# Introduction to Computing

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Introduction to Computing ­ CS101
VU
LESSON 26
ARRAYS
(WEB DEVELOPMENT LESSON 9)
During the last Lesson we had a discussion on Flow Control & Loops
We discussed the concept of flow control using the "if" and "switch" structures
And also the concept behind the "while" and "for" looping structures
We also solved simple problems using flow control and loop structures
if...else --?-- switch
If the action to be taken of the value of a single variable (or a single expression), use `switch'
When the action depends on the values of multiple variables (or expressions), use the `if...else' structure
Compound Statements:
for: Example 1
Operation
Condition
Initial count
for ( x = 1 ;
x < 6000 ;
x=x+1
){
for --?-- while
When the exact oumberenitt.wriioes (sx  nown, use the `for' loop
dn cum of erat tn i k ) ;
When the}number of iterations depend upon a condition being met, use the `while' loop
`for' loops become especially useful when used in conjunction with arrays
We'll find out about arrays today, and we'll probe their usefulness as part of `for' loop structures
Today's Topic:
Arrays
We will find out why we need arrays
We will become able to use arrays for solving simple problems
Array:
An indexed list of elements
We said that a variable is a container that holds a value.
Similarly, an Array can be considered a container as well, but this one can hold multiple values
Array:
An indexed list of elements
Example: There are many ways of assigning identifiers to the following fruit
orange
apple
watermelo
fruit2
fruit3
n
fruit[ 1 ]
fruit[ 2 ]
fruit4
strawberry
fruit[ 3 ]
fruit1
fruit[ 0 ]
Array
An indexed list of elements
fruit[ 0 ], fruit[ 1 ], fruit[ 2 ], and fruit[ 3 ] are the elements of an array
`fruit' is the identifier for that array
The length of the `fruit' array is 4, i.e. `fruit' has four elements
Array
Identifier
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Introduction to Computing ­ CS101
VU
Index
Fruit [ 0 ]
Square bracket
var student1, student2, student3, student4 ;
student1 = "Waseem" ;
student2 = "Waqar" ;
student3 = "Saqlain" ;
student4 = "Daanish" ;
document.write( student1 ) ;
document.write( student2 ) ;
document.write( student3 ) ;
document.write( student4 ) ;
student = new Array( 4 ) ; //array declaration
Can you see the
student[ 0 ] = "Waseem" ;
student[ 1 ] = "Waqar" ;
using arrays
student[ 2 ] = "Saqlain" ;
along with the
student[ 3 ] = "Daanish" ;
`for' loop?
for ( x = 0 ; x < 4 ; x = x + 1 ) {
document.write( student[ x ] ) ;
}
26.1 Arrays in JavaScript
·
In JavaScript, arrays are implemented in the form of the `Array' object
·
The key property of the `Array' object is `length', i.e the number of elements in an array
·
Two of the key `Array' methods are:
­  reverse( )
­  sort( )
·
Elements of an array can be of any type; you can even have an array containing other arrays
Declaring a New Instance of the Array Object
·
`student' is an instance of the `Array' object
·
`student' was declared such that it is of length `4'
·
That is, student is an array having 4 elements
·
The four elements of the array are: `student[ 0 ]', `student[ 1 ]', `student[ 2 ]', and `student[ 3 ]'
This is the identifier
The `new' operator creates
Pair of paren-
of the new instance
an instance
theses
The `assignment' operator
student = new Array( 4 )
This is the parent object (or
Length of the new
class) of the new instance
instance of `Array'
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Introduction to Computing ­ CS101
VU
An Object
`Instances' of an Object
All instances
of an object are objects themselves!
`Property' Values of the Instances May Differ
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Introduction to Computing ­ CS101
VU
student = new Array( 4 )
26.2 Array Identifiers
The naming rules for Array identifiers are the same as were discussed for variable identifiers
Assigning Values to Array Elements
a[ 1 ] = 5 ; //the second element
name[ 5 ] = "bhola" ;
number = 5 ;
name[ number ] = name[ 5 ] ;
for ( x = 0 ; x < 10 ; x = x + 1 ) {
y[ x ] = x * x ;
}
Remember: just like C, C++ and Java, the first element of an array has an index number equal to zero
JavaScript Arrays are Heterogeneous
Unlike many other popular languages, a JavaScript Array can hold elements of multiple data types,
simultaneously
a = new Array( 9 ) ;
b = new Array( 13 ) ;
b[ 0 ] = 23.7 ;
b[ 1 ] = "Bhola Continental Hotel" ;
b[ 2 ] = a ;
26.3 The `length' Property of Arrays
`length' is a property of
the object `d'
`d' is an instance of the
`Array' object
d = new Array ( 5 ) ;
document.write( d.length ) ;
The `length' Property of Arrays
x = new Array ( 10 ) ;
using `x.length' here
for ( x = 0 ; x < 10 ; x = x + 1 ) {
y[ x ] = x * x ;
literal `10'?
}
x = new Array ( 10 ) ;
for ( x = 0 ; x < x.length ; x = x + 1 ) {
y[ x ] = x * x ;
}
26.4 Array Methods: sort( )
26.5 Sorts the elements in alphabetical order
x = new Array ( 4 ) ;
Saqlain
Shoaib
x[ 0 ] = "Waseem" ;
Waqar
x[ 1 ] = "Waqar" ;
Waseem
x[ 2 ] = "Saqlain" ;
x[ 3 ] = "Shoaib" ;
x.sort( ) ;
for ( k = 0 ; k < x.length; k = k + 1 ) { document.write( x[ k ] + "<BR>" ) ;
}
Were the elements sorted in ascending or descending order?
What if you wanted to arrange them in the reverse order?
26.6 Array Methods: reverse( )
26.7 Reverses the order of the elements
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Introduction to Computing ­ CS101
VU
x = new Array ( 4 ) ;
Saqlain
Is this the
x[ 0 ] = "Waseem" ;
Shoaib
required
x[ 1 ] = "Waqar" ;
Waqar
result?
x[ 2 ] = "Saqlain" ;
Waseem
x[ 3 ] = "Shoaib" ;
x.reverse( ) ;
x.sort( ) ;
for ( k = 0 ; k < x.length; k = k + 1 ) { document.write( x[ k ] + "<BR>") ;
}
Array Methods: reverse( )
Reverses the order of the elements
x = new Array ( 4 ) ;
Waseem
x[ 0 ] = "Waseem" ;
Waqar
x[ 1 ] = "Waqar" ;
Shoaib
x[ 2 ] = "Saqlain" ;
Saqlain
x[ 3 ] = "Shoaib" ;
x.sort( ) ;
x.reverse( ) ;
for ( k = 0 ; k < x.length; k = k + 1 ) { document.write( x[ k ] + "<BR>") ;
}
Let's Now Do a More Substantial Example
Develop a Web page that prompts the user for 10 words, and then displays them in form of a list in two
different ways:
1.In the order in which the words were entered
2.In a sorted order
We will try to show you the complete code - the JavaScript part as well as the HTML part - for this
example
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Introduction to Computing ­ CS101
VU
26.7 Pseudo Code
1.Declare the array that will be used for storing the words
2.Prompt the user and read the user input into the elements of the array
3.Now write the array to the document
4.Sort the array
5.Write the sorted array to the document
<HTML>
<TITLE>Sort Ten Words</TITLE>
<SCRIPT>
words = new Array ( 10 ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
words[ k ] = window.prompt( "Enter word # " + k, "" ) ;
}
document.write( "UNSORTED WORDS:" + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
document.write( words[ k ] + "<BR>" ) ;
}
words.sort( ) ;
document.write( "SORTED WORDS:" + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
document.write( words[ k ] + "<BR>" ) ;
}
</SCRIPT>
<BODY>
</BODY>
</HTML>
<HTML>
<TITLE>Sort Ten Words</TITLE>
<SCRIPT>
//JavaScript Code
</SCRIPT>
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Introduction to Computing ­ CS101
VU
<BODY>
</BODY>
</HTML>
The next three slides show the JavaScript code that goes between the <SCRIPT>, </SCRIPT> tags
Pseudo Code
·Declare the array that will be used for storing the words
·Prompt the user and read the user input into the elements of the array
·Now write the array to the document
·Sort the array
·Write the sorted array to the document
words = new Array ( 10 ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) {
words[ k ] = window.prompt(
"Enter word # " + k, "" ) ;
}
This method is used for collecting data from the user. It can display a
message and provides a field in which the user can enter data
Pseudo Code
1.Declare the array that will be used for storing the words
2.Prompt the user and read the user input into the elements of the array
3.Now write the array to the document
4.Sort the array
5.Write the sorted array to the document
document.write( "Unsorted Words:" + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) { document.write( words[ k ] + "<BR>" ) ;
}
Pseudo Code
1.Declare the array that will be used for storing the words
2.Prompt the user and read the user input into the elements of the array
3.Now write the array to the document
4.Sort the array
5.Write the sorted array to the document
words.sort( ) ;
document.write( "Sorted Words:" + "<BR>" ) ;
for ( k = 0 ; k < words.length ; k = k + 1 ) { document.write( words[ k ] + "<BR>" ) ;
}
During Today's Lesson ...
·We found out why we need arrays
·We became able to use arrays for solving simple problems
Next (the 10th) Web Dev Lecture:
Functions & Variable Scope
·To become familiar with some of JavaScript's built-in functions
·To be able to understand the concept of user-defined functions and their use for solving simple
problems
·To become familiar with the concept of local and global variables
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