Irwin/McGraw-Hill Copyright© 2000 by the McGraw-Hill Companies, Inc. PowerPoint® Presentation to accompany prepared by James T. Perry University of San Diego

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Ch4: 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

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Comparison Operators SymbolMeaning >Greater than <Less than =Equal to <=Less than or equal to >=Greater than or equal to <>Not equal to

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill IF Statement Structure (1) Sleepy? Go to bed True False Form: If condition Then statement(s) End If

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill IF Statement Structure (2) Is SAT > 600? Admit Do Not Admit TrueFalse Form: If condition Then t-statement(s) Else f-statement(s) End If

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill IF Statement Structure (3) Form: If condition-1 Then If condition-2 Then t-statement-2 Else f-statement-2 End If Else f-statement-1 End If False Is SAT > 600? Do Not Admit True Is GPA > 3.75? Admit True Consider False

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Conditions  Comparing Numeric Variables & Constants  Comparing Strings  Comparing text property of text boxes  Uppercase and lowercase character comparisons  Compound conditions

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill If & opt. buttons & check boxes  If chkFlag = True Then imgFlag.Visible = True  If optDisplayForm Then frmSecond.Show

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Compound Conditions  If a1 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

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill "If" statement and option buttons

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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”

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Message box return values ConstantValueDescription vbOK1OK button pressed. vbCancel2Cancel button pressed. vbAbort3Abort button pressed. vbRetry4Retry button pressed. vbIgnore5Ignore button pressed. vbYes6Yes button pressed. vbNo7No button pressed.

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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...

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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...

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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.

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill Hands on Programming Example

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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)

Copyright© 2000 by the McGraw-Hill Companies, Inc.Irwin/McGraw-Hill 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