ASP.NET 2.0 DEMYSTIFIED phần 5 docx

There are four parts to a Do While loop: The Do While keywords The condition The code block that contains statements that are executed if the condition is true The Loop keyword Here's ho

5 Conditional Statements

Figure 5-6 The visitor is prompted to enter a starting value for the count

3 The ASP.NET engine then declares an Integer variable called i

4 The For loop is then entered The start number that the visitor entered is

a string data type (see Chapter 4) The For loop requires an Integer value Therefore, the ASPNET engine is told to convert the start number to an integer using the CM() conversion function that you learned about in Chapter 4

5 The ASP.NET engine adds 1 to the variable until the value of the variable equals 10, at which time it breaks out of the For loop Notice there is no need to insert statements within the For loop, because we're only interested

in the final value of the variable

6 The Enabled property of the Startvalue text box is set to false so that the visitor cannot change this value

7 The visible properties of the Result label and the ResultValue are set to true

so that the visitor can see these objects

8 The value of the variable is then placed into the ResultValue text box Remember that the ResultValue text box needs a string and that the variable

is an Integer Therefore, we use the CStr() conversion function to change the Integer to a string before placing it into the text box

ASPONET 2.0 Demystified

Figure 5-7 If the visitor enters 1 as the start value, the ASPNET displays 11 as the result

9 The Enabled property of the ResutlValue text box is set to false so that the visitor cannot change this value Figure 5-7 shows the web page after the visitor enters 1 as the starting value

CHAPTER 5 Conditional Statements

<form id=I1Formlt1 runat=I1servert1>

A Variation of the For Loop

The For loop increments the count variable by one each time the For loop is iter- ated However, you can increment or decrement the count variable by a particular value if you use the Step keyword in your For loop

The Step keyword tells the ASPNET engine how to increment or decrement the count variable Let's say that you want to increment the count variable by two in- stead of one Here's what you need to write:

You can count backward by using a negative value to decrement the for value Let's see how this works The next example has an unusual count range It begins with 10 and ends with 1 Notice that the Step value is -2 This means that after each loop, the value of the For variable is decreased by 2 This process continues until the value of the For variable is less than 1, at which time the loop ends and the ASPNET engine executes the statement following the next keyword

For i = 10 to 1 Step -2

'Place statements here

Next i

ASP.NET 2.0 Demystified

The Do While loop also causes the ASP.NET engine to repeatedly execute one or more statements; however, this is done differently than using a For loop The Do While loop is basically saying to the ASP.NET engine, "Do these statements while this condition is true." The condition is a conditional expression, which you learned about in Chapter 4

There are four parts to a Do While loop:

The Do While keywords The condition

The code block that contains statements that are executed if the condition

is true The Loop keyword Here's how to structure the Do While loop:

If the condition is false when the Do While loop is first encountered, then statements within the Do While loop are skipped, causing the ASP.NET engine to execute the statement below the Loop keyword

Try this example of the Do While loop This is a modification of the For loop example that you saw previously in this chapter In this example, we're still asking the visitor to enter the start value for the count However, the ASP.NET engine uses

a Do While loop to count

Here's what is happening:

1 An Integer variable called i is declared

2 The contents of the Startvalue text box, which is a string, is converted to an Integer and assigned to the variable

3 As long as the value of the variable is less than 10, then the ASP.NET engine adds 1 to the variable and assigns the sum to the variable

Conditional Statements

4 The remainder of the code is the same as in the For loop

Sub Count-Click(ByVa1 sender As Object, ByVal e As System.EventArgs) Dim i As Integer

The Do Loop While Loop

The Do Loop While loop is a variation of the Do While loop, except the ASP.NET engine doesn't evaluate the conditional expression until code within the Do Loop code block executes at least once

There are four parts to a Do Loop While loop:

The Do keyword

The code block that contains statements that are executed at least once even

if the condition is false

The Loop While keywords

The condition

Here's how to structure the Do Loop While loop:

Do

'Place statements that are executed at least once

Loop While condition

The ASP.NET engine enters the code block of the Do Loop, executes the code, and then evaluates the condition following the While keyword

If the condition is true, then statements within the Do Loop are executed again and then the ASP.NET engine reevaluates the expression

If the condition is false, then statements within the Do Loop are skipped the second time, causing the ASP.NET engine to execute the statement below the Loop While keyword

Try this example of the Do Loop While In this example, we're still asking the visitor to enter the start value for the count However, the ASP.NET engine uses

a Do Loop While to count

ASP.NET 2.0 Demystified

Here's what is happening:

1 An Integer variable called i is declared

2 The contents of the StartValue text box, which is a string, is converted to an Integer and assigned to the variable

3 The ASPNET engine adds 1 to the variable and assigns the sum to the variable

4 The ASP.NET engine then evaluates the condition

5 If the condition is true, then the ASl?.NET reenters the code block and adds

1 to the variable and assigns the sum to the variable

6 If the condition is false, then the ASPNET no longer reenters the code block but instead executes statements that follow the Loop While keywords

Sub Count-Click(ByVa1 sender As Object, ByVal e As System.EventArgs) Dim i As Integer

i = CInt (StartValue Text)

The Do Until Loop

The Do Until loop tells the ASP.NET engine to execute one or more statements until the condition is true That is, as long as the condition is false, the ASP.NET engine executes statements within the code block of the Do Until loop

There are four parts to a Do Until loop:

The Do Until keyword

The conditional expression

The code block

The Loop keyword

Here is the structure of the Do Until loop:

D o Until condition

'Place statements that are executed if the condition is false Loop

CHAPTER 5 Conditional Statements

Let's take a look at a simple example that illustrates how to use a Do Until loop This is basically the same as the Do While example, except we are using a Do Until loop The ASPNET engine is told to add l to the value of the variable and assign the sum to the variable until the value of the variable is equal to 10

Sub Count-Click(ByVa1 sender As Object, ByVal e As Systern.EventArgs) Dim i As Integer

i = CInt (Startvalue Text)

The Do Loop Until Loop

The Do Loop Until loop is a variation of the Do Until loop, except the ASP.NET engine doesn't evaluate the conditional expression until code within the Do Loop code block executes at least once

There are four parts to a Do Loop Until loop:

The Do keyword

The code block that contains statements that are executed at least once even

if the condition is false

The Loop Until keywords

The condition

Here's how to structure the Do Loop Until loop:

Do

'Place statements that are executed at least once

Loop Until condition

The ASP.NET engine enters the code block of the Do Loop and executes the code and then evaluates the condition following the Until keyword

If the condition is false, then statements within the Do Loop are executed again and then the ASPNET engine reevaluates the expression

If the condition is true, then statements within the Do loop are skipped the sec- ond time, causing the ASP.NET engine to execute the statement below the Loop Until keywords

ASPONET 2.0 Demystified

Try this example of the Do Loop Until In this example, we're still asking the visitor to enter the start value for the count However, the ASP.NET engine uses a

Do Loop Until to count

Here's what is happening:

An Integer variable called i is declared

The contents of the Startvalue text box, which is a string, is converted to an Integer and assigned to the variable

The ASPNET engine adds l to the variable and assigns the sum to the variable

The ASP.NET engine then evaluates the condition

If the condition is false, then the ASPNET reenters the code block and adds

1 to the variable and assigns the sum to the variable

If the condition is true, then the but instead executes statements

Sub Count-Click (ByVal sender As

Dim i As Integer

i = CInt(StartVa1ue.Text)

Do

i = i + l Loop Until i > 10

StartValue.Enabled = False Result.Visible = True ResultValue.Visible = True Resultvalue Text = CStr (i) ResultValue.Enab1ed = False End Sub

ASP.NET no longer reenters the code block that follow the Loop Until keywords

Object, ByVal e As System.EventArgs)

You can have a block of code executed if a condition is false by using the If

Then Else statement This statement contains two blocks of code The first block

is executed if the condition is true, and the second block executes if the condition

is false

CHAPTER 5 Conditional Statements

Sometimes you'll need to have the ASPNET engine evaluate a second condition

if the first condition is false To do this, you'll need to use the If Then Elseif

statement The Elseif portion of this statement defines another condition Only if

this condition is true are statements within the Elseif code block executed by the

ASP.NET engine

In more complex situations, you may find yourself having to make another deci-

sion if a condition is true; for instance, if the user ID is valid, then validate the user

password This situation calls for nested If Then statements The outer If Then

statement determines if the user ID is valid The inner If Then statement deter-

mines if the user password is valid

Processing a menu selection poses a challenge You could use a series of If

Then statements to compare the selection to each menu option, but then you'll end

up with a long list of If Then statements that can be difficult to read The case

statement is the better choice because it enables you to efficiently compare the se-

lection to many items

You also learned in this chapter how to have ASP.NET continually execute the

same code over and over again by using a loop You use the For loop if you know

the number of times you want the code to execute The Do While loop is used to

continue to execute code as long as a condition is true The Do Until loop continu-

ally executes code until a condition becomes true

Now that you know how to have the ASP.NET engine make decisions and exe-

cute code repeatedly, it is time to learn how to store a series of data efficiently in

your ASP.NET application by using an array

Think of an array as a group of valid user IDs that are stored in a long list that

can be assessed by using the name of the list You'll see how this is done in the next

d None of the above

2 What loop executes statements if a condition is true?

a Do While loop

b Do Until loop

ASPONET 2.0 Demystified

c Until loop

d None of the above

3 The counter range in the For loop is used to

a Increase the expression by 1

b Determine the range of values used to control the iterations of the loop

by the ASPNET engine

c Limit the number of statements that can be contained in the code block

d Limit the output of statements within the code block

4 A Case statement cannot have a default Case

6 What would you use if you want a block of statements to be executed only

if a condition isn't true?

8 What is the purpose of Elseif in an If Then Elseif statement?

a Contains statements that are executed only if the conditional expression

is true

b Defines another conditional expression the ASP.NET engine evaluates

if the first conditional expression is false

c Contains statements that are executed only if the conditional expression

is false

d Is used to nest an If statement

Conditional Statements

9 Statements within a For loop can reference the For loop variable

a True

b False

10 A Case statement is ideal to use to evaluate a menu option selected by

a visitor to your web site

3 b Determine the range of values used to control the iterations of the loop

by the ASP.NET engine

4 b False The default case is defined by the Case Else construct

5 a True The Step clause can cause values to be skipped

6 b If Then Else

7 b False It's used to execute code if none of the conditions specified

are true

8 b Defines another conditional expression the ASP.NET engine evaluates

if the first conditional expression is false

9 a True

10 a True

This page intentionally left blank

What

CHAPTER

Arrays

Suppose you had to store the name of 100 products of a sales catalog in memory

As you learned in Chapter 4, you could declare 100 variables, one for each product name; however, you’d have to come up with 100 unique variable names-and remember those names each time your ASP.NET engine needs to display products

on a web page

ASP.NET developers don’t use variables in such cases, as you probably surmise Instead they use an array An array has one name and can hold any number of prod- uct names You’ll learn about arrays and how to use them in your ASP.NET application to store and manipulate large amounts of information

Is an Array?

The ASP.NET sometimes needs to temporarily store information in memory just long enough to process a visitor’s request First you need to reserve space in mem- ory by declaring a variable such as

Dim selection AS Integer

ASP.NET 2.0 Demystified

This statement tells the ASP.NET engine to reserve a place in memory and call that place selection You use the word selection each time you want to use the value stored at that memory location You learned this in Chapter 4

An array is very similar to a variable in that an array is a place in memory that

is used to store information Unlike a variable, however, an array can have multiple variables called array elements Each array element refers to a location in memory where information is temporarily stored

An array is identified by a unique name similar to the name of a variable Each array element is identified by using the array name followed by the number of the array element This number is called an index

Think of an array as a column of a spreadsheet (see the following table) A letter identifies the column, and a number identifies each row In an array, the letter that identifies the column is the array name A row is an element of the array, and the row number is the index

We refer to the first cell of the column by combining the column name with the row number, such as A1 to refer to the first cell of the column An array works basi-

cally the same way in that you combine the name of the array with the index to reference an array element You'll see how this is done later in this chapter

The first part is the Dim keyword

The second part is the array name, which you create

The third part is the number of elements of the array

The fourth part is the data type of the array (see Chapter 4)

Here we declare an array called products that has three array elements, all of which have a String data type

Dim products(3) AS String