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Introduction to Computing

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Introduction to Computing ­ CS101
VU
LESSON 44
PROGRAMMING METHODOLOGY
(WEB DEVELOPMENT LESSON 15)
During the last Lesson we discussed Graphics & Animation
·
We became able to add and manipulate images and simple animations to a Web page
Images in HTML
·
It is quite straight forward to include gif and jpg images in an html Web page using the <IMG> tag
·
Format: <IMG src=URL, alt=text
height=pixels width=pixels
align="bottom|middle|top">
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Plea: Don't use images just for the sake of it!
Images in JavaScript
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Images in JavaScript can be manipulated in many ways using the built-in object, Image
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Properties: name, border, complete, height, width, hspace, vspace, lowsrc, src
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Methods: None
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Event handlers: onAbort, onError, onLoad, etc.
Image Preloading
·
The primary use for an Image object is to download an image into the cache before it is actually
needed for display
·
This technique can be used to create smooth animations or to display one of several images based
on the requirement
The Image Pre-Loading Process
1.
An instance of the Image object is created using the new keyword
2.
The src property of this instance is set equal to the filename of the image to be pre-loaded
3.
That step starts the down-loading of the image into the cache without actually displaying it
4.
When a pre-loaded image is required to be displayed, the src property of the displayed image is set
to the src property of the pre-fetched image
Animated Gifs
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We could have saved the 16 gif images of the previous example in a single file in the form of an
animated gif, and then used it in a regular <IMG> tag to display a moving image
·
However, JavaScript provides better control over the sequencing and the gap between the
individual images
·
Example
Today's Goals
(Programming Methodology)
·
To understand effective programming practices that result in the development of correct programs
with minimum effort
·
To become familiar with testing & debugging
programming
methodology?
The process used by an individual or a team for
developing programs
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Introduction to Computing ­ CS101
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Good programming
methodology?
A methodology that enables the lowest-cost and
on-schedule development of programs that are
correct, easy to maintain & enhance
correct
program?
A program with correct
syntax & semantics
readable
program?
A program that is easy to read &
understand, and therefore, easy to
maintain & enhance
Bubble Sort
swapFlag = true ;
while ( swapFlag == true ) {
swapFlag = false ;
for ( k = 0 ; k < ht.length - 1 ; k++ ) {
if ( ht[ k ] < ht[ k + 1 ] ) {
temp = ht[ k + 1 ] ;
ht[ k + 1 ] = ht[ k ] ;
ht[ k ] = temp ;
swapFlag = true ;
}  }
}
for ( j = 0 ; j < 100000 ; j++ ) {
for ( k = 0 ; k < ht.length - 1 ; k++ ) {
if ( ht[ k ] < ht[ k + 1 ] ) {
temp = ht[ k + 1 ] ;
ht[ k + 1 ] = ht[ k ] ;
ht[ k ] = temp ;
}
}
}
44.1 Design Guidelines
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Break your code down into short and simple functions (e.g. take the 3 swap statements out from the
last example and put them into a function of their own)
·
Do not use global variables
44.2 Coding Guidelines
·
Always use semicolons to end statements
·
Indent blocks of code (2 to 5 spaces)
·
Identifiers:
­  Use the camelBack scheme
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Introduction to Computing ­ CS101
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­
Make them descriptive but concise
­
Variables: nouns
­
Functions: verbs
·
Comment liberally
44.3 Guidelines for Developing Short Programs
·
Read, understand the problem
·
Do you have all the required
data?
No: Get it
Else assume it. State it explicitly
Example: Problem Statement
·
Develop a Web page that displays an order taking form
·
It takes the number of items required for each product, multiplies with the prices, sums them up,
adds the GST, and displays the total value of the order
Guidelines for Developing Short Programs
·
Read, understand the problem
·
Do you have all the required data?
No: Get it
Else assume it. State it explicitly
·
Do the design
Developing Short Programs
·
Read, understand the problem
·
Do you have all the required data?
No: Get it
Else assume it. State it explicitly
·
Do the design
·
Write test cases
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Introduction to Computing ­ CS101
VU
Developing Short Programs
44.4 Design & Code Reviews
·
Probably the most efficient way of improving the a program
·
Another pair of eyeballs may not have the same problem, especially if they were not involved in
building the design or code
Two Popular Review Methods
1.  Give the problem statement, design, and code (that includes all assumptions) to a peer, and ask
him/her to see if things have been done properly
2.  Walk a peer or a group of peers through the problem, the design, and the code yourself
3.  Which of the two is better?
44.5 Testing & Debugging
·
Testing: The tasks performed to determine the existence of defects
·
Debugging: The tasks performed to detect the exact location of defects
·
Defects are also called bugs or errors
·
Let us now look at one of their classifications
Types of Errors
·
Syntax errors
·
Semantic errors
·
Run-time errors
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Introduction to Computing ­ CS101
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Syntax Errors
·
They are caused by the code that somehow violates the rules of the language
·
Easy to detect and fix errors
·
The browser stops code interpretation on detecting one of these
Examples:
Syntax
­a = b + * c ;
error?
­receiver = reciever + 2
Semantic Errors
Occur when a statement executes and has an effect not intended by the ptiogrammerr
Run- r me Erro
·
·
Hard to detect during normal testing
·
Often times occur only in unusual & infrequent circumstances
·
The `+' operator often results in unintended consequences. Remedy: Convert, before use
Run-Time Errors
·
Occur when the program is running and tries to do something that is against the rules
Example: Accessing a non-existent variable, property, method, object, etc (e.g. a method name is
misspelled)
·
Sources of these can be determined by a careful reading of the code, but unfortunately, not always!
Debugging
Tools:
Internet
Options...:
Advanced:
name = "Bhola ;
Syntax
Error
checkPulse( ) ;
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Introduction to Computing ­ CS101
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Run-time
Error
x = 1.3 ;
x.upperCase( ) ;
income = document.myForm.salary.value +
document.myForm.bonus.value ;
Semantic Error
Common Mistakes
if ( today = "holiday" )
mood = "good" ;
function doThis ( tiger ) {
box[ 0 ] = tiger ;
x = box[ 0 ] ;
return x ;
box = new array( 10 ) ;
box = new Array( 10 ) ;
box( 0 ) = 43 ;
44.6 Helpful Editors
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Using smart editors (e.g. DreamWeaver, nedit) can help in avoiding many types of syntax errors
·
They can, for example:
­  Automatically color different parts of statements in different colors, e.g. comments in Gray, strings
in Green, HTML tags in Blue
­  Auto indent
­  Visually indicate the presence of mismatched parentheses, curly braces or square brackets
During Today's Lesson ...
·
We looked at a few effective programming practices that result in the development of correct
programs with minimum effort
·
We also became familiar with testing & debugging
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Introduction to Computing ­ CS101
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Final Lecture:
Review & Wrap-Up
·
To review a selection from the interesting ideas that we explored over the last 44 lectures
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Table of Contents:
  1. INTRODUCTION
  2. EVOLUTION OF COMPUTING
  3. World Wide Web, Web’s structure, genesis, its evolution
  4. Types of Computers, Components, Parts of Computers
  5. List of Parts of Computers
  6. Develop your Personal Web Page: HTML
  7. Microprocessor, Bus interface unit, Data & instruction cache memory, ALU
  8. Number systems, binary numbers, NOT, AND, OR and XOR logic operations
  9. structure of HTML tags, types of lists in web development
  10. COMPUTER SOFTWARE: Operating Systems, Device Drivers, Trialware
  11. Operating System: functions, components, types of operating systems
  12. Forms on Web pages, Components of Forms, building interactive Forms
  13. APPLICATION SOFTWARE: Scientific, engineering, graphics, Business, Productivity, Entertainment, Educational Software
  14. WORD PROCESSING: Common functions of word processors, desktop publishing
  15. Interactivity to Forms, JavaScript, server-side scripts
  16. ALGORITHMS
  17. ALGORITHMS: Pseudo code, Flowcharts
  18. JavaScript and client-side scripting, objects in JavaScript
  19. Low, High-Level, interpreted, compiled, structured & object-oriented programming languages
  20. Software Design and Development Methodologies
  21. DATA TYPES & OPERATORS
  22. SPREADSHEETS
  23. FLOW CONTROL & LOOPS
  24. DESIGN HEURISTICS. Rule of thumb learned through trial & error
  25. WEB DESIGN FOR USABILITY
  26. ARRAYS
  27. COMPUTER NETWORKS: types of networks, networking topologies and protocols
  28. THE INTERNET
  29. Variables: Local and Global Variables
  30. Internet Services: FTP, Telnet, Web, eMail, Instant messaging, VoIP
  31. DEVELOPING PRESENTATIONS: Effective Multimedia Presentations
  32. Event Handlers
  33. GRAPHICS & ANIMATION
  34. INTELLIGENT SYSTEMS: techniques for designing Artificial Intelligent Systems
  35. Mathematical Functions in JavaScript
  36. DATA MANAGEMENT
  37. DATABASE SOFTWARE: Data Security, Data Integrity, Integrity, Accessibility, DBMS
  38. String Manipulations:
  39. CYBER CRIME
  40. Social Implications of Computing
  41. IMAGES & ANIMATION
  42. THE COMPUTING PROFESSION
  43. THE FUTURE OF COMPUTING
  44. PROGRAMMING METHODOLOGY
  45. REVIEW & WRAP-UP of Introduction to Computing