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Understanding Conditional Logic and Input Validation in Visual Basic Programming

This chapter covers essential principles of decision-making in programming using Visual Basic. It explores the structure and types of If statements, including single-line and multi-line forms. Learn how to evaluate conditions using relational operators, nested structures, and logical combinations. Gain insights into validating numeric fields and using message boxes for user interaction. Additionally, this chapter addresses debugging techniques, including breakpoints and stepping through code. Enhance your programming skills by mastering these crucial concepts for developing reliable applications.

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Understanding Conditional Logic and Input Validation in Visual Basic Programming

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  1. 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. 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

  4. Comparison Operators Symbol Meaning > Greater than < Less than = Equal to <= Less than or equal to >= Greater than or equal to <> Not equal to

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

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

  7. 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 True Is SAT > 600? False True Is GPA > 3.75? Do Not Admit Consider Admit IF Statement Structure (3)

  8. Conditions • Comparing Numeric Variables & Constants • Comparing Strings • Comparing text property of text boxes • Uppercase and lowercase character comparisons • Compound conditions

  9. If & opt. buttons & check boxes • If chkFlag = True Then imgFlag.Visible = True • If optDisplayForm Then frmSecond.Show

  10. 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

  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”

  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

  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.

  15. 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 ...

  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...

  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

  19. Hands on Programming Example

  20. 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)

  21. 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

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