VB Decisions

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VB Decisions Conditions
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Covered Tipics

• Conditions defined
• Relational operators
• Logical Operators
• If Blocks
• Read Create Flowcharts (Logic)

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A Condition

• Definition
• Condition is an expression involving relational
  operators (such as gt or ) or logical operators
  (and or) that is either true or false.
• Example
• Frog Frog
• 12 gt 8

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Relational Conditions

• gt greater than
• lt less than
• equal to
• ltgt not equal to
• gt greater than or equal to
• lt less than or equal to

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Examples of Conditions

• txtName.text ltgt "Frockmeister"
• Val(txtAge.Text) gt 25
• optPrintForm.Value True
• chkDiscardStyles.Value False
• (Val(txtInterestRate.Text) / 12) lt 0.05

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Mathematical Relational Operators
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Is the Condition True

• 1 lt 1
• 1 lt 1
• car lt cat
• Dog gt dog
• Remember the ASCII Table

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Are These Conditions True?

• Let a 4 and b 3
• Let c hello and d bye
• (a b) lt (2 a)
• (Len(c) b) (a / 2)
• C lt (good d)
• The function Len() returns a numeric value that
  indicated the number of characters in the string,
  inside the function.

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Logical Operators

• The three main logical operators are
• And
• Or
• Not
• Some programming conditions require more
  complicated comparison then just true or false

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Logical Operators allow us to..

• Evaluate condition1 and condition2 based on the
  following
• condition1 and condition2
• condition1 or condition2
• condition1 not condition2

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With Relational and Logical Conditions We Can

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

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Comparing Numbers Strings

• If txtPrice.Text
• curRetailPrice Then ...
• If Val(txtPrice.Text) curRetailPrice Then ...
• Compare these two slightly different conditions
• If txtLastName.Text Smith
• If Lcase(txtLastName.Text) smith

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Are these conditions T/F?

• Let the string variable answ Y
• (answ Y) or (answ y)
• (answ Y) and (answ y)
• Not (answ y)

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Using Parentheses

• Using parentheses can improve readability.
• Place parentheses in this equation to help
  evaluate it.
• a lt b c Or d lt e And Not f g

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Answer

• a lt (b c) Or (d lt c) And (Not f g)
• A condition involving numeric variables is
  different from an algebraic truth.
• The condition (a b) lt 2 a is not a valid
  algebraic truth because it is not true for all
  values of a and b
• VB will evaluate it as true if it is correct for
  the current values of the variables

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Compound Conditions

• If a1 lt a2 or a3 gt a4 and a5 lt a6 Then
• And has precedence over Or
• All comparison operators have precedence over all
  logical operators
• Use parentheses () to alter the order of
  evaluation

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Parentheses

• Parentheses alter the order of evaluation, are
  good for explicitly documenting groupings, and
  are good if you are unsure of precedence.
• If (curValue gt 100) and (curValue lt 200) Then
• lblMessage.Caption "Value
• between 101 and 199"
• End If

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IF Statement Structure (1)

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IF Statements Structure (2)
If School Day
False
True
Stay In Bed
Go to Class
Action
Action
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If Examples in VB

• If Then (all on one line)
• If strName "Jamie" Then lblMessage.Caption _
• "Hello Jamie"
• Block If Then End If
• If strName "Sam" Then
• lblMessage.Caption "Hello Sam"
  another statement
• End If

Time
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Block IfThenElseEnd If

• If strName "Connie" Then
• lblMessage.Caption "Hello Connie" another
  statement
• Else
• lblMessage.Caption "Hello Stranger"
  another statement
• End If

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IfthenElseIfthenend if
If School Day
True
False
If Sunny
Stay In Bed
False
True
Play Tennis
Go to Class
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Example of Nested IF
If costs revenue Then picResult.Print Break
even Else If costs lt revenue Then profit
revenue costs picResult.Print Profit is
_ FormatCurrency(profit) Else
loss costs revenue picResult.Print Loss is
_ FormatCurrency(loss) End
if End if
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If Block and Logic Operators

• The If Block can be combined with logic operators
  to compare more than one condition
• If (answer gt .5) and (answer lt 1) Then
• picSolution.Print Good
• Else
• picSolution.Print No
• End if

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Additional Information

• It is not necessary to indent If Blocks
• We indent to improve readability
• Some times the use of relational operators make
  the If Block structure easier to understand
• If condition1 Then
• If condition2 Then
• action
• End if
• End if
• If condition1 and condition2 then
• action
• End if

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The Select Case Block

• Executes a statement based on the value of a
  variable
• Select Case bytMonth
• Case 1, 3, 5, 7, 8, 10, 12
• number_of_days 31
• Case 2
• number_of_days 28
• Case 4, 6, 9, 11
• number_of_days 30
• End Select

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Looking for a Number

• Select Case Score Case is 0
• ' Score is still 0
• lblMsgbox ("Keep trying!") Case 1To 4
• ' Score is between 1 and 4.
• lblMsgbox ("Making a good start!") Case 5,
  6,7
• ' Score is either 5, 6, or 7.
• lblMsgbox ("Nearly there!") Case Is gt 7
  And Number lt 11
• ' Score is 8 ,9, or 10
• lblMsgbox ("Getting close! ") Case Else
• ' Score is one of the other values.
• lblMsgbox (Number is to large! ") End
  Select

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The Problem

• In New Jersey the state income tax is based on a
  persons base income at three different interval
  levels
• The tax on Income lt 20,000
• is taxed at 2 of income
• The tax on Income gt 20,000 but lt 50,000
• is 400 plus 2.5 of (income -20,000)
• The tax on income gt 50,000
• is 1150 plus 3.5 of (income - 50,000)

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For Next Block

• For index lower limit to upper limit
• Coding statements
• This could be code to print the records in a
  file.
• Print one record from a file
• May contain condition blocks
• Next index

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Action of For...Next structure
Counterltending value
Countergtending value
For counter start To end statement1 statement2
. . .
For counter start To end statement1 statement2
. . .
Increment counter
Next counter
Next counter statement
The Index can also be called the counter.
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Iterations of a For...Next loop

• Private Sub cmdForNextTest_Click()
• Dim X As Integer define the index
• For X 1 To 3 1 lower limit
• picDisplay.Print X, 3 upper limit
• Next X
• End Sub

What will be the value of X when the loop
terminates? What will be inside the picture box?
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Problem Check

• Any Questions about the homework problem?