Javascript.ppt

 2004 Prentice Hall, Inc. All rights reserved.
1
Chapter 10 - JavaScript: Functions
Outline
10.1 Introduction
10.2 Program Modules in JavaScript
10.3 Programmer-Defined Functions
10.4 Function Definitions
10.5 Random-Number Generation
10.6 Example: Game of Chance
10.7 Another Example: Random Image Generator
10.8 Scope Rules
10.9 JavaScript Global Functions
10.10 Recursion
10.11 Recursion vs. Iteration
10.12 Web Resources

2
Objectives
• In this tutorial, you will learn:
– To understand how to construct programs modularly from
small pieces called functions.
– To be able to create new functions.
– To understand the mechanisms used to pass information
between functions.
– To introduce simulation techniques that use random-number
generation.
– To understand how the visibility of identifiers is limited to
specific regions of programs.

3
10.1 Introduction
• Software design
– Break software up into modules
• Easier to maintain and debug
– Divide and conquer

4
• Modules in JavaScript
– Functions
– Methods
• Belong to an object
– JavaScript includes many useful pre-defined methods
• Combine with programmer-defined methods to make a
program

5
• Functions
– Started by function call
– Receive necessary information via arguments (parameters)
– Boss-Worker relationship
• Calling function
• Called function
• Return value when finished
• Can have many tiers

6
boss
worker1 worker2 worker3
worker4 worker5
Fig. 10.1 Hierarchical boss-function/worker-function relationship.

7
• Function calls
– Name
– Left parenthesis
– Arguments separated by commas
• Constants, variables or expressions
– Right parenthesis
– Examples:
total += parseFloat( inputValue );
total += parseFloat( s1 + s2 );

8
10.3 Programmer-Defined Functions
• Defining functions
– All variables declared in function are called local
• Do not exist outside current function
– Parameters
• Also local variables
– Promotes reusability
• Keep short
• Name clearly

9
• Format of a function definition
function function-name( parameter-list )
{
declarations and statements
}
– Function name any valid identifier
– Parameter list names of variables that will receive arguments
• Must have same number as function call
• May be empty
– Declarations and statements
• Function body (“block” of code)

10
• Returning control
– return statement
– Can return either nothing, or a value
return expression;
– No return statement same as return;
– Not returning a value when expected is an error

11
• Writing a function to square two numbers
– for loop from 1 to 10
– Pass each number as argument to square
– return value of argument multiplied by itself
– Display result

 2004 Prentice Hall, Inc.
All rights reserved.
Outline
12
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>A Programmer-Defined square Function</title>
11
12 <script type = "text/javascript">
13 <!--
14 document.writeln(
15 "<h1>Square the numbers from 1 to 10</h1>" );
16
17 // square the numbers from 1 to 10
18 for ( var x = 1; x <= 10; ++x )
19 document.writeln( "The square of " + x + " is " +
20 square( x ) + "<br />" );
21
SquareInt.html
(1 of 2)
Calling function square and passing it the value of x.

Outline
13
22 // The following square function's body is executed
23 // only when the function is explicitly called.
24
25 // square function definition
26 function square( y )
27 {
28 return y * y;
29 }
30 // -->
31 </script>
32
33 </head><body></body>
34 </html>
SquareInt.html
(2 of 2)
Variable y gets the value of variable x.
The return statement passes the value of y * y
back to the calling function.

14
Fig. 10.2 Using programmer-defined function square.

15
• Finding the maximum of 3 numbers
– Prompt for 3 inputs
– Convert to numbers
– Pass to maximum
– Math.max

Outline
16
4
5 
6 
7
9 <head>
10 <title>Finding the Maximum of Three Values</title>
11
13 <!--
14 var input1 =
15 window.prompt( "Enter first number", "0" );
16 var input2 =
17 window.prompt( "Enter second number", "0" );
18 var input3 =
19 window.prompt( "Enter third number", "0" );
20
21 var value1 = parseFloat( input1 );
Maximum.html
(1 of 2)
Prompt for the user to input three integers.

Outline
17
24
25 var maxValue = maximum( value1, value2, value3 );
26
27 document.writeln( "First number: " + value1 +
28 "<br />Second number: " + value2 +
29 "<br />Third number: " + value3 +
30 "<br />Maximum is: " + maxValue );
31
32 // maximum method definition (called from line 25)
33 function maximum( x, y, z )
34 {
35 return Math.max( x, Math.max( y, z ) );
36 }
37 // -->
38 </script>
39
40 </head>
41 <body>
42 <p>Click Refresh (or Reload) to run the script again</p>
43 </body>
44 </html>
Maximum.html
(2 of 2)
Call function maximum and pass it the value of
variables value1, value2 and value3.
Variables x, y and z get the value of variables
value1, value2 and value3, respectively.
Method max returns the larger of the two
integers passed to it.

18
Fig. 10.3 Programmer-defined maximum function (1 of 2).

19
Fig. 10.3 Programmer-defined maximum function (2 of 2).

20
• Random-number generation introduces element of
chance
– Math.random
var randomValue = Math.random();
– Floating point value between 0 and 1
– Adjust range by scaling and shifting
– Math.floor
• Always round down
Math.floor( 1 + Math.random() * 6 )

Outline
21
4
5 
6 
7
9 <head>
10 <title>Shifted and Scaled Random Integers</title>
11
13 <!--
14 var value;
15
17 "<table border = "1" width = "50%">" );
19 "<caption>Random Numbers</caption><tr>" );
20
RandomInt.html
(1 of 2)

Outline
22
21 for ( var i = 1; i <= 20; i++ ) {
22 value = Math.floor( 1 + Math.random() * 6 );
23 document.writeln( "<td>" + value + "</td>" );
24
25 // write end and start <tr> tags when
26 // i is a multiple of 5 and not 20
27 if ( i % 5 == 0 && i != 20 )
28 document.writeln( "</tr><tr>" );
29 }
30
31 document.writeln( "</tr></table>" );
32 // -->
33 </script>
34
35 </head>
36 <body>
38 </body>
39 </html>
RandomInt.html
(2 of 2)
The for loop creates 20
table cells (4 rows x 5
columns).
Each cell is populated
with a random number
generated by method
random.
Method floor rounds the number
generated by method random down.

23
Fig. 10.4 Random integers, shifting and scaling.

Outline
24
4
5 
6 
7
9 <head>
10 <title>Roll a Six-Sided Die 6000 Times</title>
11
13 <!--
14 var frequency1 = 0, frequency2 = 0,
15 frequency3 = 0, frequency4 = 0,
16 frequency5 = 0, frequency6 = 0, face;
17
18 // summarize results
19 for ( var roll = 1; roll <= 6000; ++roll ) {
20 face = Math.floor( 1 + Math.random() * 6 );
21
RollDie.html
(1 of 3)
This expression uses method random to
generate a random number between 1 and 6.

Outline
25
RollDie.html
(2 of 3)
22 switch ( face ) {
23 case 1:
24 ++frequency1;
25 break;
26 case 2:
27 ++frequency2;
28 break;
29 case 3:
30 ++frequency3;
31 break;
32 case 4:
33 ++frequency4;
34 break;
35 case 5:
36 ++frequency5;
37 break;
38 case 6:
39 ++frequency6;
40 break;
41 }
42 }
43
When the controlling expression, face,
matches a case label, the respective
frequency variable is incremented.

Outline
26
RollDie.html
(3 of 3)
44 document.writeln( "<table border = "1"" +
45 "width = "50%">" );
46 document.writeln( "<thead><th>Face</th>" +
47 "<th>Frequency<th></thead>" );
48 document.writeln( "<tbody><tr><td>1</td><td>" +
49 frequency1 + "</td></tr>" );
50 document.writeln( "<tr><td>2</td><td>" + frequency2 +
51 "</td></tr>" );
53 "</td></tr>" );
55 "</td></tr>" );
57 "</td></tr>" );
59 "</td></tr></tbody></table>" );
60 // -->
61 </script>
62
63 </head>
64 <body>
66 </body>
67 </html>
The results of rolling the die
600 times are displayed in a
table.

27
Fig. 10.5 Rolling a six-sided die 6000 times.

28
• Craps
– Click Roll Dice
– Text fields show rolls, sum and point
– Status bar displays results

29
• Uses XHTML forms
– Gather multiple inputs at once
– Empty action attribute
– name attribute allows scripts to interact with form
• Event handling and event-driven programming
– Assign a function to an event
– Onclick
• Constants
– Variable that cannot be modified
– Part of many languages, not supported in JavaScript
• Name “constant” variables with all capital letters
– Make values easier to remember/change

30
• Changing properties
– Access with dot (.) notation
– value property of text fields
– status property of window

Outline
31
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>Program that Simulates the Game of Craps</title>
11
13 <!--
14 // variables used to test the state of the game
15 var WON = 0, LOST = 1, CONTINUE_ROLLING = 2;
16
17 // other variables used in program
18 var firstRoll = true, // true if first roll
19 sumOfDice = 0, // sum of the dice
20 myPoint = 0, // point if no win/loss on first roll
21 gameStatus = CONTINUE_ROLLING; // game not over yet
22
Craps.html
(1 of 5)

Outline
32
23 // process one roll of the dice
24 function play()
25 {
26 if ( firstRoll ) { // first roll of the dice
27 sumOfDice = rollDice();
28
29 switch ( sumOfDice ) {
30 case 7: case 11: // win on first roll
31 gameStatus = WON;
32 // clear point field
33 document.craps.point.value = "";
34 break;
35 case 2: case 3: case 12: // lose on first roll
36 gameStatus = LOST;
37 // clear point field
38 document.craps.point.value = "";
39 break;
40 default: // remember point
41 gameStatus = CONTINUE_ROLLING;
42 myPoint = sumOfDice;
43 document.craps.point.value = myPoint;
44 firstRoll = false;
45 }
46 }
Craps.html
(2 of 5)
If the value of firstRoll is true, then function
rollDice is called.
If function rollDice returns a value of 7
or 11, the player wins and the break
statement causes program control
proceeds to the first line after the switch
structure.
If function rollDice returns a 2, 3 or 12, the
player loses and the break statement causes
control to proceed to first line after the switch
structure.

Outline
33
47 else {
48 sumOfDice = rollDice();
49
50 if ( sumOfDice == myPoint ) // win by making point
51 gameStatus = WON;
52 else
53 if ( sumOfDice == 7 ) // lose by rolling 7
54 gameStatus = LOST;
55 }
56
57 if ( gameStatus == CONTINUE_ROLLING )
58 window.status = "Roll again";
59 else {
60 if ( gameStatus == WON )
61 window.status = "Player wins. " +
62 "Click Roll Dice to play again.";
63 else
64 window.status = "Player loses. " +
65 "Click Roll Dice to play again.";
66
67 firstRoll = true;
68 }
69 }
70
Craps.html
(3 of 5)
If the value returned by function rollDice
equals the value of variable myPoint, the player
wins because the point has been reached.
If the values returned by function rollDice
equals 7, the player loses.
window method status displays a
message in the status bar of the browser.
If the value of firstRoll is false,
function rollDice is called to see if the
point has been reached.

Outline
34
Craps.html
(4 of 5)
71 // roll the dice
72 function rollDice()
73 {
74 var die1, die2, workSum;
75
76 die1 = Math.floor( 1 + Math.random() * 6 );
77 die2 = Math.floor( 1 + Math.random() * 6 );
78 workSum = die1 + die2;
79
80 document.craps.firstDie.value = die1;
81 document.craps.secondDie.value = die2;
82 document.craps.sum.value = workSum;
83
84 return workSum;
85 }
86 // -->
87 </script>
88
89 </head>
Function rollDice is called to simulate
the rolling of two dice on the craps table.
Methods random and floor are used to
generate the values for the two dice.
Referencing the names of form elements
in the XHTML document, the values of
the dice are placed in their respective
form fields.

Outline
35
Craps.html
(5 of 5)
90 <body>
91 <form name = "craps" action = "">
92 <table border = "1">
93 <caption>Craps</caption>
94 <tr><td>Die 1</td>
95 <td><input name = "firstDie" type = "text" />
96 </td></tr>
97 <tr><td>Die 2</td>
98 <td><input name = "secondDie" type = "text" />
99 </td></tr>
100 <tr><td>Sum</td>
101 <td><input name = "sum" type = "text" />
102 </td></tr>
103 <tr><td>Point</td>
104 <td><input name = "point" type = "text" />
105 </td></tr>
106 <tr><td><input type = "button" value = "Roll Dice"
107 onclick = "play()" /></td></tr>
108 </table>
109 </form>
110 </body>
111 </html>

36
Fig. 10.6 Craps game simulation.
A text XHTML GUI component
A button XHTML GUI component
Browser’s status bar

37

38

39
10.7 Another Example: Random Image
Generator
• Randomly selecting an image
– Images have integer names (i.e., 1.gif, 2.gif, …, 7.gif)
– Generate random number in proper range
– Update src property

Outline
40
RandomPicture.html
(1 of 1)
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 
18 </script>
19
20 </head>
21
22 <body>
24 </body>
25 </html>
Inserting a random number into the image’s src
property with document.write and Math.random

41
10.7 Another Example: Random Image
Generator
Fig. 10.7 Random image generation using Math.random.

42
10.8 Scope Rules
• Scope
– Portion of program where identifier can be referenced
– Inside function is local or function scope
• Identifiers exist only between opening and closing braces
• Local variables hide global variables

43
10.8 Scope Rules
• Scope demonstration
– Global variable x initialized to 1
– start has local variable x initialized to 5
– functionA has local variable x initialized to 25
– functionB has no local variable x
– Observe output of each function

Outline
44
4
5 
6 
7
9 <head>
10 <title>A Scoping Example</title>
11
13 <!--
14 var x = 1; // global variable
15
16 function start()
17 {
18 var x = 5; // variable local to function start
19
20 document.writeln( "local x in start is " + x );
21
22 functionA(); // functionA has local x
23 functionB(); // functionB uses global variable x
24 functionA(); // functionA reinitializes local x
25 functionB(); // global variable x retains its value
Scoping.html
(1 of 3)
To begin the program, variable x is initialized to 1.
Function start changes the value of x to 5.

Outline
45
Scoping.html
(2 of 3)
26
28 "<p>local x in start is " + x + "</p>" );
29 }
30
31 function functionA()
32 {
33 var x = 25; // initialized each time
34 // functionA is called
35
36 document.writeln( "<p>local x in functionA is " +
37 x + " after entering functionA" );
38 ++x;
39 document.writeln( "<br />local x in functionA is " +
40 x + " before exiting functionA" + "</p>" );
41 }
42
Function functionA changes the value of x to 25.
The value of x is incremented.

Outline
46
Scoping.html
(3 of 3)
43 function functionB()
44 {
45 document.writeln( "<p>global variable x is " + x +
46 " on entering functionB" );
47 x *= 10;
48 document.writeln( "<br />global variable x is " +
49 x + " on exiting functionB" + "</p>" );
50 }
51 // -->
52 </script>
53
54 </head>
55 <body onload = "start()"></body>
56 </html>
Function functionB multiplies the value of x by 10.

47
10.8 Scope Rules
Fig. 10.8 Scoping example.

48
• Global object
– Always available
– Provides 7 methods
– Do not need to explicitly reference Global before method
call
– Also holds all global variables, user defined functions

49
Global function Description
escape This function takes a string argument and returns a
string in which all spaces, punctuation, accent
characters and any other character that is not in the
ASCII character set (see Appendix D, ASCII
Character Set) are encoded in a hexadecimal format
(see Appendix E, Number Systems) that can be
represented on all platforms.
eval This function takes a string argument representing
JavaScript code to execute. The JavaScript
interpreter evaluates the code and executes it when
the eval function is called. This function allows
JavaScript code to be stored as strings and executed
dynamically.
isFinite This function takes a numeric argument and returns
true if the value of the argument is not NaN,
Number.POSITIVE_INFINITY or
Number.NEGATIVE_INFINITY; otherwise, the
function returns false.
isNaN This function takes a numeric argument and returns
true if the value of the argument is not a number;
otherwise, it returns false. The function is
commonly used with the return value of parseInt
or parseFloat to determine whether the result is a
proper numeric value.
Fig. 10.9 JavaScript global functions.

50
Global function Description
parseFloat This function takes a string argument and attempts
to convert the beginning of the string into a floating-
point value. If the conversion is unsuccessful, the
function returns NaN; otherwise, it returns the
converted value (e.g., parseFloat( "abc123.45" )
returns NaN, and parseFloat( "123.45abc" )
returns the value 123.45).
parseInt This function takes a string argument and attempts
to convert the beginning of the string into an integer
value. If the conversion is unsuccessful, the function
returns NaN; otherwise, it returns the converted
value (e.g., parseInt( "abc123" ) returns NaN, and
parseInt( "123abc" ) returns the integer value
123). This function takes an optional second
argument, from 2 to 36, specifying the radix (or
base) of the number. Base 2 indicates that the first
argument string is in binary format, base 8 indicates
that the first argument string is in octal format and
base 16 indicates that the first argument string is in
hexadecimal format. See see Appendex E, Number
Systems, for more information on binary, octal and
hexadecimal numbers.
unescape This function takes a string as its argument and
returns a string in which all characters previously
encoded with escape are decoded.
Fig. 10.9 JavaScript global functions.

51
10.10 Recursion
• Recursive functions
– Call themselves
• Recursion step or recursive call
• Part of return statement
– Must have base case
• Simplest case of problem
• Returns value rather than calling itself
– Each recursive call simplifies input
• When simplified to base case, functions return

52
10.10 Recursion
• Factorials
– Product of calculation n · (n - 1) · (n - 2) · … · 1
– Iterative approach:
var factorial = 1;
for ( var counter = number; counter >= 1; --counter )
factorial *= counter;
– Note each factor is one less than previous factor
• Stops at 1: base case
• Perfect candidate for recursive solution

53
10.10 Recursion
5!
5 * 4!
4 * 3!
3 * 2!
2 * 1!
1
5!
5 * 4!
4 * 3!
3 * 2!
2 * 1!
1
(a) Procession of recursive calls. (b) Values returned from each recursive call.
5! = 5 * 24 = 120 is returned
4! = 4 * 6 = 24 is returned
2! = 2 * 1 = 2 is returned
3! = 3 * 2 = 6 is returned
1 returned
Fig. 10.10 Recursive evaluation of 5!.
Final value = 120

Outline
54
4
5 
6 
7
9 <head>
10 <title>Recursive Factorial Function</title>
11
12 <script language = "javascript">
13 document.writeln( "<h1>Factorials of 1 to 10</h1>" );
15 "<table border = '1' width = '100%'>" );
16
17 for ( var i = 0; i <= 10; i++ )
18 document.writeln( "<tr><td>" + i + "!</td><td>" +
19 factorial( i ) + "</td></tr>" );
20
21 document.writeln( "</table>" );
22
FactorialTest.html
(1 of 2)
Calling function factorial and passing
it the value of i.

Outline
55
23 // Recursive definition of function factorial
24 function factorial( number )
25 {
26 if ( number <= 1 ) // base case
27 return 1;
28 else
29 return number * factorial( number - 1 );
30 }
31 </script>
32 </head><body></body>
33 </html>
FactorialTest.html
(2 of 2)
Variable number gets the value of variable i.
Call to function factorial and passing it 1
less than the current value of number .

56
10.10 Recursion
Fig. 10.11 Factorial calculation with a recursive function.

57
• Iteration
– Explicitly uses repetition structures to achieve result
– Terminates when loop-continuation condition fails
– Often faster than recursion
• Recursion
– Repeats through function calls
– Terminates when base case reached
– Slower due to function call overhead
• Each call generates new copy of local variables
– Easy to read and debug when modeling problem with
naturally recursive solution

58
Chapter Recursion examples and exercises
10 Factorial function
Sum of two integers
Multiply two integers
Raising an integer to an integer power
Visualizing recursion
12 Printing a string input at the keyboard backward
13 Navigating the object hierarchy in Dynamic HTML
Fig. 10.12 Recursion examples and exercises in the text.

Javascript.ppt

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