<Previous Lesson

Introduction To Computing

Next Lesson>

Lesson#29

History and Avolution of Internet

Who runs the Internet?


Who owns it?

Today’s Goal:

Introduction to the Internet



We looked at the role Internet plays in today’s computing
We reviewed some of the history and evolution of the Internet

Next Lecture:

Internet Services



We will try to familiarize ourselves with with some of the Internet services:
–http (surfing, shopping, searching)
–eMail
–ftp
–News groups, message boards, forums
–Instant messaging
–Multimedia delivery
function writeList( heading, words ) {
document.write( heading + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
document.write( words[ k ] + "<BR>" ) ;
}
}

29.3 Function Identifiers



The naming rules for function identifiers are the same as were discussed for variable and array identifiers

29.4 Arguments of a Function



A comma-separated list of data
Arguments define the interface between the function and the rest of the Web page
Arguments values are passed to the function by value (some popular languages pass arguments ‘by
reference’ as well)
To ensure that a function is defined before it is called up, define all functions in the HEAD portion of Web
pages
Keyword
Function
identifier
Pair of parenthesis
Function ‘arguments’ separated by
commas
Function definition
enclosed in a pair
of curly braces
function popUp( message ) {
window.alert( message ) ;
}
popUp( “Warning!” ) ;
A function call appearing
as a complete statement

function popUp( message ) {
window.alert( message ) ;
}
popUp( “Warning!” ) ;
function popUp( message ) {
window.alert( message ) ;
}
a = popUp( “Warning!” ) ;
document.write( a ) ;
What will get written by
this statement?
undefined
function add( a, b ) {
c = a + b ;
return c ;
}
sum = add( 2, 4 ) ;
document.write( sum ) ;
function add( a, b ) {
c = a + b ;
return c ;
}
document.write( add( 2, 4 ) ) ;
What would this
modifica-tion do?
function add( a, b ) {
c = a + b ;
return c ;
}
sum = add( 2, 4 ) ;
document.write( sum ) ;

What Would this Statement Do?



factorial( factorial ( 3 ) ) ;
This is termed as the
recursive
use of a function

Methods



Methods are functions
They are
unusual in the sense that they are stored as properties of objects

Object


: A named collection of properties (data, state) & methods (instructions, behavior)

29.5 Event Handlers



Special-purpose functions that come predefined with JavaScript
They are
unusual in the sense that they are many times called in the HTML part of a Web page and not the
<SCRIPT> … </SCRIPT> part
More on event handlers in a future lecture

Predefined, Top-Level or Built-In Functions



Event handlers are not the only functions that come predefined with JavaScript. There are many others.
Practically, there is no difference between predefined functions and those that are defined by the
programmer (termed as user-defined or custom functions)
There are many of them, but here we discuss only two: parseInt( ), parseFloat( )

parseInt( )



Syntax: parseInt ( string )
string1 = “3.14159” ;
prop
1
prop
2
prop
5
nam
e
prop
3
prop
4
A collection of
properties & methods
All objects have the “name”
property: it holds the name of
the object (collection)
method 3
method 1
method 2
The dictionary meaning of ‘Parse’: To
breakdown into simpler components and
analyze

 
document.write( parseInt( string1 ) ) ;
document.write( “<BR>” ) ;
string2 = “$100.00” ;
document.write( parseInt( string2 ) ) ;
document.write( “<BR>” ) ;
string3 = “ 23 ” ;
document.write( parseInt( string3 ) ) ;
1. Parses the string argument; returns an integer
2. If it encounters a non-numeral - anything other than (+,-) or (0-9) - it ignores it and all succeeding
characters, and returns the integer value parsed up to that point
3. If the first character cannot be converted to a number, parseInt returns NaN
4. parseInt truncates numbers to integer values
5. Leading and trailing spaces are ignored

parseFloat( )



Syntax: parseFloat ( string )
string1 = “3.14159” ;
document.write( parseFloat( string1 ) ) ;
document.write( “<BR>” ) ;
string2 = “$100.00” ;
document.write( parseFloat( string2 ) ) ;
document.write( “<BR>” ) ;
string3 = “ 23E-15 ” ;
document.write( parseFloat( string3 ) ) ;
1. Parses the string argument; returns a FP number
2. If it encounters a character other than
A sign (+,-)
A numeral (0-9)
A decimal point
An exponent
it returns the value up to that point, ignoring that and all succeeding characters
3. If the first character cannot be converted to a number, parseFloat returns NaN
4. Leading and trailing spaces are ignored

29.6 Scope of Variable



Defining the space in which a variable is effective is known as
defining the scope of a variable. A variable can be either
local or global in scope.

Local and Global Variables



Local or Function-level Variable
Effective only in the function in which they are declared
Global Variables
Visible everywhere on the Web page
function factorial( n ) {
product = 1 ;
for ( k = 1 ; k <= n ; k = k + 1 ) {
product = product * k
}
return product ;
}
n = window.prompt( "Enter a number ", "" ) ;
document.write( “k = ”, k ) ;
document.write( “<BR>” ) ;
3
NaN
23

Example


 
document.write(n, "! = ", factorial( n ) ) ;
function factorial( n ) {
product = 1 ;
for ( k = 1 ; k <= n ; k = k + 1 ) {
product = product * k
}
return product ;
}
n = window.prompt( "Enter a number ", "" ) ;
document.write( “k = ”, k ) ;
document.write( “<BR>” ) ;
document.write(n, "! = ", factorial( n ) ) ;
function factorial( n ) {
product = 1 ;
for ( k = 1 ; k <= n ; k = k + 1 ) {
product = product * k
}
return product ;
}
n = window.prompt( "Enter a number ", "" ) ;
10! = 3628800
k = 11

document.write(n, "! = ", factorial( n ) ) ;
document.write( “<BR>” ) ;
document.write( “k = ”, k ) ;
function factorial( n ) {
var k ;
product = 1 ;
for ( k = 1 ; k <= n ; k = k + 1 ) {
product = product * k
}
return product ;
}
n = window.prompt( "Enter a number ", "" ) ;
document.write(n, "! = ", factorial( n ) ) ;
document.write( “<BR>” ) ;
document.write( “k = ”, k ) ;

 

‘k’ is a Local Variable



‘k’ is not declared or used in the main code
Instead, it is declared within the function ‘factorial’
only
‘k’ i local to the ‘factorial’ function, and does not hold any meaning outside that function
function factorial( n ) {
var k, product ;
product = 1 ;
for ( k = 1 ; k <= n ; k = k + 1 ) {
product = product * k
}
return product ;
}
n = window.prompt( "Enter a number ", "" ) ;
document.write(n, "! = ", factorial( n ) ) ;
document.write( “<BR>” ) ;
document.write( product ) ;

Local Variables



Declaring variables (using the var keyword) within a function, makes them local•They are available only
within the function and hold no meaning outside of it

Global Variables



All other variables used in a Web page (or window) are global
They can be manipulated from the main code as well as from any of the functions
They include:
–All variables declared in the main code
–All variables used but not declared in the main code
–All variables used but not declared in any of the functions defined on the Web page (or window)
function writeList( heading, words ) {
document.write( heading + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
document.write( words[ k ] + "<BR>" ) ;
}
}
words = new Array ( 10 ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
words[ k ] = window.prompt( "Enter word # " + k, "" ) ;
}
writeList( “Unsorted Words:”, words ) ;
words.sort( ) ;
writeList( “Sorted List:”, words ) ;
words.reverse( ) ;
writeList( “Reverse-Sorted List:”, words ) ;

Local –vs- Global



Global variables can make the logic of a Web page difficult to understand
Global variables also make the reuse and maintenance of your code much more difficult
Here ‘product’ has been made a local
variable as well
Would the functionality
change if we delete the
argument ‘words’ from these
4 places?

During Today’s Lecture …



We looked at functions and their use for solving simple problems
We became familiar with a couple of JavaScript’s built-in functions
We became familiar with the concept of local and global variables

Next Web Dev Lecture:


Event Handling



We’ll learn to appreciate the concept of event driven programming
We will produce solutions for simple problems using various event handlers

HEURISTIC:



If it’s possible to define a variable as local,
do it!

var u ;
document.write( m ) ;
var c, d ;
x = y * y ;
r = s ;
var a, b ;
p = q + 2 ;
y
r
q d
x
s
p c
m b
u a
Glob Local

Variables declared within functions are local; all others global

<Previous Lesson

Principles of Management

Next Lesson>

Home

Lesson Plan

Topics

Go to Top

Copyright © 2008-2013 zainbooks All Rights Reserved
Next Lesson
Previous Lesson
Lesson Plan
Topics
Home
Go to Top