument
of the parseInt method. Method parseInt takes a string representation of an integer
as an argument and returns the corresponding integer value.
Function (and method) arguments may be constants, variables or expressions. If s1 =
"22.3" and s2 = "45", then the statement
total += Lang.parseInt( s1 + s2 );
evaluates the expression s1 + s2, concatenates the strings s1 and s2 (resulting in the
string "22.345"), converts the result into an integer and adds the floating-point number
to variable total.