JavaScript Arrays

IPA
1st
Semester, 2007-2008
Internet 1
Ch. 12
JavaScript: Arrays
attasr@ipa.edu.sa

09/30/15 © Reem Al-Attas 2
Introduction
Arrays
 Data structures of related items
 Also called Collections
 Dynamic

Arrays
Arrays in JavaScript
 Each element referenced by a number
 Start at “zeroth element”
 Subscript or index
 Accessing a specific element
 Name of array
 Brackets
 Number of element
 Arrays know their length
 length property

Arrays
c[ 6 ]
-45
6
0
72
1543
-89
0
62
-3
1
6453
78
Name of array c[ 0 ]
c[ 1 ]
c[ 2 ]
c[ 3 ]
c[ 11 ]
c[ 10 ]
c[ 9 ]
c[ 8 ]
c[ 7 ]
c[ 5 ]
c[ 4 ]
Position number (index
or subscript) of the
element within array c
Fig. 11.1 A 12-element array.

Arrays
Operators Associativity Type
() [] . left to right highest
++ -- ! right to left unary
* / % left to right multiplicative
+ - left to right additive
< <= > >= left to right relational
== != left to right equality
&& left to right logical AND
|| left to right logical OR
?: right to left conditional
= += -= *= /= %= right to left assignment
Fig. 11.2 Precedence and associativity of the operators discussed so far.

Declaring and Allocating Arrays
Arrays in memory
 Objects
 Operator new
 Allocates memory for objects
 Dynamic memory allocation operator
var c;
c = new Array( 12 );

Examples Using Arrays
Arrays grow dynamically
 Allocate more space as items are added
Must initialize array elements
 Default value is undefined
 for loops convenient
 Referring to uninitialized elements or elements
outside array bounds is an error

1 <?xml version = "1.0"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
4
5 
6 
7
8 <html xmlns = "http://www.w3.org/1999/xhtml">
9 <head>
10 <title>Initializing an Array</title>
11
12 <script type = "text/javascript">
13 <!--
14 // this function is called when the <body> element's
15 // onload event occurs
16 function initializeArrays()
17 {
18 var n1 = new Array( 5 ); // allocate 5-element Array
19 var n2 = new Array(); // allocate empty Array
20
21 // assign values to each element of Array n1
22 for ( var i = 0; i < n1.length; ++i )
23 n1[ i ] = i;
Array n1 has five elements.
The for loop initializes the elements in n1 to
their subscript numbers (0 to 4).
Array n2 is an empty array.

24
25 // create and initialize five-elements in Array n2
26 for ( i = 0; i < 5; ++i )
27 n2[ i ] = i;
28
29 outputArray( "Array n1 contains", n1 );
30 outputArray( "Array n2 contains", n2 );
31 }
32
33 // output "header" followed by a two-column table
34 // containing subscripts and elements of "theArray"
35 function outputArray( header, theArray )
36 {
37 document.writeln( "<h2>" + header + "</h2>" );
38 document.writeln( "<table border = "1" width =" +
39 ""100%">" );
40
41 document.writeln( "<thead><th width = "100"" +
42 "align = "left">Subscript</th>" +
43 "<th align = "left">Value</th></thead><tbody>" );
The for loop adds five elements to Array n2 and initialize
each element to its subscript number (0 to 4).
Each function displays the
contents of its respective Array
in an XHTML table.
The first time function ouputArray is called,
variable header gets the value of “Array n1
contains” and variable theArray gets the
value of n1.
The second time function ouputArray is
called, variable header gets the value of
“Array n2 contains” and variable
theArray gets the value of n2.

44
45 for ( var i = 0; i < theArray.length; i++ )
46 document.writeln( "<tr><td>" + i + "</td><td>" +
47 theArray[ i ] + "</td></tr>" );
48
49 document.writeln( "</tbody></table>" );
50 }
51 // -->
52 </script>
53
54 </head><body onload = "initializeArrays()"></body>
55 </html>

Possible to declare and initialize in one
step
 Specify list of values
 Initializer list
var n = [ 10, 20, 30, 40, 50 ];
var n = new Array( 10, 20, 30, 40,
50 );
 Also possible to only initialize some values
 Leave uninitialized elements blank
 Uninitialized elements default to “undefined”
var n = [ 10, 20, , 40, 50 ];

4
5 
6 
7
9 <head>
10 <title>Initializing an Array with a Declaration</title>
11
13 <!--
14 function start()
15 {
16 // Initializer list specifies number of elements and
17 // value for each element.
18 var colors = new Array( "cyan", "magenta",
19 "yellow", "black" );
20 var integers1 = [ 2, 4, 6, 8 ];
21 var integers2 = [ 2, , , 8 ];
22
23 outputArray( "Array colors contains", colors );
24 outputArray( "Array integers1 contains", integers1 );
25 outputArray( "Array integers2 contains", integers2 );
26 }
Array integers1 is initialized using an initializer list.
Two values are not supplied for integers2,
which will be displayed as undefined.

27
28 // output "header" followed by a two-column table
29 // containing subscripts and elements of "theArray"
31 {
32 document.writeln( "<h2>" + header + "</h2>" );
33 document.writeln( "<table border = "1"" +
34 "width = "100%">" );
35 document.writeln( "<thead><th width = "100" " +
36 "align = "left">Subscript</th>" +
37 "<th align = "left">Value</th></thead><tbody>" );
38
40 document.writeln( "<tr><td>" + i + "</td><td>" +
41 theArray[ i ] + "</td></tr>" );
42
44 }
45 // -->
46 </script>
47
48 </head><body onload = "start()"></body>
49 </html>

 for…in statement
 Perform an action for each element in an array
 Iterates over array elements
 Assigns each element to specified variable one at a
time
 Ignores non-existent elements

SumArray.html
(1 of 2)
4
5 
6 
7
9 <head>
10 <title>Sum the Elements of an Array</title>
11
13 <!--
14 function start()
15 {
16 var theArray = [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];
17 var total1 = 0, total2 = 0;
18
20 total1 += theArray[ i ];
21
22 document.writeln( "Total using subscripts: " + total1 );
23
The for loop sums the values contained in the 10-
element integer array called theArray.

24 for ( var element in theArray )
25 total2 += theArray[ element ];
26
27 document.writeln( "<br />Total using for...in: " +
28 total2 );
29 }
30 // -->
31 </script>
32
34 </html>
Variable element is assigned a subscript
in the range of 0 up to, but not including,
theArray.length.

Arrays can provide shorter and cleaner
substitute for switch statements
 Each element represents one case

RollDie.html
(1 of 2)
4
5 
6 
7
9 <head>
10 <title>Roll a Six-Sided Die 6000 Times</title>
11
13 <!--
14 var face, frequency = [ , 0, 0, 0, 0, 0, 0 ];
15
16 // summarize results
17 for ( var roll = 1; roll <= 6000; ++roll ) {
18 face = Math.floor( 1 + Math.random() * 6 );
19 ++frequency[ face ];
20 }
21
Referencing Array frequency replaces the switch
statement used in Chapter 10’s example.

RollDie.html
(2 of 2)
22 document.writeln( "<table border = "1"" +
23 "width = "100%">" );
24 document.writeln( "<thead><th width = "100"" +
25 " align = "left">Face<th align = "left">" +
26 "Frequency</th></thead></tbody>" );
27
28 for ( face = 1; face < frequency.length; ++face )
29 document.writeln( "<tr><td>" + face + "</td><td>" +
30 frequency[ face ] + "</td></tr>" );
31
33 // -->
34 </script>
35
36 </head>
37 <body>
38 <p>Click Refresh (or Reload) to run the script again</p>
39 </body>
40 </html>

Random Image Generator Using
Arrays
Cleaner approach than previous version
 Specify any file name rather than integers 1-7
 Result of Math.random call is index into array
of image file names

RandomPicture2
.html
(1 of 2)
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
3 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
4
5 
6 
7
9 <head>
10 <title>Random Image Generator</title>
11
13 <!--
14 var pictures =
15 [ "CPE", "EPT", "GPP", "GUI", "PERF", "PORT", "SEO" ];

RandomPicture2
.html
(2 of 2)
16
17 document.write ( "<img src = "" +
18 pictures[ Math.floor( Math.random() * 7 ) ] +
19 ".gif" width = "105" height = "100" />" );
20 // -->
21 </script>
22
23 </head>
24
25 <body>
26 <p>Click Refresh (or Reload) to run the script again</p>
27 </body>
28 </html>

Random Image Generator Using
Arrays

References and Reference
Parameters
Two ways to pass parameters
 Pass-by-value
 Pass copy of original value
 Default for numbers and booleans
 Original variable is unchanged
 Pass-by-reference
 How objects are passed, like arrays
 Pass location in memory of value
 Allows direct access to original value
 Improves performance

Passing Arrays to Functions
Name of array is argument
 Not necessary to also pass size of array
 Arrays know their size
 Passed by reference
 Individual elements are passed by value if numbers
or booleans
 Array.join
 Creates string containing all array elements
 Specify separator

PassArray.html
(1 of 3)
4
5 
6 
7
9 <head>
10 <title>Passing Arrays and Individual Array
11 Elements to Functions</title>
12
14 <!--
15 function start()
16 {
17 var a = [ 1, 2, 3, 4, 5 ];
18
19 document.writeln( "<h2>Effects of passing entire " +
20 "array call-by-reference</h2>" );
21 outputArray(
22 "The values of the original array are: ", a );
23
24 modifyArray( a ); // array a passed call-by-reference
25
The first call to function outputArray displays the
contents of the Array a before it is modified.
Function modifyArray multiplies each element by 2.

PassArray.html
(2 of 3)
26 outputArray(
27 "The values of the modified array are: ", a );
28
29 document.writeln( "<h2>Effects of passing array " +
30 "element call-by-value</h2>" +
31 "a[3] before modifyElement: " + a[ 3 ] );
32
33 modifyElement( a[ 3 ] );
34
35 document.writeln(
36 "<br />a[3] after modifyElement: " + a[ 3 ] );
37 }
38
39 // outputs "header" followed by the contents of "theArray"
41 {
42 document.writeln(
43 header + theArray.join( " " ) + "<br />" );
44 }
45
Again, function outputArray is called to show
that the contents of Array a have been modified.
Method join takes as its argument a string
containing a separator that should be used to
separate the elements of the array in the string
that is returned.
Function modifyElement multiplies the
contents of a[ 3 ] by 2.
The value of a[ 3 ] is output to show its
contents before it is modified.

PassArray.html
(3 of 3)
46 // function that modifies the elements of an array
47 function modifyArray( theArray )
48 {
49 for ( var j in theArray )
50 theArray[ j ] *= 2;
51 }
52
53 // function that attempts to modify the value passed
54 function modifyElement( e )
55 {
56 e *= 2;
57 document.writeln( "<br />value in modifyElement: " + e );
58 }
59 // -->
60 </script>
61
63 </html>
Multiply each element in theArray by 2.

Passing Arrays to Functions

Sorting Arrays
Sorting
 Important computing task
 Array.sort
 Defaults to string comparison
 Optional comparator function
 Return negative if first argument less than second
 Return zero if arguments equal
 Return positive if first argument greater than second

Sort.html
(1 of 2)
4
5 
6 
7
9 <head>
10 <title>Sorting an Array with Array Method sort</title>
11
13 <!--
14 function start()
15 {
16 var a = [ 10, 1, 9, 2, 8, 3, 7, 4, 6, 5 ];
17
18 document.writeln( "<h1>Sorting an Array</h1>" );
19 outputArray( "Data items in original order: ", a );
20 a.sort( compareIntegers ); // sort the array
21 outputArray( "Data items in ascending order: ", a );
22 }
Method sort takes as its optional argument the name of a
function that compares two arguments and returns a value
of –1, 0 or 1.

Sort.html
(2 of 2)
23
24 // outputs "header" followed by the contents of "theArray"
26 {
27 document.writeln( "<p>" + header +
28 theArray.join( " " ) + "</p>" );
29 }
30
31 // comparison function for use with sort
32 function compareIntegers( value1, value2 )
33 {
34 return parseInt( value1 ) - parseInt( value2 );
35 }
36 // -->
37 </script>
38
40 </html>
Function compareIntegers calculates the difference
between the integer values of its arguments.

Sorting Arrays

Searching Arrays: Linear Search
and Binary Search
 Searching
 Look for matching key value
 Linear search
 Iterate through each element until match found
 Inefficient
 Worst case scenario, must test entire array
 Binary search
 Requires sorted data
 Cuts search range in half each iteration
 Efficient
 Only look at small fraction of elements

LinearSearch.html
(1 of 3)
4
5 
6 
7
9 <head>
10 <title>Linear Search of an Array</title>
11
13 <!--
14 var a = new Array( 100 ); // create an Array
15
16 // fill Array with even integer values from 0 to 198
17 for ( var i = 0; i < a.length; ++i )
18 a[ i ] = 2 * i;
19
Array a is initiated with 100 elements.
Array a is populated with the even integers 0 to 198.

LinearSearch.html
(2 of 3)
20 // function called when "Search" button is pressed
21 function buttonPressed()
22 {
23 var searchKey = searchForm.inputVal.value;
24
25 // Array a is passed to linearSearch even though it
26 // is a global variable. Normally an array will
27 // be passed to a method for searching.
28 var element = linearSearch( a, parseInt( searchKey ) );
29
30 if ( element != -1 )
31 searchForm.result.value =
32 "Found value in element " + element;
33 else
34 searchForm.result.value = "Value not found";
35 }
36
Get value of search key from the input field in
the XHTML form.
Calling function linearSearch and passing it the
Array a and the value of variable searchKey as
an integer.

LinearSearch.html
(3 of 3)
37 // Search "theArray" for the specified "key" value
38 function linearSearch( theArray, key )
39 {
40 for ( var n = 0; n < theArray.length; ++n )
41 if ( theArray[ n ] == key )
42 return n;
43
44 return -1;
45 }
46 // -->
47 </script>
48
49 </head>
50
51 <body>
52 <form name = "searchForm" action = "">
53 <p>Enter integer search key<br />
54 <input name = "inputVal" type = "text" />
55 <input name = "search" type = "button" value = "Search"
56 onclick = "buttonPressed()" /><br /></p>
57
58 <p>Result<br />
59 <input name = "result" type = "text" size = "30" /></p>
60 </form>
61 </body>
62 </html>
Variable theArray gets the value of
Array a and variable key gets the
value of variable searchKey.Function linearSearch compares each
each element with a search key.

and Binary Search

BinarySearch.html
(1 of 5)
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
4
5 
6 
7
9 <head>
10 <title>Binary Search</title>
11
13 <!--
14 var a = new Array( 15 );
15
16 for ( var i = 0; i < a.length; ++i )
17 a[ i ] = 2 * i;
18
Array a is initialized with 15 elements.

BinarySearch.html
(2 of 5)
19 // function called when "Search" button is pressed
20 function buttonPressed()
21 {
22 var searchKey = searchForm.inputVal.value;
23
24 searchForm.result.value =
25 "Portions of array searchedn";
26
27 // Array a is passed to binarySearch even though it
28 // is a global variable. This is done because
29 // normally an array is passed to a method
30 // for searching.
31 var element =
32 binarySearch( a, parseInt( searchKey ) );
33
34 if ( element != -1 )
35 searchForm.result.value +=
36 "nFound value in element " + element;
37 else
38 searchForm.result.value += "nValue not found";
39 }
40
Function binarySearch receives two arguments:
the Array a and the search key, searchKey.

BinarySearch.html
(3 of 5)
41 // Binary search
42 function binarySearch( theArray, key )
43 {
44 var low = 0; // low subscript
45 var high = theArray.length - 1; // high subscript
46 var middle; // middle subscript
47
48 while ( low <= high ) {
49 middle = ( low + high ) / 2;
50
51 // The following line is used to display the
52 // part of theArray currently being manipulated
53 // during each iteration of the binary
54 // search loop.
55 buildOutput( theArray, low, middle, high );
56
57 if ( key == theArray[ middle ] ) // match
58 return middle;
59 else if ( key < theArray[ middle ] )
60 high = middle - 1; // search low end of array
61 else
62 low = middle + 1; // search high end of array
63 }
If the key matches the middle element of a
subarray, the subscript of the current element is
returned.
If key is less than the middle element, the high
subscript is set to middle – 1.
If key is greater then the middle elements, the
high subscript is set to middle + 1.

BinarySearch.html
(4 of 5)
64
65 return -1; // searchKey not found
66 }
67
68 // Build one row of output showing the current
69 // part of the array being processed.
70 function buildOutput( theArray, low, mid, high )
71 {
72 for ( var i = 0; i < theArray.length; i++ ) {
73 if ( i < low || i > high )
74 searchForm.result.value += " ";
75 // mark middle element in output
76 else if ( i == mid )
77 searchForm.result.value += theArray[ i ] +
78 ( theArray[ i ] < 10 ? "* " : "* " );
79 else
80 searchForm.result.value += theArray[ i ] +
81 ( theArray[ i ] < 10 ? " " : " " );
82 }
83
84 searchForm.result.value += "n";
85 }
86 // -->
87 </script>
88 </head>
89
Function buildOutput creates the markup that
displays the results of the search.

90 <body>
91 <form name = "searchForm" action = "">
92 <p>Enter integer search key<br />
93 <input name = "inputVal" type = "text" />
94 <input name = "search" type = "button" value =
95 "Search" onclick = "buttonPressed()" /><br /></p>
96 <p>Result<br />
97 <textarea name = "result" rows = "7" cols = "60">
98 </textarea></p>
99 </form>
100 </body>
101</html>

and Binary Search

Multidimensional Arrays
Two-dimensional arrays analogous to
tables
 Rows and columns
 Specify row first, then column
 Two subscripts

a[ 1 ][ 0 ] a[ 1 ][ 1 ] a[ 1 ][ 2 ] a[ 1 ][ 3 ]
Row 0
Row 1
Row 2
Column 0 Column 1 Column 2 Column 3
Row subscript (or index)
Array name
Column subscript (or index)
a[ 0 ][ 0 ] a[ 0 ][ 1 ] a[ 0 ][ 2 ] a[ 0 ][ 3 ]
a[ 2 ][ 0 ] a[ 2 ][ 1 ] a[ 2 ][ 2 ] a[ 2 ][ 3 ]
Fig. 11.12 Two-dimensional array with three rows and four columns.

Declaring and initializing multidimensional
arrays
 Group by row in square brackets
 Treated as arrays of arrays
 Creating array b with one row of two elements
and a second row of three elements:
var b = [ [ 1, 2 ], [ 3, 4, 5 ] ];

Also possible to use new operator
 Create array b with two rows, first with five
columns and second with three:
var b;
b = new Array( 2 );
b[ 0 ] = new Array( 5 );
b[ 1 ] = new Array( 3 );

InitArray3.html
(1 of 2)
4
5 
6 
7
9 <head>
10 <title>Initializing Multidimensional Arrays</title>
11
13 <!--
14 function start()
15 {
16 var array1 = [ [ 1, 2, 3 ], // first row
17 [ 4, 5, 6 ] ]; // second row
18 var array2 = [ [ 1, 2 ], // first row
19 [ 3 ], // second row
20 [ 4, 5, 6 ] ]; // third row
21
22 outputArray( "Values in array1 by row", array1 );
23 outputArray( "Values in array2 by row", array2 );
24 }
Array array1 provides six initializers in
two rows.
Array array2 provides six initializers in
three rows.
Function outputArray displays each array’s
elements in a Web page.

InitArray3.html
(2 of 2)
25
27 {
28 document.writeln( "<h2>" + header + "</h2><tt>" );
29
30 for ( var i in theArray ) {
31
32 for ( var j in theArray[ i ] )
33 document.write( theArray[ i ][ j ] + " " );
34
35 document.writeln( "<br />" );
36 }
37
38 document.writeln( "</tt>" );
39 }
40 // -->
41 </script>
42
44 </html>
Referencing the multidimensional
array theArray.

Different array manipulations
using for and for/in
var total = 0;
for (var row = 0; row < a.length; ++row )
for (var col = 0; col < a[ row ].length; ++col )
total += a[ row ][ col ];
identical to
var total = 0;
for (var row in a )
for (var col in a[ row ] )
total += a[ row ][ col ];
Both statements total the elements of the array,
one row at a time

Building an Online Quiz
Radio buttons
 Represented as an array
 Name of radio buttons is name of array
 One element per button
 checked property is true when selected
XHTML Forms
 Contain controls, including radio buttons
 action property specifies what happens when
submitted
 Can call JavaScript code

Quiz.html
(1 of 2)
1 <?xml version = "1.0" encoding = "utf-8"?>
2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
3 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
4
5 
6 
7
9 <head>
10 <title>Online Quiz</title>
11
12 <script type = "text/JavaScript">
13
14 function checkAnswers()
15 {
16 // determine whether the answer is correct
17 if ( myQuiz.radiobutton[ 1 ].checked )
18 document.write( "Congratulations, your answer is correct" );
19 else // if the answer is incorrect
20 document.write( "Your answer is incorrect. Please try again" );
21 }
22
23 </script>
24
25 </head>
Determining the value of property
checked.

Quiz.html
(2 of 2)
26
27 <body>
28 <form id = "myQuiz" action = "JavaScript:checkAnswers()">
29 <p>Select the name of the tip that goes with the image shown:<br />
30 <img src="EPT.gif" width="108" height="100" alt="mystery tip"/>
31 <br />
32
33 <input type = "radio" name = "radiobutton" value = "CPE" />
34 <label>Common Programming Error</label>
35
36 <input type = "radio" name = "radiobutton" value = "EPT" />
37 <label>Error-Prevention Tip</label>
38
39 <input type = "radio" name = "radiobutton" value = "PERF" />
40 <label>Performance Tip</label>
41
42 <input type = "radio" name = "radiobutton" value = "PORT" />
43 <label>Portability Tip</label><br />
44
45 <input type = "submit" name = "submit" value = "Submit" />
46 <input type = "reset" name = "reset" value = "Reset" />
47 </p>
48 </form>
49 </body>
50 </html>
Call the checkAnswers function
when the form is submitted.

Building an Online Quiz

Assignment 9
 1) Exercise # 11.10. Due Date for A # 9:
 Next Monday before
your lecture.

JavaScript Arrays

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