1. Decisions and Conditions
Chapter 4
Decisions and Conditions

2. Decisions and Conditions
Block If statements
Nested If statements
Evaluating Conditions and Relational Operators
Testing Values with option buttons & check boxes
Validating input numeric fields
Creating Message boxes
Calling event procedures
Debugging with breakpoints & stepping execution

3. Alternative courses of action are required in a program
Decision Making
Alternative courses of action are required in a program
The IF statement provides one of these structures
There are two types of IF statements
Single-line IF statement
Multi-line IF statement
Single-line IF statement example:
If txtUserResponse = “Yes” Then curTaxableIncome = 0
Multi-line IF statement example:
If intDependentNumber > 2 Then
lblAnswer.caption = “Two Dependents”
Else
lblAnswer.cpation = “Not Two Dependents”
End If

4. Symbol Meaning > Greater than < Less than = Equal to
Comparison Operators
Symbol Meaning
> Greater than
< Less than
= Equal to
<= Less than or equal to
>= Greater than or equal to
<> Not equal to
Expressions containing comparison operators always result in an answer that is either true or false.
Example expressions using Visual Basic controls:
txtName.text <> "Frockmeister"
Val(txtAge.Text) > 25
optPrintForm.Value = True
chkDiscardStyles.Value = False
txtInterestRate.Text / 12 <= 0.05

5. IF Statement Structure (1)
Sleepy?
Go to bed
True
False
Form:
If condition Then
statement(s)
End If

6. IF Statement Structure (2)
Is SAT >
600?
Admit
Do Not
True
False
Form:
If condition Then
t-statement(s)
Else
f-statement(s)
End If
If Val(txtSat.text) > 600 Then
Msgbox “Admit”
Else
Msgbox “Do Not Admit”
End If

7. IF Statement Structure (3)
Form:
If condition-1 Then
If condition-2 Then
t-statement-2
Else
f-statement-2
End If
f-statement-1
End If
False
Is SAT >
600?
Do Not
Admit
True
Is GPA > 3.75?
Consider
If Val(txtSat.text) > 600 Then
If Val(txtGPA) > 3.75 Then
Msgbox “Admit”
Else
Msgbox “Consider”
End if
Msgbox “Do Not Admit”
End If

8. Comparing Numeric Variables & Constants Comparing Strings
Conditions
Comparing Numeric Variables & Constants
Comparing Strings
Comparing text property of text boxes
Uppercase and lowercase character comparisons
Compound conditions
When comparing values from text boxes with numeric values, be sure to first convert the text box data to a value. For example:
If txtPrice.Text = curRetailPrice Then ...
can result in an inaccurate comparison of a string and a currency value. Better to do this:
If Val(txtPrice.Text) = curRetailPrice Then ...
Also, if the text box is empty, then Val(textbox.text) is equal to zero. That can be very handy. It eliminates one possible error when you use a value in calculations
When comparing strings, make sure you “negate” any potential problems with case not matching. Generally, you simply want the spelling to match. Compare these two slightly different conditions:
If txtLastName.Text = “Smith”
or If Lcase(txtLastName.Text) = “smith”
You can use the logical operators of And, Or, and Not to form compound expressions such as:
If (Ucase(txtLastName) = “SMITH” And Val(txtAge) > 25) Then...
If (Val(txtAge.Text) = 0 Or txtLastName.Text = “”) Then ...
Using parentheses around the conditional statement is optional but desirable

9. If & opt. buttons & check boxes
If chkFlag = True Then
imgFlag.Visible = True
If optDisplayForm Then
frmSecond.Show
When using option button controls, you don't have to compare it against True or False in a conditional expression. Because the value of 0 is considered False and all other values (1, for example) are considered True. So, instead of writing this:
If optPrintSelection = True Then ···
you can write the following equivalent statement:
If optPrintSelection Then ···
The preceding is an example of an implied condition
This can work for check boxes also, except check boxes can also have the value of 2 (undefined)
When comparing a literal string to a text box value, always compare “apples to apples” by converting both to either uppercase or lowercase. That way you avoid a mismatch based solely on capitalization an not spelling:
If txtLastName.Text = "Smith" Then …
will evaluate to false if the txtLastName contains SmITH or smith.
Better way is to always do this type of operation:
If Ucase(txtLastName.Text) = "SMITH" Then ··· or
If Lcase(txtLastName.Text) = "smith" Then ...

10. If a1 <= a2 or a3 > a4 and a5 < a6 Then
Compound Conditions
If a1 <= a2 or a3 > a4 and a5 < a6 Then
And has precedence over Or
All comparison operators have precedence over all logical operators
Use parentheses to alter the order of evaluation
Parentheses alter the order of evaluation and are good for explicitly documenting groupings or are good if you are unsure of precedence.
If (curValue > 100) and (curValue < 200) Then
lblMessage.Caption = "Value between 101 and 199"
End If
The following is a project called SimpleSample.vbp to illustrate the example on pages

11. "If" statement and option buttons

12. Displaying Messages in Message boxes
Special window displaying message to user
Form:
MsgBox “message” [,buttons][, “t.b. caption”]
Example:
MsgBox “Numeric ID only”, vbOkOnly, “Error”
Buttons/Icons you can display:
Intrinsic Name Value Description
vbOKOnly 0 OK button only (default).
vbOKCancel 1 OK and Cancel buttons.
vbAbortRetryIgnore 2 Abort, Retry, and Ignore buttons.
vbYesNoCancel 3 Yes, No, and Cancel buttons.
vbYesNo 4 Yes and No buttons.
vbRetryCancel 5 Retry and Cancel buttons.
vbCritical 16 Critical message.
vbQuestion 32 Warning query.
vbExclamation 48 Warning message.
vbInformation 64 Information message.
vbDefaultButton1 0 First button is default (default).
vbDefaultButton Second button is default.
vbDefaultButton Third button is default.
vbApplicationModal 0 Application modal message box vbSystemModal System modal message box.

13. Displaying a Message String
Use & to concatenate strings
(“Concatenate” means join end to end)
The VB intrinsic constant vbCRLF creates a new line in a string
MsgBox stMessage, vbOKOnly, stTitle
Dim stMessageString as String
Dim stTitleBarString as String
stMessageString = "The top salary is: " & vbCrLf & curTopSal & " for 2001"
stTitleBarString = "Information for You"
MsgBox stMessageString, vbOkOnly, stTitleBarString

14. Message box return values
Constant Value Description
vbOK 1 OK button pressed.
vbCancel 2 Cancel button pressed.
vbAbort 3 Abort button pressed.
vbRetry 4 Retry button pressed.
vbIgnore 5 Ignore button pressed.
vbYes 6 Yes button pressed.
vbNo 7 No button pressed.
When you use the function form of the message box, you can save the returned value like this:
iReturnedValue = MessageBox("Click a Button", vbYesNoCancel,"Test")

15. Checking a data type: IsNumeric & IsDate
Input Validation
Checking a data type: IsNumeric & IsDate
IsNumeric checks & returns true or false
If IsNumeric(txtQty.Text) Then
lblDue.Caption = curPrice + Val(txtQty)
Validating value ranges
If Val(txtHours.Text) > 10 And _
Val(txtHours.Text) <= 80 Then ...
Note that IsNumeric works on the Variant data type.
The information in all text boxes is of type Variant—you can enter numbers of text values in a text box.
Testing a text box with IsNumeric is always a good idea to prevent “surprises.”

16. Data Validation
IsDate returns true or false depending on whether or not a value is a date
If IsDate(txtData) Then …
the VarType function return a number that corresponds to the data type stored in a variant.
If VarType(varValue) = 0 Then...
Here are the VarType values returned and each data type. Keep this for reference.
Value Name Contents
0 Empty No data in the text box (.Text property is variant)
1 Null Variant has no value--not empty
2 Integer Whole number between and 32767
3 Long Whole number between -2Billion and +2Billion
4 Single Decimal number
5 Double Either a very big dec. numb. Or huge # dec. places
6 Currency Decimal number with 4 decimal places
7 Date/Time Value holding a particular date & time
8 String Text
9 OLE Object (ignore for now)
10 Error (ignore for now)
11 Boolean True or false value
12 Variant An array of variants
13 Object Code lump--a non-OLE object (ignore for now)
14 Decimal A power of 10 type number
17 Byte A binary value
8192 Array An ordered table of values

17. Using LostFocus to Validate
LostFocus is a way to validate check boxes before allowing user to go on.
But is it a good way?
Consider the code
If txtUserName.Text = "" Then txtUserName.SetFocus
Problem: some object gets focus and you cannot have allow both objects to be tested using the LostFocus event.

18. Calling Event Procedures
An event procedure is a subprocedure that reacts to a specific event such as a button click.
You can call any given event procedure from multiple locations, as long as the procedure is in the same form or is public
Example: Call cmdCalculate_Click
Suffix is event, prefix is object name
The word call is optional. You can call an event procedure (or any procedure for that matter) by simply using its name.

19. Hands on Programming Example

20. Debug projects by setting code breakpoints
Debugging VB Projects
Debug projects by setting code breakpoints
Run the project,
Step through the code window at break time by pressing F8,
Point to variables above executed code lines to view their current values (see program shot in Notes portion of this slide)
breakpoint
value of variable

21. You can choose "Step into" or "Step over"
Debugging continued
You can choose "Step into" or "Step over"
Step into traces all code execution, including traversing down into subprocedures
Step over stays with current subprocedure only, not showing any called routines' code