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CS101 - Introduction to Computing - Lecture Handout 44

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Programming Methodology

(Web Development Lecture 15)

During the last lecture 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">
  • Plea: Don’t use images just for the sake of it!

Images in JavaScript

  • Images in JavaScript can be manipulated in many ways using the built-in object, Image
  • Properties: name, border, complete, height, width, hspace, vspace, lowsrc, src
  • Methods: None
  • 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 preloaded
  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

  • 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?

programming methodology

Good prrogrrammiing methodollogy?

Good prrogrrammiing methodollogy

correct program?

correct program

readable program?

readable program

swapFlag = true ;
while ( swapFlag == true ) {
swapFlag = false ;
for ( k = 0 ; k < ht.length - 1 ; k++ ) {readable program 1
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 ;
}
}
}

Design Guidelines

  • 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

Coding Guidelines

  • Always use semicolons to end statements
  • Indent blocks of code (2 to 5 spaces)
  • Identifiers:
    • Use the camelBack scheme
    • Make them descriptive but concise
    • Variables: nouns
    • Functions: verbs
  • Comment liberally

Guidelines for Developing Short Programs

Guidelines for Developing Short Programs

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

Guidelines for Developing Short Programs 1

Developing Short Programs

Developing Short Programs

Developing Short Programs 1

Developing Short Programs

Developing Short Programs 2

Design & Code Reviews

  • Probably the most efficient way of improving the a program
  • Being humans, at time we see what is supposed to be there instead of what is actually there
  • 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?

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

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:
–a = b + * c ;
–receiver = reciever + 2Syntax Errors

Semantic Errors

  • Occur when a statement executes and has an effect not intended by the programmer
  • 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

Debugging

Debugging 1

Debugging 2

income = document.myForm.salary.value +
document.myForm.bonus.value ;

Common Mistakes

if ( today = “holiday” )
mood = “good” ;

Common Mistakes

Helpful Editors

  • 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 Lecture …

  • 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

Final Lecture:

Review & Wrap-Up

  • To review a selection from the interesting ideas that we explored over the last 44 lectures