Computer-made Decisions

1. Computer-made Decisions Chapter 3 & 4

2. Overview • Variable Scope • Conditionals • Relational Operators • AND, OR, NOT • If Statements • MsgBox function

3. Conditionals • Sometimes it is necessary to alter one’s behavior, depending on existing conditions • Examples: • If the light is red, stop • If it is Labor Day, don’t come to class, otherwise attend lecture and lab • If there is a quiz on Monday, then read the book Sunday

4. Conditionals • True or False conditions are created • when comparing variables with relational operators • by evaluating a boolean variable • Relational Operators > Greater than < Less than = Equal to <> Not equal to >= Greater than or equal to <= Less than or equal to

5. Compound Conditions • You may join separate conditions with the ‘AND’ and the ‘OR’ to create compound conditions • The AND is data reducing; it reduces choices that fit • The OR is data expanding; it increases the choices that fit Blue Cars AND Four Doors is ????? Blue Cars OR Four Doors is ????? Blue Cars A B C Four Doors

6. Compound Conditions Union = Both Alternation = Either • F And T Or T F T T T T F T F F F F T F

7. Compound Conditions • Evaluate Negations (NOTs) first • Evaluate ANDs; left to right • Evaluate ORs after all ANDs; left to right • Parentheses can, as usual override this precedence

8. The If Test • A conditional statement that permits portions of code to be executed only if and when some special condition applies • I'd like root beer if they have it, otherwise I'll have a cola • If it is raining, bring an umbrella

9. If Test If (Condition) Then ' (do true stuff) Else ' (do false stuff) End If

10. If Test (short form) If (Condition) Then ' (do true stuff) End If use only if there is nothing to do in the Else branch

11. IF Test Example Private Sub cmdDo_Click() If cmdDo.Caption = "Talk" Then lblBox.Caption = "ding ding" cmdDo.Caption = "clear" Else 'in this case, cmdDo = "clear" lblBox.Caption = "" cmdDo.Caption = "Talk" End If End Sub

12. IF Test Example << alternate on clicking >>

13. IF Test Example Private Sub cmdBlink_Click() If lblBox.Visible = True then lblBox.Visible = False Else lblBox.Visible = True End If End Sub Since lblBox.Visibleis boolean why not use If lblBox.Visible then

14. ElseIf Clause Ifcondition1Then statements to process when condition1 is true ElseIf condition2Then statements to process when condition1 is false and condition2is true Else statements to process when condition1 is false and condition2is false End If 'ElseIf is much like select-case

15. Nested Selection Structure • A nested selection structure is one in which either the true path or the false path includes yet another selection structure • Any of the statements within either the true or false path of one selection structure may be another selection structure

16. Nested If Example • Write an if statement that assigns a sales tax rate to the sngTax variable • The tax rate is determined by the state code stored in the intCode variable: • Codes of 1 and 3 represent a 4% rate • A code of 2 represents a 5% rate • All other codes represent a 2% rate

17. Nested If Example If intCode = 1 Or intCode = 3 Then sngTax = .04 Else If intCode = 2 Then sngTax = .05 Else sngTax = .02 End If End If

18. Nested If Example • Write a selection structure that assigns a bonus to the sngBonus variable • The bonus is determined by the salesperson’s code (intCode) and, in some cases, by the sales amount (sngSales) • If the code is 1 • and the salesperson sold at least $10,000, then the bonus is $500 • otherwise these salespeople receive $200 • If the code is 2 • and the salesperson sold at least $20,000, then the bonus is $600; • otherwise these salespeople receive $550 • All others receive $150

19. Nested If Example If intCode = 1 Then If sngSales >= 10000 Then sngBonus = 500 Else sngBonus = 200 End If ElseIf intCode = 2 Then If sngSales >= 20000 Then sngBonus = 600 Else sngBonus = 550 End If Else sngBonus = 150 End If

20. Nested If Example If intCode = 1 And sngSales >= 10000 Then sngBonus = 500 Else If intCode = 1 And sngSales < 10000 Then sngBonus = 200 Else If intCode = 2 And sngSales >= 20000 Then sngBonus = 600 Else If intCode = 2 And sngSales < 20000 Then sngBonus = 550 Else sngBonus = 150 End If End If End If End If

21. ElseIf Example If intCode = 1 And sngSales >= 10000 Then sngBonus = 500 ElseIf intCode = 1 And sngSales < 10000 Then sngBonus = 200 ElseIf intCode = 2 And sngSales >= 20000 Then sngBonus = 600 ElseIf intCode = 2 And sngSales < 20000 Then sngBonus = 550 Else sngBonus = 150 End If

22. The Message Box • VB allows the user to generate message boxes as a part of a user program. When invoked they look something like this:

23. Format of MsgBox • A simple example of the MsgBox function Dim intX As Integer intX = MsgBox("Read the Password")

24. The Details Dim intX As Integer intX = MsgBox("Do this now", _ vbCritical, "Prompting Issue")

25. The value of “intZ” above depends on which response is chosen. “Yes” - intZ = 6 “No” - intZ = 7 (the numerical values are defined by Visual Basic.) Response Choices Dim intZ As Integer intZ = MsgBox(“Should I Do This”, _ vbYesNo ,”Prompting Issue”)

26. Icon Choices vbExclamation vbQuestion vbCritical

27. Possible values for the 2nd argument