Chapter 3

Control Structures

The If…Then Statement

The If…Then statement is a Decision statement = that executes a set of

statements when a condition is True. Takes the form:

If condition Then

statements

End If

If…Then cont…

Condition = What the computer evaluates to be either True or False

Statements = What the computer does if the condition is True

Example:If intGuess = 7 Then

Me.lblMessage.Text = “You guessed it!”End If

Relational Operators

Relational Operators can be used to form Boolean expressions (remember Boolean is either True or False)

Operator Meaning

= Equal to

< Less than

<= Less than or equal to

> Greater than

>= Greater than or equal to

<> Not equal to

If…Then…Else Statement

Includes an optional Else clause that is executed when the If condition evaluates to False.

Takes the form:If condition Then

statementsElse

statementsEnd If

Nested If…Then…Else Statement

An If…Then…Else statement can contain another If…Then…Else or If…Then statement, which is said to be nested.

Nested statements should be indented for good programming style.

Continued on next page..

Nested If…Then…Else Statement cont…

Takes the form:If condition Then

statementsElse

If condition Thenstatements

Elsestatements

End IfEnd If

The If…Then…ElseIf Statement

Used to decide among three or more actions Takes the form:

If condition Thenstatements

ElseIf condition Thenstatements

…Else

statementsEnd If

The If…Then…ElseIf Statement cont…

There can be multiple ElseIf clauses, and the last Else clause is optional.

Comments are a must to keep complicated code much more readable. This is especially important for the last branch of a decision structure which usually does not include an explicit condition.

The Select…Case Statement

The Select…Case Statement is a decision structure that uses the result of an expression to determine which block of code to execute.

The Select…Case Statement is sometimes preferable to the If…Then…ElseIf statement because code may be easier to read.

Select…Case Statement cont…

Select…Case takes the form:

Select expression

Case value

statements

…

Case Else

statements

End Select

Example of Select…Case

Example:Select Case intScore

Case 0, 10 ‘Score is 0 or 10

statements

Case 20 To 25 ‘Score is 20,21,22,23,24

statements or 25

Case Else ‘Score other than 0,10,20,

statements 21,22,23,24 or 25

End Select

Select…Case Statement cont…

The Select…Case must evaluate to a built-in data type. Ex. Integer, single, …

Their can be multiple Case clauses, and the Case Else clause is optional.

IMPORTANT: The value type should match the expression type and can be a single value, a list separated by commas, of a range separated by the keyword To.

The Select…Case Is Statement

Compares the result of an expression to a range of values when a relational operator is part of the value.

Example:Select Case intScore

Case Is < 10statements

Case Is < 25statements

Case Is >= 25statements

End Select

Generating Random Numbers

The phrase Randomize() must be included at the beginning of the procedure that is using the random number.

To generate a random number use the built in Rnd() function

The formula for a random number is: (High Number – Low Number) * Rnd() + Low Number

Which is set equal to some variable.

Message Box

Can be displayed to alert the user of invalid data or as a reminder of options required for an application to continue.

How to code a message box? MessageBox.Show(“message”,”title”) The “title” is optional.

Counter Variables

A counter is a variable storing a number that is incremented by a constant value.

Counters are useful for keeping track of the number of times a user clicks a button, enters a guess, or types a password.

Updating a counter takes the form:

counter = counter + constant

Counter Variables cont…

Counter is the numeric variable that is updated.

Constant is the number that is added to the current value of counter.

A counter should be initialized when it is declared and then updated by an unchanging amount.

Static Variables

Anytime a counter variable is declared it should be declared as a static variable instead of dimensional.

A variable declared as a local variable is redeclared every time through the event procedure unless it is declared as a static.Ex:

Static intCounter As Integer

Static Variables cont…

Extends the lifetime of a local variable. The value of a static variable is not

redeclared and reinitialized each time the Click event occurs.

Useful when assigning random numbers to variables.

The CheckBox Control

Name = prefix = chk Text = text displayed next to the box Checked = can be set to either True or False to

display the check box with or without a check, respectively.

Related check boxes are sometimes placed together in a GroupBox object. As with radio buttons, a group box should be added to the form before adding check boxes.

Line-Continuation Character

Statements that are long can be typed onto two or more lines when the line-continuation character is used.

The underscore ( _ ) is the line-continuation character.

Must have a space before it and nothing after it and cannot occur within quotation marks.

Logical Operators

A Boolean expression can be formed using the logical operators And and Or.

A logical operator joins two expressions to create an expression that evaluates to either True or False.

A third logical operator used is Not. Not changes an expression either from

True to False or from False to True.

Logical Operators cont…

Logical OperatorsAnd OrExpression1 Expression2 Result Expression1 Expression2 Result True True True True True True True False False True False

True False True False False True True False False False False False False

NotExpression Result True False False True

String Concatenation

Two or more strings can be joined together in a process call concatenation.

The & operator can be used to concatenate string. The & operator is used in an expression similar to the following:newString = string1 & string2

newString is a String variable that will store the result of the expression. string1 and string2 are String variables or string enclosed in quotation marks.

& cont…

Example

Dim strFirstName As String

Dim strLastName As String

Dim strFullName as String

strFirstName = “Al”

strLastName = “Bundy”

strFullName = strFirstName & “ “ & strLastName

StrFullName will = Al Bundy

& cont…

Concatenation can also be used to combine two labels into one.

Ex: Me.lblMessage.text = “Your answer is “ & sngAnswer & “ inches”