csis 138 javascript class3
Post on 10-Dec-2014
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1
JavaScript – Class 3
• String object• toLowerCase()• toUpperCase()• Switch statement• Arrays• loops
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String Object – comparing strings
var answer = “xyz”;
stringName.toLowerCase();
stringName.toUpperCase();
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String Object
var answer = prompt("What breed of dog was Lassie?", "");
if (answer == "collie") { document.write("Yes, that is correct!"); }
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var answer = prompt("What breed of dog was Lassie?", "");
if (answer.toLowerCase() == "collie") { document.write("Yes, Lassie was a collie!"); } else { document.write("No, you are wrong!"); }
comparison
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Switch Statement
For checking a condition when there are a large number of possibilities Can be used in place of an if else if
What it contains: • the test statement - the expression/condition to test
• the case statements - the possible answers
• the break statements – stops checking after a match is
• the default statement –executes if none of the possible answers are found
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var theDay= prompt("What day is the fourth of July?", "");
switch (theDay.toLowerCase()) → the test expression– what we are comparing{ case “monday”:execute some code; break;
case “tuesday” : execute some code; break;
default: → - the default statement - if nothing matches, this will execute execute some code; break;
}
See pages 4-6 handout and sample code files
Double quotesfor a literal/string
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the Array Object – (chapter 2)
• Similar to a variable except that it can hold more than one item of data at a time
• Each piece of data stored is referred to as an element
• Each element is referenced by a number or an index
• The index number is enclosed in brackets – [ ]
• Arrays start with the number 0
• Arrays have a length property, which is the number of elements in the array
• Arrays are defined with the JavaScript key word “new”
• The length of the array minus 1 gives the number of the last index
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Declaring an Array
var myArray = new Array() or var myArray = new Array(4)
We are creating a “new” instance of the “Array()” object
keyword keyword
Array object – upper case “A”
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Listing the elements in an array
var dogs = new Array(); dogs[0] = "Missy" ; dogs[1] = "Letty" ; dogs[2] = "Andy" ; dogs[3] = "Terri" ;
var dogs = new Array(“Missy”, “Letty”, “Andy”, “Terri”);______________________________________________
Returning the value of the array elements:
document.write("my first dog is " + dogs[0] + "<br>"); document.write("my second dog is " + dogs[1] + "<br>"); see page 8
index position 0
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Loops
A loop is a type of JavaScript statement that repeats a block of code over and over again if a condition evaluates to true
Two kinds of loops -– a “for” loop and a “while” loop
• Use the “for” loop when you know, or can determine by a calculation, the number of times you want to repeat the code
• Use the “while” loop when you do not know how many times you will need to repeat the block of code
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The “for” loop – Commonly used with an array
• Repeats a block of code for a certain number of times
• When you know or can determine how many times to repeat the code
• Involves some kind of counter that is incremented as long as the condition is true
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The “for in” loop – ONLY used with an array
• Repeats a block of code for a certain number of times
• When you know or can determine how many times to repeat the code
• Involves some kind of counter that is incremented as long as the condition is true
var elementIndex; var myArray();
for (elementIndex in myArray) { document.write(myArray[elementIndex] + “<br>”); }
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for loop - Pages 9 -11
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for in loop - Page 12
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for in loop - Page 12
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while loop
• Allows you to test a condition and keep on looping as long as the condition is true; will stop when the condition is false • If the condition is false to begin with, it will never loop • Used when you don’t know the number of times you need to loop • You must increment the counter in the code to execute or it will loop indefinitely and the computer will crash ☺
var counter; counter = prompt("Enter a number from 1 to 10", ""); while (counter <= 10) { document.write("this is count number " + counter + "<br><br>" ); counter++ }
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do while loop
Same as the while loop except •The test condition is at the bottom• Will always loop at least once
var counter; counter = prompt("Enter a number from 1 to 10", ""); do { document.write("this is count number " + counter + "<br><br>" ); counter++ }
while (counter <= 5)
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Assignment 3 Looping through an array of URLs Concatenating a variable with HTML to produce a hypertext link
<a href=”http://variable”>variable</a>
document.write(‘<a href=”http://’ + variable + ‘”>’ + variable + ‘</a>’);
note the double quotes inside the document.write()
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