Statements, Conditionals & Loops

1. Statements, Conditionals & Loops • Statement Syntax • Procedure Calls • Referencing Objects • If-Then Conditional Statement • Select Case Statement • Loops • While-Wend Loops • Programming Principles Jeffrey P. Landry, University of South Alabama

2. Assignment statement: Let varname = value Equal sign is assignment operator Let Name$ = “Name: “ & txtEmpName.Text Expressions have a data type string, numeric, boolean (true/false, etc.) conditional expressions in If-Then statements must evaluate to a boolean value Type compatibility between varname and value A procedure call is a statement Call cmdQuit_Click() - Statement Syntax Jeffrey P. Landry, University of South Alabama

3. Statement Syntax (cont.) • Use underscore for statement continuation • Response = MsgBox(“Invalid input.”, _ vbExclamation, “Error”) • No line termination character in VB • Must have a complete statement • not an expression on a line by itself • remember pairs of delimiters (parens, quotes) • Common syntax errors • End of statement • Expecting ‘)’ Jeffrey P. Landry, University of South Alabama

4. Function calls can appear in expressions but they must return a value function must match expected type Wrong: if Len(txtLast) Then… Correct: if Len(txtLast) = 0 Then... Subroutines called on a line by itself Call CalculatePay CalculatePay Procedure Calls Jeffrey P. Landry, University of South Alabama

5. Referencing Objects • Objects can be assigned values or referenced • Properties have a data type • variant type for some properties • watch for type incompatibility errors • Refer to objects in code using the object.property notation • if txtLast.Text = “” Then … • fMainMenu.Enabled= False • Also the form.control.property notation • Form1.Label3.Caption = “My Name: ” • Default form and default properties Jeffrey P. Landry, University of South Alabama

6. Format End line with Then Indent 2 spaces before {statements} If, Else & End If all line up Only one set of statements gets executed If-Then’s can be nested, or use the ElseIf structure If condition Then {statements} Else {statements} End If If condition1 Then {statements} ElseIf condition2 Then {statements} Else {statements} End If If-Then Conditional Statement Jeffrey P. Landry, University of South Alabama

7. An alternative to nested If-Then’s Actual values of testexpression are matched to expressionlists following the Case statements Only code in (one) appropriate block executes Select Case testexpression [Case expressionlist [statements1]]… [Case Else [elsestatements]] End Select Select Case Index Case 0 fAdd.Show Case 1 fPrint.Show Case Else End Select Select-Case Statement Jeffrey P. Landry, University of South Alabama

8. Select-Case Statement (cont.) • Several forms of the Case statement • Case “No” ‘* string values • Case 3 ‘* numeric values • Case 3 To 7 ‘* range of values • Case Is > 7 • Case Is <= 2 ‘* comparisons • Case 3,5,8 • Case 3 To 7, 9 ‘* lists of values Jeffrey P. Landry, University of South Alabama

9. For / Counted Loop Dim intCount as Integer For intCount = 1 to 3 Step 1 Print intCount Next intCount Do While Loop Dim intCount as Integer intCount = 1 Do While intCount <= 3 Print intCount intCount = intCount + 1 Loop (Pre-Test Loop) Do Until Loop Dim intCount as Integer intCount = 1 Do Print intCount intCount = intCount + 1 Loop until intCount > 3 (Post Test Loop) Loops Jeffrey P. Landry, University of South Alabama

10. An example of a conditional loop statements repeatedly execute until the condition is satisfied condition must be a boolean expression See help on Do-Loop statement for more flexibility Read about For-Next counting loops While condition {statements} Wend Let Count = 0 Let Sum = 0 Open “Grades.txt” For Input As #1 While Not EOF(1) Input #1, Name$, Grade Let Sum = Sum + Grade Let Count = Count + 1 Wend picOut.Print “Average=“;Sum/Count While-Wend Loops Jeffrey P. Landry, University of South Alabama

11. Programming Principles • Use incremental development strategy • Top-down design & development • Debug & comment as-you-go • Don’t code on speculation -- pseudocode instead • Meaningful identifiers in mixed case style • cmdQuit, EmployeePayRate, LastName • Make use of proper indenting & spacing • No GoTo statements! • GoTo’s create ‘spaghetti’ code Jeffrey P. Landry, University of South Alabama