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Statements, Conditionals & Loops. Statement Syntax Procedure Calls Referencing Objects If-Then Conditional Statement Select Case Statement Loops While-Wend Loops Programming Principles. Assignment statement: Let varname = value Equal sign is assignment operator

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Statements, Conditionals & Loops

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Statements conditionals loops l.jpg

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


Statement syntax l.jpg

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()

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Statement Syntax

Jeffrey P. Landry, University of South Alabama


Statement syntax cont l.jpg

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


Procedure calls l.jpg

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


Referencing objects l.jpg

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


If then conditional statement l.jpg

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


Select case statement l.jpg

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


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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


Loops l.jpg

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


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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


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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


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