Objectives

1
Objectives

• Use block Ifs to control the flow of logic.
• Understand and use nested Ifs.
• Read and Create flowcharts indicating the logic
  in a selection process.
• Evaluate conditions using the relational
  operators.
• Combine conditions using And and Or.
• Test the Value property of option buttons and
  check boxes.
• Perform validation on numeric fields.
• Call event procedures from other procedures.
• Create message boxes to display error conditions.
• Apply the message box constants.
• Debug projects using breakpoints, stepping
  program execution, and displaying intermediate
  results.

2
If Statement

• Allows for an alternative path of execution to be
  carried out, depending on the outcome of a
  condition.
• If...Then...ElseIf condition Then
  if_statement(s)Else else_statement(s)End If
• The if_statement(s) are executed when the
  condition evaluates to True.The else_statements
  are executed when the condition evaluates to
  False.

3
If Statement (Continued)

• A Flow chart is sometimes used to graphically
  express the flow of control.

4
If Statement (Continued)
If x lt 10 Then lblMsg.Caption "OK"Else
lblMsg.Caption "No"End If
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If Statement (Continued)
If x lt 10 Then lblMsg.Caption "OK"End If
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If Statement (Continued)

• If...Then...ElseIf...Then...ElseIf condition1
  Then if_statement(s)ElseIf condition2 Then
  elseIf_statement(s)Else else_statementsEnd
  If
• The if_statement(s) are executed when condition1
  evaluates to True.The elseIf_statements are
  executed when condition2 evaluates to True.The
  else_statements are executed when both conditions
  evaluate to False.

7
If Statement (Continued)
If x lt 10 Then lblMsg.Caption "OK"ElseIf x
gt 10 Then lblMsg.Caption "No"Else
lblMsg.Caption "Yes"End If
8
Relational Operators

• You may use any of the following relational
  operators to form the condition in an If
  statement. Each evaluates to True or False.
• gt greater than x gt 10
• lt less than x lt 10
• equal to txtName.Text "Jane"
• ltgt not equal to txtName.Text ltgt "Jane"
• gt greater than or equal to x gt 10
• lt less than or equal to x lt 10
• The operators are binary operators, they require
  two arguments (same type), a left argument and a
  right argument.

9
Relational Operators (Continued)

• Numeric Relational ExamplesAssume the following
  variables have been declared and assigned the
  following values

intAlpha
intBravo
intCharlie
5
4
-5
intAlpha intBravo evaluates
False
intCharlie lt 0 evaluates
True
intBravo gt intAlpha evaluates
False
intCharlie lt intBravo evaluates
True
intAlpha gt 5 evaluates
True
intAlpha ltgt intCharlie evaluates
True
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Relational Operators (Continued)

• When comparing strings, each character is
  compared based on its internal ASCII (American
  Standard Code for Information Interchange) value.
  Refer to table on page 129.
• The comparison begins with the first character in
  each string and continues with each successive
  character.
• Once two corresponding characters are found not
  equal, the comparison ends and one string is
  declared less than the other.

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Relational Operators (Continued)

• String Relational Examples

JOHN lt JOAN evaluates
False
HOPE lt HOPELESS evaluates
True
300ZX gt Porsche evaluates
False
Apple apple evaluates
False
Banana gt apple evaluates
False
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Relational Operators (Continued)

• Relational ExamplesAssume the following
  variables have been declared and assigned the
  following values

intCount1
intCount2
intCount3
txt4.Text
txt5.Text
5
5
-5
"Bit"
"Bite"
intCount1 gt intCount2 evaluates
True
intCount3 lt 0 evaluates
True
intCount3 lt intCount2 evaluates
True
intCount1 ltgt intCount2 evaluates
False
intCount1 2 gt intCount2 2 evaluates
False
txt4.Text lt txt5.Text evaluates
True
txt4.Text ltgt txt5.Text evaluates
True
txt4.Text gt "D" evaluates
False
"2" ltgt "Two" evaluates
True
"" lt "?" evaluates
True
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Boolean Expressions

• An expression that evaluates to either True or
  False.
• True is assigned the value -1, False the value 0.
• Every expressions that evaluates to zero (0) is
  considered False, and every expression that
  evaluates to non-zero is considered True.
• ExampleIf blnFlag True Then ...is
  equivalent toIf blnFlag Then ...

14
Comparing .Text Properties

• CautionThe .Text property of a text box is a
  Variant, so when compared against another text
  box, VB uses string comparison.When compared to
  a numeric variable or constant, VB uses numeric
  comparison.

txt1.Text txt2.Text txt1.Text gt
txt2.Text
1 1
True
2 100
True
100 -100
False
0 ltblankgt
True
txt1.Text txt2.Text Val(txt1.Text) gt
Val(txt2.Text)
1 1
False
2 100
False
100 -100
True
0 ltblankgt
False
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Comparing .Text Properties (Continued)

• String comparison are case sensitive, i.e. "a" is
  not the same as "A".
• Use the functions UCase(string) and LCase(string)
  to convert the string argument to either
  uppercase or lowercase respectively.
• ExamplesUCase("Hello") returns "HELLO"
• UCase("HELLO") returns "HELLO"
• LCase("HELLO") returns "hello"
• LCase("hello") returns "hello"

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Logical Operators

• Used to combine two or more relational
  expressions into a compound expression.
• Or (binary) num1 0 Or num2 gt 5
• And (binary) num1 0 And num2 gt 5
• Not (unary) Not num1 0

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Logical Operators (Continued)

• OrEvaluates to True if one or both conditions
  are true, False otherwise.
• AndEvaluates to True if both conditions are
  true, False otherwise.
• NotEvaluates to True if condition is false,
  False if true.
• Order of PrecedenceNotAndOr

18
Nested If Statements

• You may have an If statement within another If
  statement, either in the If part or the Else
  part.
• Example 1 If within the If partIf condition
  Then If condition Then ' do something
  End If ' do somethingElse ' do
  somethingEnd If

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Nested If Statements (Continued)

• Example 2 If within the Else partIf condition
  Then ' do somethingElseIf condition Then '
  do somethingElse ' do somethingEnd If

20
The MsgBox Statement

• Used to display informative messages to the user.
• SyntaxMsgBox Prompt, Button, Title
• Example 1 Displaying a simple messageMsgBox
  "This is the prompt", vbInformation, "This is the
  Title"

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The MsgBox Statement (Continued)

• Available buttons to choose from.
• vbCritical
• vbExclamation
• vbInformation
• vbQuestion

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The MsgBox Statement (Continued)

• To add a newline to the prompt, use the constant
  vbCrLf.
• Any text following the newline character will
  appear on the next line in the message box
  prompt.
• Example2 Using the newline characterMsgBox
  "Hello, y'all" vbCrLf "How are you?"

23
Input Validation

• Question Do users always follow instructions for
  inputting data?
• Answer Of course not!
• Input validation deals with making sure the input
  values are tested during input, rather than pop
  up later as errors.
• As a rule-of-thumb, the more validation you
  include the better the program will be, but at
  the same time, more code will be required.
• The benefits do outweigh the costs, and you
  should always strive to validate your input.

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Input Validation Numeric Values

• To assure a number has been entered, use VB's
  IsNumeric function.
• The function returns True if its argument is
  numeric, else it returns False.
• SyntaxIsNumeric(expression)
• Example 1 Checking numeric valuesIf
  IsNumeric(txtQuantity.Text) Then intQuantity
  Val(txtQuantity.Text) lblDue.Caption
  curPrice intQuantityElse MsgBox "Nonnumeric
  data entered",,"Invalid Data"End If

25
Calling Event Procedures

• If code you need is already written in an event
  procedure, you may wish to call the event
  procedure rather than retyping the code.
• You may call any event procedure any number of
  times.
• SyntaxProcedureName
• Example 1 calling an event procedure
• cmdCalculate_ClickcmdDoIt_ClickcmdDoIt_Click

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Hands-On Project

• Step 1 Create the GUI

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Hands-On Project (Continued)

• Step 2 - Set the Properties
• Object Property Setting
• Form Name FBilling Caption R 'n R for
  Reading 'n Refreshment
• Frame1 Caption Order Information
• Frame2 Caption Coffee Selections
• Frame3 Caption ltblankgt
• option1 Name optCappuccino Caption Cappuccin
  o Value True
• option2 Name optEspresso Caption Espresso
• option3 Name optLatte Caption Latte

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Hands-On Project (Continued)

• Step 2 - Set the Properties (Continued)
• Object Property Setting
• option4 Name optIcedLatte Caption Iced
  Latte
• option5 Name optIcedCappuccino Caption Iced
  Cappuccino
• Label1 Caption Quantity
• Text1 Name txtQuantity Text ltblankgt
• Check1 Name chkTax Caption Takeout?
• Label2 Caption Item Amount
• Label3 Caption SubTotal
• Label4 Caption Tax (if Takeout)

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Hands-On Project (Continued)

• Step 2 - Set the Properties (Continued)
• Object Property Setting
• Label5 Caption Total Due
• Label6 Name lblItemAmount Caption ltblankgt B
  orderStyle 1 - Fixed Single
• Label7 Name lblSubTotal Caption ltblankgt Bor
  derStyle 1 - Fixed Single
• Label8 Name lblTax Caption ltblankgt BorderSt
  yle 1 - Fixed Single
• Label9 Name lblTotal Caption ltblankgt Border
  Style 1 - Fixed Single

30
Hands-On Project (Continued)

• Step 2 - Set the Properties (Continued)
• Object Property Setting
• Command1 Name cmdCalculate Caption Calculate
  Selection Default True
• Command2 Name cmdClear Caption Clear for
  Next Item Cancel True
• Command3 Name cmdNewOrder Caption New Order
• Command4 Name cmdSummary Caption Summary
• Command5 Name cmdExit Caption Exit

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Hands-On Project (Continued)

• Step 3 - Write the Code
• Declare some module-level variables and constants
• Private Const mcurTaxRate As Currency
  0.08Private Const mcurCappuccinoPrice As
  Currency 2.0Private Const mcurEspressoPrice As
  Currency 2.25Private Const mcurLattePrice As
  Currency 1.75Private Const mcurIcedPrice As
  Currency 2.50
• Private mcurSubTotal As CurrencyPrivate
  mcurTotal As CurrencyPrivate mcurGrandTotal As
  CurrencyPrivate mintCustomerCount As Integer

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• cmdCalculate_Click()Determine price per
  cup.Multiply price by quantity.Calculate tax if
  needed.Calculate total subtotal tax.Display
  the values.

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• Private Sub cmdCalculate_Click()' Calculate and
  display the current amounts, add to' totals.
• Dim curPrice As CurrencyDim intQuantity As
  IntegerDim curTax As CurrencyDim curItemAmount
  As Currency
• ' Determine the current priceIf
  optCappuccino.Value True Then curPrice
  mcurCappuccinoPriceElseIf optEspresso.Value
  True Then curPrice mcurEspressoPriceElseIf
  optLatte.Value True Then curPrice
  mcurLattePrice

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• ElseIf optIcedCappuccino.Value True Or _
  optIcedLatte.Value True Then curPrice
  curIcedPriceEnd If
• ' calculate the item amount and update order
  subtotalIf IsNumeric(txtQuantity.Text) Then
  intQuantity Val(txtQuantity.Text) '
  calculate item amount curItemAmount curPrice
  intQuantity ' update order subtotal
  mcurSubTotal mcurSubTotal curItemAmount

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• ' calculate the tax if a takeout If
  chkTax.Value vbChecked Then curTax
  mcurSubTotal mcurTaxRate End If
• ' update order total mcurTotal
  mcurSubTotal curTax
• ' display the results lblItemAmount.Caption
  FormatCurrency(curItemAmount) lblSubTotal.Capt
  ion FormatCurrency(mcurSubTotal) lblTax.Caption
  FormatCurrency(curTax) lblTotal.Caption
  FormatCurrency(mcurTotal)

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• Else ' non-numeric quantity entered MsgBox
  "Quantity must be numeric", vbExclamation,
  "Non-numeric Entry" txtQuantity.SetFocusEnd
  If
• End Sub

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• cmdClear_Click()Clear the coffee
  selections.Clear the quantity and the item
  amount.Disable the takeout check box.Set the
  focus to the quantity.

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• Private Sub cmdClear_Click()' Clear appropriate
  controlsoptCappuccino.Value True
• lblItemAmount.Caption vbNullString
• chkTax.Enabled False
• With txtQuantity .Text vbNumllString .SetFocu
  sEnd With
• End Sub

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• cmdExit_Click()Terminate the project
• Private Sub cmdExit_Click()
• ' terminate the projectEnd
• End Sub

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• cmdNewOrder_Click()Clear the previous
  order.Accumulate total sales and count.Set
  subtotal and total due to 0.Enable takeout check
  box.

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• Private Sub cmdNewOrder_Click()' prepare form
  for a new order' clear the item
  amountscmdClear_Click
• ' clear total amountslblSubTotal.Caption
  vbNullStringlblTax.Caption vbNullStringlblTota
  l.Caption vbNullString
• ' update grand totalsIf mcurSubTotal ltgt 0
  Then mcurGrandTotal mcurGrandTotal
  mcurTotal mcurSubTotal 0 mcurTotal
  0 mintCustomerCount mintCustomerCount 1End
  If

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• ' enable the check boxWith chkTax .Enabled
  True .Value vbUncheckedEnd With
• End Sub

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• cmdSummary_Click()Display the average.Display
  the number of Customers.

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• Private Sub cmdSummary_Click()' calculate the
  average and display the totalsDim curAverage As
  CurrencyDim strMessageString As StringDim
  strFormattedAvg As String
• ' do we have any customers?If mintCustomerCount
  gt 0 Then ' count the last order if still
  displayed, ' since it would not have been
  counted. If mcurTotal ltgt 0 Then cmdNewOrder_Cli
  ck End If
• curAverage mcurGrandTotal / mintCustomerCount

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Hands-On Project (Continued)

• Step 3 - Write the Code (Continued)
• ' format the numbers strFormattedAvg
  FormatCurrency(curAverage)
• ' concatenate the message string strMessageStri
  ng "Number of Orders "
  mintCustomerCount vbCrLf _
  "Average Sale " strFormattedAvg
• MsgBox strMessageString, vbInformation,
  "Coffee Sales Summary"Else MsgBox "No data to
  summarize", vbExclamation, "Coffee
  Sales Summary"End If
• End Sub

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Debugging your Code

• The process of eradicating your bugs.
• VB offers a very nice debugger to aid us in
  detecting and correcting errors in the code.
• The debugger will detect run-time and logic
  errors.
• You can use the Immediate/Debug window to
  interact with your application during debugging.
• Terms to be familiar with
• step into steps into a procedure and executes
  each line.
• step over executes entire procedure as one
  statement.
• breakpoint a marked location in the code where
  you would like to enter Break mode.