1. VB Decisions & Conditions
2/21/2019
Prepared By: Deborah Becker

2. Prepared By: Deborah Becker
Covered Tipics
Conditions defined
Relational operators
Logical Operators
If Blocks
Read & Create Flowcharts (Logic)
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3. Prepared By: Deborah Becker
A Condition
Definition
Condition is an expression involving relational operators (such as > or =) or logical operators (and or) that is either true or false.
Example:
“Frog” = “Frog”
12 > 8 =
2/21/2019
Prepared By: Deborah Becker

4. Relational Conditions
> greater than
< less than
= equal to
<> not equal to
>= greater than or equal to
<= less than or equal to
Expressions containing comparison operators always result in an answer that is either true or false.
Example expressions using Visual Basic controls:
txtName.text <> "Frockmeister"
Val(txtAge.Text) > 25
optPrintForm.Value = True
chkDiscardStyles.Value = False
txtInterestRate.Text / 12 <= 0.05
2/21/2019
Prepared By: Deborah Becker

5. Examples of Conditions
txtName.text <> "Frockmeister"
Val(txtAge.Text) > 25
optPrintForm.Value = True
chkDiscardStyles.Value = False
(Val(txtInterestRate.Text) / 12) <= 0.05
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Prepared By: Deborah Becker

6. Mathematical Relational Operators
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Is the Condition True
1 <= 1
1 < 1
“car” < “cat”
“Dog” > “dog”
Remember the ASCII Table
2. False < means less than. No number can be less than itself
True Characters of a string are compared one at a time, working from left to right. The first two characters match and the third decides if the condition is true. R precedes T in the alphabet.
True Uppercase letters precede lowercase letters in the ANSI table.
2/21/2019
Prepared By: Deborah Becker

8. Are These Conditions True?
Let a = 4 and b = 3
Let c = “hello” and d = “bye”
(a + b) < (2 * a)
(Len(c) – b) = (a / 2)
C < (“good” & d)
* The function Len() returns a numeric value that indicated the number of characters in the string, inside the function.
1. True = 7 and 2 * 4 = 8
2. True Len(c) = 5 – 3 = 2 and (4 / 2) = 2
False H follows G in the ANSI table
2/21/2019
Prepared By: Deborah Becker

9. Prepared By: Deborah Becker
Logical Operators
The three main logical operators are
And Or
Not
Some programming conditions require more complicated comparison then just true or false
2/21/2019
Prepared By: Deborah Becker

10. 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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Prepared By: Deborah Becker

11. 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
When comparing values from text boxes with numeric values, be sure to first convert the text box data to a value. For example:
If txtPrice.Text = curRetailPrice Then ...
can result in an inaccurate comparison of a string and a currency value. Better to do this:
If Val(txtPrice.Text) = curRetailPrice Then ...
Also, if the text box is empty, then Val(textbox.text) is equal to zero. That can be very handy. It eliminates one possible error when you use a value in calculations
When comparing strings, make sure you “negate” any potential problems with case not matching. Generally, you simply want the spelling to match. Compare these two slightly different conditions:
If txtLastName.Text = “Smith”
or If Lcase(txtLastName.Text) = “smith”
You can use the logical operators of And, Or, and Not to form compound expressions such as:
If (Ucase(txtLastName) = “SMITH” And Val(txtAge) > 25) Then...
If (Val(txtAge.Text) = 0 Or txtLastName.Text = “”) Then ...
Using parentheses around the conditional statement is optional but desirable
2/21/2019
Prepared By: Deborah Becker

12. 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”
2/21/2019
Prepared By: Deborah Becker

13. Are these conditions T/F?
Let the string variable answ = “Y”
(answ = “Y”) or (answ = “y”)
(answ = “Y”) and (answ = “y”)
Not (answ = “y”)
True the condition become “Y” = “Y”
False the condition would be false for every condition fo answ. You can not set a variable = to two separate values simultaneously
True because “Y” = “y” is false
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Prepared By: Deborah Becker

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Using Parentheses
Using parentheses can improve readability.
Place parentheses in this equation to help evaluate it.
a < b + c Or d < e And Not f = g
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Answer
a < (b + c) Or (d < c) And (Not f = g)
A condition involving numeric variables is different from an algebraic truth.
The condition (a + b) < 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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Prepared By: Deborah Becker

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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
Parentheses alter the order of evaluation and are good for explicitly documenting groupings or are good if you are unsure of precedence.
If (curValue > 100) and (curValue < 200) Then
lblMessage.Caption = "Value between 101 and 199"
End If
The following is a project called SimpleSample.vbp to illustrate the example on pages
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Prepared By: Deborah Becker

17. Prepared By: Deborah Becker
Parentheses
Parentheses alter the order of evaluation, are good for explicitly documenting groupings, and are good if you are unsure of precedence.
If (curValue > 100) and (curValue < 200) Then
lblMessage.Caption = "Value
between 101 and 199"
End If
2/21/2019
Prepared By: Deborah Becker

18. IF Statement Structure (1)
Sleepy?
Go to bed
True
False
Form:
If condition Then
statement(s)
End If
2/21/2019
Prepared By: Deborah Becker

19. IF Statements Structure (2)
If School Day
False
True
Stay In Bed
Go to Class
Action
Action
The if block allows a program to decide on a course of action.
In the If statement You are always testing the condition to determine if it is True or False
If no Else statement is included then the statement is only executed if the condition is true.
2/21/2019
Prepared By: Deborah Becker

20. Prepared By: Deborah Becker
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
In the If Statement, when the condition is true, the Then clause is executed; when the condition is false, only the Else clause, if present, is executed.
The if block must contain an End If. It is always two words
Time
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Prepared By: Deborah Becker

21. Block If…Then…Else…End If
If strName = "Connie" Then
lblMessage.Caption = "Hello Connie" [another statement]
Else
lblMessage.Caption = "Hello Stranger" [another statement]
End If
The if then else clause can contain an else condition
If today is cloudy then
go to class
Else
go to beach
End if
2/21/2019
Prepared By: Deborah Becker

22. If…then…ElseIf…then…end if
If School Day
True
False
If Sunny
Stay In Bed
False
True
Play Tennis
Go to Class
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Prepared By: Deborah Becker

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Example of Nested IF
If costs = revenue Then
picResult.Print = “Break even”
Else
If costs < revenue Then
profit = revenue – costs
picResult.Print “Profit is “; _ FormatCurrency(profit)
Else loss = costs – revenue
picResult.Print “Loss is “; _ FormatCurrency(loss)
End if
Elseif is written as one word
End if is always two words.
2/21/2019
Prepared By: Deborah Becker

24. If Block and Logic Operators
The If Block can be combined with logic operators to compare more than one condition
If (answer >= .5) and (answer <= 1) Then
picSolution.Print “Good”;
Else
picSolution.Print “No”
End if
2/21/2019
Prepared By: Deborah Becker

25. 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
If condition1 and condition2 then
2/21/2019
Prepared By: Deborah Becker

26. Prepared By: Deborah Becker
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
If more branches are required then a ‘Select Case’ command can be used. Notice that in the following example, several values can trigger the same case statement. For example, the months 1, 3, 5, 7, 8, 10 and 12 all are 31 days long. Instead of writing 12 separate case statements, all these values can be included in one case by using commas.
The ‘End Select’ statement defines the end of the Case block. To ‘trap’ any values not picked up by earlier cases used the ‘Case Else’ construct.
Case Else MsgBox "ERROR: Month must be between 1 and 12!“
End Select
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Prepared By: Deborah Becker

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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 > 7 And Number < 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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Prepared By: Deborah Becker

28. Prepared By: Deborah Becker
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 <= $20,000
is taxed at 2% of income
The tax on Income > $20,000 but <= $50,000
is $400 plus 2.5% of (income -$20,000)
The tax on income > $50,000
is $1150 plus 3.5% of (income - $50,000)
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Prepared By: Deborah Becker

29. Prepared By: Deborah Becker
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30. Prepared By: Deborah Becker
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
2/21/2019
Prepared By: Deborah Becker

31. Action of For...Next structure
Counter<=ending value
Counter>ending 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.
2/21/2019
Prepared By: Deborah Becker

32. Iterations of a For...Next loop
Private Sub cmdForNextTest_Click()
Dim X As Integer ‘ define the index
For X = 1 To ‘ 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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Prepared By: Deborah Becker

33. Any Questions about the homework problem?
Problem Check
Any Questions about the homework problem?
2/21/2019
Prepared By: Deborah Becker