Instructor: Announcements
Nelson Padua-Perez ([e-mail]) No posting of code in the forum Check class announcements daily You must implement programming projects by yourself
1
JavaScript Reference
https://developer.mozilla.org/en/Core_JavaScript_1.5_Reference
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JavaScript (If Statement)
What is the difference between === and == ? Let's compare 20 === "20" vs. 20 == "20"
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JavaScript (Logical Operators)
Used with comparison operators to create more complex expressions Operators Logical and (&&) expr1 && expr2 Expression is true if and only if both expressions are true otherwise is false You can have more than two expressions Example: LogicalOp1.html Logical or (||) expr1 || expr2 Expression is false if and only if both expressions are false otherwise is true You can have more than two expressions Example: LogicalOp2.html Logical Not (!) !expr Inverts the boolean value of the expression
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Precedence/Associativity
Remember you can use parenthesis to impose a particular order for the evaluation of an expression
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Cascaded If Statement Idiom
You can combine if statements to handle different cases This approach to organize if statements to handle different cases is called the Cascaded If Statement Cascaded If statement general form: If (expr1) { // Statements executed if expr1 is true } else if (expr2) { // Statements executed if expr2 is true } else if (expr3) { // Statements executed if expr3 is true } else { // If none of the above expressions is true }
Notice it is not a JavaScript statement Once one of the cases is executed no other case will be executed You don't need to use { } if you only have one statement More efficient than having multiple if statements Example: See CascadedIf.html
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while Statement
while statement Control statement which allows JavaScript to repeat a set of statements Basic Form while (expression) { statement(s) // executed as long as expression is true } { } not needed if you only have one statement You can have other types of statements (including whiles) in a while Common mistake: to add a semicolon after closing parenthesis Example: Numbers.html Example: EvenNumbers.html Example: NumbersTable.html Example: SqrtTable.html
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Trace Tables
Mechanism to keep track of values in a program Allows you to understand the program behavior We could create a trace table for EvenNumbers.html
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Combination of Statements
Keep in mind that you can have any combination of conditionals, and iteration (while) statements For example: Conditionals insides loops of Conditionals inside conditionals Loops inside conditionals Loops inside of loops
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Infinite Loops
An infinite loop occurs when the expression controlling the loop never becomes false Example1 int x = 30; while(x > 0) { document.writeln("<li>Element</li>"); }
Example2 int x = 7; // how about x = 8 while (x != 0) { document.writeln("<li>Element</li>"); x=x 2; } How can we detect infinite loops?
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Programming Errors
Syntax Error: (Compile-time error) The program violates the language's grammar Semantic Error: The program fails to accomplish what we want Debugging: The process of finding and fixing errors. Extremely hard for large software systems. Tools for debugging: Trace tables Output statements Debuggers Analogy:
Taco tom ate. Syntactically therefore semantically incorrect. A taco ate tom. Syntactically correct however semantically incorrect. Tom ate a taco Syntactically and semantically correct (what we want!)
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How to find problems in your code
Computer programming is not about writing code and letting someone else find the problems (bugs) it may have You have to learn how to find problems in your code Approach
Use Error Console to see possible error Use JavaScript Lint http://www.javascriptlint.com/online_lint.php
How about logical errors?
12
Introduction to Debugging
How
to debug your code?
Introducing the alert() function
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Suggestions for Solving Problems Using a Programming Language
Design Make sure you first come up ...