ISM 3230 Introduction to Programming 

Index: 
  

• 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

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

Comparison Operators

• 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

IF Statement Structure (One-way)

IF Statement Structure (Two-way)

Example: If Val(txtSat.text) > 600 Then Msgbox “Admit” Else Msgbox “Do Not Admit” End If

IF Statement Structure (Multi-way)

If Val(txtSat.text) > 600 Then If Val(txtGPA) > 3.75 Then Msgbox "Admit" Else Msgbox "Consider" End if Else Msgbox "Do Not Admit" End If

Conditions

Comparing Numeric Variables & Constants

• 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

Comparing Strings - the .Text property of text boxes -

• 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”

  If Lcase(txtLastName.Text) = “smith”

ASCII Code Table

Uppercase / Lowercase Comparisons

• 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. A better way is to always do this type of operation:

If Ucase(txtLastName.Text) = "SMITH" Then ···

If Lcase(txtLastName.Text) = "smith" Then ...

Compound Conditions

• 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 ...

  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 if you are unsure of precedence.

   

  If (curValue > 100) and (curValue < 200) Then

  lblMessage.Caption = "Value between 101 and 199"

  End If

If ... / Option Buttons / Check Boxes

Examples:

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. This is 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)

Option Button Example	Check Box Example

Displaying Messages in Message boxes

A Message Box is a special window displaying a message to the user.

Form:

MsgBox “message” [,buttons][, “t.b. caption”]

Example:

MsgBox “Numeric ID only”, vbOkOnly, “Error”

Buttons you can use:

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

• 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")

Message Box Return Values

Input Validation

• Checking a data type: IsNumeric and IsDate
• IsNumeric checks & returns true or false

  If IsNumeric(txtQty.Text) Then

  lblDue.Caption = curPrice + Val(txtQty)

• Note that IsNumeric works on the Variant data type.
• The information in all text boxes is of type Variant—you can enter numbers or text values in a text box.
• Testing a text box with IsNumeric is always a good idea to prevent “surprises.”

• Validating value ranges

  If Val(txtHours.Text) > 10 And Val(txtHours.Text) <= 80 Then ...

• 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...

   

  

   

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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
• Note that the word Call is optional. You can call an event procedure (or any procedure for that matter) by simply using its name.

  cmdCalculate_Click

Hands on Programming Example

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)
• 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

Chapter Summary