Controling program Flow

Outline

The If Keyword

Select case block

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Controling program Flow

Controling program Flow

Making decisions using the If keyword

At some point in the code you may need to test some condition and the, depending on the results of that test, perform different operations. The decision structures in Visual Basic are:

1. The If …Then statement

Syntax:

If [condition] Then [statement]

If [condition] Then

[statements]

End If

2. The If … Then … Else statement
Syntax:

If [condition1] Then
[statementblock-1]
ElseIf condition2 Then
[statementblock-2]] ...
Else
[statementblock-n]]
End If

Example: This example demonstrates the use of variables, decision statements and operators usage.

Dim Number As Integer, UserNumber As String 'declare the variables
Randomize 'throw the dice
'we select a random number and then we make it an integer
Number = Int(Rnd * 100) + 1 'select the number and exculde 0
Do 'here starts the main program loop
     DoEvents
     UserNumber = InputBox("Enter a number", "Guess the number")
     If UserNumber <> "" Then 'we verify if the user has entered a value
         If IsNumeric(UserNumber) Then
         'we use the IsNumeric function to verify if the
         'user has entered an valid numeric value
             If UserNumber = Number Then
             'if the number equals the number
             'the computer has selected
                 MsgBox "You have guessed the number!"
                 Exit Sub
             ElseIf UserNumber > Number Then
             'If the number is too big prompt the
             'user that he has entered a wrong value
                 MsgBox "Your number is too big."
             ElseIf UserNumber < Number Then
             'If the number is too small prompt the
             'user that he has entered a wrong value
                 MsgBox "Your number is too small."
             End If
         End If
     Else
         Exit Sub 'we have an invalid value so exit the program
     End If
Loop

The Select Case Statement

Select case almost alike If … Then … Else statement. This statement is used when you have to evaluate one expression and depending on the result, perform a certain action.
The syntax is very simple and easy to understand:

Select Case [expression to test]
Case [expressionlis1]
[statement list1]
Case [expressionlis2]
[statement list2]
…
Case [expressionlisn]
[statement listn]
Case Else
[statement]
End Select

Each expressionlist is a list of one or more values. If there is more than one value in a single list, the values are separated by commas. Each statementblock contains zero or more statements. If more than one Case matches the test expression, only the statement block associated with the first matching Case will execute. Visual Basic executes statements in the Case Else clause (which is optional) if none of the values in the expression lists matches the test expression.

Note: Select case function evaluates an expression once at the beginning, while the If…Then…Else evaluates an expression for each ElseIf statement. So if you have only one expression is recommended that you use the Select Case block instead of the If..Then..Else block, because it’s faster and more easy to write.

In the next example we will use:

Constants, Enumeration of constants, Select case block

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