Decisions and Conditions

1. Decisions and Conditions Control statements - seqeuncing - selection - repetition (picture later) - abstraction (procedure calls)

2. If-statements - a decision is made by the program - a given condition is either True or False Example If the street is blocked then take a detour Endif Example If English T122 is not full then sign up for English T122 Else check History T122 End If

3. True False street is blocked take a detour The flowchart for these examples True branch False branch (This is called a null branch)

4. True False Eng T122 is not full True branch False branch check on Hist T122 sign up for Eng T122

5. Syntax for flowcharts an individual instruction decision box if..then.. - general form If condition Then statement End If The statement can be any Visual Basic statement, including another If statement.

6. Example If a student’s GPA is 3.5 or more, write that they are on the Dean’s List. True False GPA >= 3.5 Write “Dean’s List” If sGpa >= 3.5 Then lbl.HonorMessage.Caption = “Dean’s List” End If

7. True False Grade = “P” Write “Pass” Write “Fail” if..then..else If condition Then statement 1 Else statement 2 End If Example A student takes a course on a Pass/Fail basis; a report prints “P” for pass and “F” for fail. If sGrade = “P” Then lblMessage.Caption =“Pass” Else lblMessage.Caption = “Fail” End If

8. Note: indentations are required for readability Definition A boolean expression is a statement which has the value True or False. Example sGPA >= 3.5 sGrade = “P” Another way to write the statement If sGpa >= 3.5 Then lbl.HonorMessage.Caption = “Dean’s List” End If is Dim bFlag As Boolean bFlag = (sGpa >= 3.5) If bFlag Then lbl.HonorMessage.Caption = “Dean’s List” End If

9. or If bFlag = True Then lbl.HonorMessage.Caption = “Dean’s List” End If The statement bFlag = (sGpa >= 3.5) places the value, either True or False, of the boolean expression sGpa >= 3.5 into the boolean variable bFlag. The advantage is that a boolean expression can be long and complicated. Using a flag will simplify the code.

10. False True Terminology A boolean expression or condition is also known as a predicate. Definition The valus of all predicates and variables in a program is called the state of the program. Note The state of the progrm is a description of what the program has done at any given moment. Flowcharting if-statements General If..Then General If..Then..Else False True

11. Note: Advantage of flowcharts - detail Disadvantage of flowcharts - detail Flowcharts are good for descrbing small pieces of code, not entire programs. Conditions Relational operators = < > < = (less than or equal) > = (greater than or equal) < > (not equal) Example Let iSal_1 = 10000, iSal_2 = 20000, iSal_3 = 30000. (a) iSal_1 = iSal_2 is False (b)iSal_2 < iSal_3 is True

12. Using if statements with option buttons and check boxes - use if-statements for option buttons and check buttons - put the code into the appropriate command button Example P. 108 Example P. 120 Private subCalculate_Click()) Nested if-statements Example P. 108, 109 If iTemp > 32 Then If iTemp > 80 Then lblComment.Caption = “Hot” Else lblComment.Caption = “Moderate” End If Else lblComment.Caption = “Freezing” End If

13. Use Elseif for several If-statements Example P. 109 If iTemp <= 32 Then lblComment.Caption = “Freezing” Elseif iTemp > 80 Then lblComment.Caption = “Hot” Else lblComment.Caption = “Moderate” End If Note Avoid too much nesting. This is cahracterized by “wide” flowcharts.

14. Example Find the largest of three numbers, A, B, C. First flowchart If iA < iB Then If iB < iC Then write iC Else write iB End If Else If iA < iC Then write iC Else write iA End If End If True False A < B True True False False A < C B < C Write A Write C Write B Write C

15. Max = A False True Max < B Max = B True Max<C Max = C Second flowchart The idea is to declare an extra variable iMax, which can simplify the flowchart considerably iMax = A If iMax< B Then iMax = B End If If iMax < C Then iMax = C End If write iMax In general, avoid wide flowcharts.

16. Comparing strings ASCII code P. 104 - 105 - all symbols, including numbers, are represented as symbols - strings are compared left to right (alphabetical order) Example “Bill” < “Hillary” “2” < “A” “A” < “a” Comparing the text property of text boxes P. 107 - values entered in text boxes are compared as strings - their data type is variant Example Suppose cSalFirst is 20000 and cSalSecond is 1000000. Then cSalFirst > cSalSecond is True val(cSalFirst) > val(cSalSecond) is False

17. Comparing uppercase and lowercase characters Example Let sName_1 be “Smith” and sName_2 be “jones”. Then (sName_1 > sName_2) is False because the ASCII valus of “S” is less than the ASCII value of “j”. Use ucase or lcase to change the words all to compare the uppercase or lowercase value of the words. Example ucase(sName_1) > ucase(sName_2) is True. Compound conditions The logical operators, in hierarchical order, are not, and, and or. Example (4 >= 5) and (8 = (3 + 5)) (4 >= 5) or (8 = (3 + 5)) not (- 4 > 0)

18. Hierarchical order of all operators ( ) NOT * / AND + - OR relational operators Example Suppose iNum holds the value 1. 0 < iNum and iNum < 2 gives a syntax error! The operator of highest precedence is AND. The compiler evaluates iNum AND iNum. But iNum is of type integer, so this expression is not well-defined. It should be written (0 < iNum) and (iNum < 2) which is true. See handout

19. Control arrays (Fig. 4.7 - 4.9) - list of controls with the same Name - elements of the list are distinguished by some number - syntax - give the first control a name - give the second control the same name - a message box will ask if you want a control array - select Yes - the Caption may be different - in code, refer to the elements of the control array by name and number Example P. 113

20. Example Message formatter The option buttons have Caption Red, Green, Blue, and Black. Suppose their names are all optColor. The code would be If optColor(0).Value = Checked Then lblMessage.Forecolor = vbRed Elseif optColor(1).Value = Checked Then lblMessage.Forecolor = vbGreen Elseif optColor(2).Value = Checked Then lblMessage.Forecolor = vbBlue Else lblMessage.Forecolor = vbBlack End If Finding the highest or lowest value previous

21. Input validation (for numeric data) Checking for correct data type Example P. 115 If isNumeric (txtQuantity.Text) Then lblDue.Caption = cPrice * val (txtQuantity) End If Checking for the range of values - the programmer checks that the date entered is reasonable Example Check that the number of hours per week do not exceed 168. If iHours > 168 Then some action Else cPay = iHours * cWage End If

22. Message boxes Definition A message box is a box within which the program displays a message to the user. (Fig. 4.10) The general form is MsgBox “string” [,buttons/icon][,“caption on title bar”] (Everything between the square brackets is optional.) The possible buttons/icons are vbOkOnly vbCritical vbQuestion vbExclamation vbInformation Example If iHours > 168 Then MsgBox “Please enter a numeric value”, vbOkOnly, “Error” Else cPay = iHours * cWage End If

23. Note: The exceptional case (the case that generates the error message) should come first, if possible. Example If not IsNumeric (txtQuantity.Text) Then MsgBox “Please enter a numeric value”, vbOkOnly, “Error” Else lblDue.Caption = cPrice * Val(txtQuantity.Text) End If This is better than writing what the book writes, which is If IsNumeric (txtQuantity.Text) Then lblDue.Caption = cPrice * Val(txtQuantity.Text) Else MsgBox “Please enter a numeric value”, vbOkOnly, “Error” End If

24. The message string To include long, formatted messages, do something like formatted string = Format(formatted string, etc.) string = formatted string & “xxxxxxxxxx” MsgBox formatted string, vbOkOnly, name of box Example Dim stFormattedString as String Dim stMessageString as String stFormattedString = Format$(iHighValue, “Standard”) stMessageString = “Your value is higher than” & stFormattedString MsgBox = stMessageString, vbCritical, “Value out of bounds”

25. To have a message extending over several lines, use - line feed - insert a new line - the code is Chr (10) - carriage return - place the cursor at the beginning of this new line - the code is Chr (13) Note: Chr is a method which takes an integer and returns the corresponding ASCII character (if any). Example Dim stNewline as String stNewLine = Chr(10) & Chr (13) Place stNewLine into a string when you want a new line in the message box.

26. Note: Chr is a method which takes an integer and returns the corresponding ASCII character, if any Example lblMessage.Caption = Chr (90) displays Z Note that Chr (500) is undefined, and causes an error. Debugging tools The Debug toolbar - select View - select Toolbar - select Debug Definition A break is a pause in the execution of the program. Forcing a break Method 1 - write Stop in the code

27. Method 2 - set a break using the debugger - set the cursor where you want the break to occur - select Toggle Breakpoint - remove the break SAME STEPS - remove all breaks - select Debug - select Remove all breaks The Immediate Window - select View and then select Immediate WIndow - allows the user to execute a single intruction - done during Break time - enter code (either type or copy it) - hit Return

28. Watch - examine a variable or expression during program execution -two methods for watches Method 1 The Watch Pane (Fig. 4.15) - this is set during design time - allows the user to examine values and predicates - select View and Watch Window - type in a variable or expression and select options -look at the window in Break time Method 2 The Instant Watch - under Code select View - highlight the variable or expression you wish to examine - under Debug select Instant Watch - Break the code (as before)

29. Stepping through code - execute one or more lines of code individually - Step into - Break the code - under Debug select Step Into