Java script objects 1

Lecture3

Data Type Conversion (변하게 하다, 전환하다)

To understand what is data type conversion, and why we need data type conversion, let
us take an example.

<html>
<body>
<script language="JavaScript" type="text/javascript">

var firstNumber = prompt("Enter the first number","");
var secondNumber = prompt("Enter the second number","");
var theTotal = firstNumber + secondNumber;
document.write(firstNumber + " added to " + secondNumber + " equals " +
theTotal);

</script>
</body>
</html>

What is the output, if user enters firstNumber=2 and secondNumber=3, Output should
be 5.

But if you will execute above program, you will be disappointed (실망시키다) to find that
output is 23.

Why is output 23 instead of 5?
Ans: We know that function prompt () returns a string, and if we use '+' operator with
strings, they are concatenated (사슬같이 잇다 ).

To solve (<문제 등을> 풀다, 해석하다) this problem we will take help of data type
conversion. Data type conversion means convert data type.

What is the solution?
Solution is convert String to int or float. There are 2 conversion functions parseInt() and
parseFloat().

parseInt(): This functions takes a string and convert it to an integer.
For example:
parseInt(“123”) = 123
parseInt(“123Abc”) = 123

parseFloat(): This function take a string and convert it to a real number.

What is the output for the following?
1. parseInt(“ABC”)
2. parseInt(“ABC123”)

Find it and I will ask you in the class.

Now, modify (수정하다 ) the above code so that it gives correct output i.e 5.

<html>
<body>

var theTotal = parseInt(firstNumber) + parseInt(secondNumber);
theTotal);

</script>
</body>
</html>

What is NaN?

If you write document.write(parseInt(“abc”)), output will be NaN. NaN means Not a
Number.

If you use parseInt() or parseFloat() and input argument is a string that is empty or
doesn't start with a number, output is NaN.

There is a function in Javascript isNaN(x), which checks whether 'x' is NaN or not.
For example:
isNaN(“123”) returns false
isNaN(“ABC”) returns true
isNaN(“123ABC”) returns true

So, Before converting any String to a number we can check beforehand (미리) whether
string consist ((부분·요소로) 되어[이루어져] 있다 ) of numbers or not. If string consist of
numbers, we can convert it otherwise ([접속사적으로] 만약 그렇지 않으면) display an error
message.

Below is a program which checks for user input. If user input is not valid, we generate
an error message.

<html>
<body>

var ifn,isn;
if (isNaN(firstNumber))
alert("this is Not a Number");
else
ifn = parseInt(firstNumber);
if (isNaN(secondNumber))
alert("this is not a number");
else
isn = parseInt(secondNumber);
var theTotal = ifn+ isn;
theTotal);

</script>
</body>
</html>

Multi Dimensional Array

In my previous lecture, I taught you about 1-dim arrays. 1-dim arrays are those that
have only one index.
Like 1-dim arrays, there are 2-dim, 3-dim and multi-dim arrays.

2-dim arrays are those that have 2-dimensions.

First answer this question, What is the output of following code

<html>
<body>
var a = new Array("a",2,"b",3,"c",4);
for (i=0;i<a.length;i++)
document.write("a["+i+"] = "+a[i]+"<br>");
</script>
</body>
</html>

Two things are worth (…의 가치가 있는 ) notice (통지, 통보) in the above program
1. Unlike (같지 않은, 다른, 닮지 않은 ) arrays in other languages, arrays in Javascript can
store values of different data types. For example:

var a = new Array("a",2,"b",3,"c",4);

2. I told in my previous lecture also, that new keyword create an object. This means, 'a'
is an object of Array.
And length is a variable of Array Object in Javascript, So a.length returns length of
array 'a'.

To understand 2-dim arrays, Let us take an example.

Suppose, we want to store a company's employee information in an array. Each
employee has
-name
-age
-address

One way to do this, is store the information in a 1-dim array sequentially (잇달아 일어나
는, 연속하는, 순차적인 ). For example:

Index Data Stored
0 Name1
1 Age1

Index Data Stored
2 Address1
3 Name2
4 Age2
5 Address2
6 Name3
7 Age3
8 Address3

var employee = new
Array(“Huh”,23,”suwon”,”cho”,30,”hwaseong”,”Chong”,28,”Guro”,”chae”,40,”bundang”);

or

employee[0] = “huh”;
employee[1] = 23;
employee[2] = “suwon”;

employee[3] = “cho”;
employee[4] = 30;
employee[5] = “hwaseong”;

employee[6] = “chong”;
employee[7] = 28;
employee[8] = “guro”;

employee[9] = “chae”;
employee[10] = 40;
employee[11] = “bundang”;

employee is a 1-dim array, employee[0], employee[1], employee[2] stores information
about one employee.
empoyee[3], employee[4], employee[5] stores information about 2nd employee.

2nd way of doing this, is store the information in a 2-dim arrays.

Index 0 1 2
0 Name1 Name2 Name3
1 Age1 Age2 Age3
2 Address1 Address2 Address3

var employee = new Array(); // employee is an array
employee[0] = new Array(); // employee[0] is also an array

The first index (0) belongs ((…의) 소유물이다 ) to the employee array; the second index
(0) belongs to the employee[0] array.

employee[0][0] = “huh”;
employee[0][1] = “23”;
employee[0][2] = “suwon”;

employee[1][0] = “cho”;
employee[1][1] = “30”;
employee[1][2] = “hwaseong”;

employee[2][0] = “chong”;
employee[2][1] = “28”;
employee[2][2] = “guro”;

employee[3][0] = “chae”;
employee[3][1] = “40”;
employee[3][2] = “bundang”;

Here is the complete program

<html>
<body>


var employee = new Array();

employee[0][0] = “huh”;
employee[0][1] = “23”;
employee[0][2] = “suwon”;
employee[1] = new Array();
employee[1][0] = “cho”;
employee[1][1] = “30”;
employee[1][2] = “hwaseong”;

employee[2] = new Array();
employee[2][0] = “chong”;
employee[2][1] = “28”;
employee[2][2] = “guro”;

document.write("Name : " + employee[1][0] + "<br>");
document.write("Age : " + employee[1][1] + "<br>");
document.write("Address : " + employee[1][2]);

</script>

</body>
</html>

Answer these?

var myArray = new Array(); Is it 1-dim or 2-dim array or 3-dim or 4-dim or 5-dim?
myArray[0] = new Array(); Is it 1-dim or 2-dim array or 3-dim or 4-dim or 5-dim?

myArray[0][0] = new Array(); Is it 1-dim or 2-dim array or 3-dim or 4-dim or 5-dim?
myArray[0][0][0] = new Array(); Is it 1-dim or 2-dim array or 3-dim or 4-dim or 5-dim?
myArray[0][0][0][0] = new Array(); Is it 1-dim or 2-dim array or 3-dim or 4-dim or 5-dim?

Javascript-An Object based language

In this topic, we will look at most important aspect of Javascript programing 'Objects'.
Javascript itself consist of Objects and browser also made of collection of
object.

For example:
One Javascript object that you have used is Array.
You have also used 2 objects document and window, these are objects provided by
browser.
We can create our own objects.

A brief introduction about objects

Those who have been doing programming in C++ or Java already know about objects, I
still fell the need to give an introduction about objects for those who are first time using
it.

To understand what are objects, we take an example. We will take an example of car.

How do we define a car? We define a car as a blue car with 4 wheel drive, automatic
gear, power steering, etc. Color, 4 wheel, automatic gear, 4 wheel drive, power steering
are properties of a car. If I make an object of type Car, color, automatic gear, 4 wheel
drive, power steering will be attributes or property (재산, 자산 ) of object. In Javascript,
we represent attributes as properties for an object.

How do we use a car? We turn in the key, press the pedal, change the gear, and then
move the steering. This can also be defined as behavior (행동, 거동, 행실, 품행 ) of car. In
Javascript, we represent the behavior as methods or functions of an object.

We can pass arguments to a function, for example if gear() is a function, then one of its
argument can be gear number, gear 1 or 2 or 3 or 4 or reverse.
Similarly, speed() function can return me the speed of the car.
Or OilLeft() function can return the amount of oil left in the car.

If we want to define an object for car, we need to defined attributes and functions for
the car object. In Object based programming, we try to model real life things by an
Object. And objects are defined by their attributes and functions.

Basic definition of an Object is “A thing with attributes and functions”.

Take an example, We used array.length which returns length of the array, now array is
an object and length is an attribute of array object. Length attribute of array object,
returns number of elements stored in the array.

Let us do to anatomy (해부;해부학;해부술 ) of Array object.

We create an array by the statement
var num = new Array(7, 6, 5, 4, 3, 2, 1, 0, 8, 9);

'num' is a variable name. In the above example, right hand side of the statement create

an array in memory.

Starting address of the array is 100. 'num' stores only the starting address of array i.e.
'num' stores value 100.

new is a keyword in Javascript for creating object. In the above example, we are
creating an object of type Array. Object is created in memory and its starting address is
100 which is stored in variable 'num'.

Let us take another example of using Date Object

var d = new Date(“1 Jan 2000”);

'd' is a variable which can store an integer, character, string etc, in the above example
variable 'd' stores address of an object of type Date.

'new' is a keyword that creates an object. In the above example we are creating an
object of type Date. The object of type Date takes some space in memory and has a
starting address. 'd' stores starting address of object of type Date.

Let us try to understand more about objects and variables.
I am starting with an example of simple variables.

1. var a = 10;
2. var b = a;
3. document.write(a); // Output is 10
4. document.write(b); // Output is 10
5. b = 20;
6. document.write(a); // Output is 10
7. document.write(b); // Output is 20

a and b are 2 different variables.
Line 1, variable a stores value 10.
Line 2, variable b also stores the value value stored in a (10). So b also stores value 10.
Line 3 output is 10 and line 4 output is 20.

Line 5, we change the value stored in b. New value stored in b is 20 now. But value
stored in a does not change which is still 10.
Line 6 output is 10, because value stored in a does not change.
Line 7 output is 20, because value stored in b changes from 10 to 20.

The above was a simple example using variables. Next we take an example using
objects.

1. var a = new Array(7, 6, 5);

2. var b = a;

3. document.write(a[0] + “ ”+ a[1]+ “ ”+ a[2]); // output is 7 6 5
4. document.write(b[0] + “ ”+ b[1]+ “ ”+ b[2]); // output is yeh
5. b[1] = 3;

6. document.write(a[0] + “ ”+ a[1]+ “ ”+ a[2]); // output is 7 3 5
7. document.write(b[0] + “ ”+ b[1]+ “ ”+ b[2]);// output is 7 3 5

Line 1, creates an object of type array and stores the address of object in a. Suppose
Object of type array has starting address 100, then a stores 100.

Line 2, variable b gets the value stores in a, which is 100. i.e. B = 100.

Line 3, output is 7 6 5.

Line 4, output is 7 6 5, because b also contain the address 100 which is same as
address stored in a.

Line 5. We change the value if b[1] to 2, because a and b both contain the same address
b[1] and a[1] refer to same location of memory.

Line 6 output is 7 3 5 and line 7 output is 7 3 5.

Using an Object Properties and Methods

Every object provide (지급하다) some methods and properties. For example, we can find the length of the array,
For example:

var a = new Array( 7 , 6 , 5);
var len = a.length; // output is 3

length is a property of Array object. It returns the length of the array.
Address of Array object is stored in the variable a. So a can access all the properties of Array object.

a . name of property

Simillary,
a . name of function(), here function can be replaced by actual function name.
For example
Date d = new Date(“15 Aug 1953”)
dd = d.getDate(); // dd = 31

getDate() is a function of Date Object.

Primitives and Objects

You should now have a good idea about the difference between primitive (원시의, 초기의;
태고의, 옛날의 ) data, such as numbers and strings, and object data, such as Dates and
Arrays.
We have string, number and boolean data types. In fact there are String, Number, and
Boolean objects corresponding (유사한 ) to the three string, number, and Boolean
primitive data types. For example, to create a String object containing the text "I'm a
String object," we can use
var myString = new String("I'm a String object");

The String object has the length property just as the Array object does. This returns the
number of characters in the String object. For example
var lengthOfString = myString.length;

would store the data 19 in the variable lengthOfString.
But what if we had declared a primitive string called mySecondString holding the text
"I'm a _primitive string" like this:
var mySecondString = "I'm a primitive string";

and wanted to know how many characters could be found in this primitive string?

So, for our primitive string mySecondString, we can use the length property of the
String object to find out the number of characters it contains. For example
var lengthOfSecondString = mySecondString.length;

would store the data 22 in the variable lengthOfSecondString
If we declare a primitive string and then treat it as an object, such as by trying to access
one of its methods or properties, JavaScript would know that the operation we're trying
to do won't work if it's a primitive string. It will only work if it's an object; for example, it
would be valid if it were a String object. In this case, JavaScript converts the plain
text string into a temporary String object, just for that operation.

Java script objects 1

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