Review for Exam 2 School of Business Eastern Illinois University © Abdou Illia, Spring 2003 (Week 10, Friday 3/21/2003) - IF Blocks - Do Loops - Select Case Blocks

2 IF Blocks n Represent computers abilities to make decisions n Allow a program to decide on a course of action – Based on whether a certain condition is true or false n Simplest form (Single-alternative decision): If Condition Then Statement(s) End If Syntax If Credits < 10 Then Tuition = 100 * Credits End If Example n Condition is an expression that is evaluated by the computer n If Condition is true, Statement(s) are executed;otherwise Statement(s) are skipped

3 Nested IF Blocks n Nested IF Blocks (IF Blocks contained inside others): If condition1 Then [Statement(s)] If condition2 Then Statement(s) End If Syntax If Credits > 10 Then Tuition = 1000 If Credits > 15 Then Message = "Over load" End If Example n Note: In the nested If Blocks, each If must have its own End If statement CreditsTuitionMessage Fill in this table indicating the values of variables Tuition and Message for each value of Credits

4 Conditions n Expression involving: – Relational operators (, >=, ….) – Logical operators (And, Or, Not) letterGrade = txtGrade.Text If ( letterGrade = "A") Or (letterGrade = "B") Then picOutput.Print "Good Work" ElseIf (letterGrade = "C") Then picOutput.Print " Average Work" Else picOutput.Print "Poor Work" End If

5 Relational operators n Applied to numbers. Also applied to strings

6 Relational operators n Computer uses a special coding system to compare character strings n String1 is said to be less than String2 if String1 precede String2 alphabetically according to ANSI table ( See Appendix A ). "Chase" < "Chaz" True " Cat" < "Cat" True "Pay" < "Pay " True "Jones" <> "James" True "Hope" < "Hopeful" True Example: Comparing character strings

7 Logical operators n The three Logical Operators are: Not, And, Or n Not: Negates a single expression. Example: Suppose answer = "Y" Not (answer = "y") is true n And: Combines two expressions; each expression must be True for the entire expression to be True. Example: Suppose answer = "Y" (answer = "Y") And (answer = "y") is False n Or: Combines two expressions; either expression (or both expressions) must be True for the entire expression to be True. Example: Suppose answer = "Y" (answer = "Y") Or (answer = "y") is True

8 VB operators hierarchy n First, Arithmetic operations (^, *, /, +, -) are carried out n Second, Expressions involving relational operators (>, <, =, …) are evaluated to True or False n Third, Logical operators are applied in the order Not, And, Or. n In the event of a tie, the leftmost operator is applied. Note: Expressions in parentheses evaluated first

9 Determine whether the following conditions are true or false? n 1 <= 1 Answer: True n "car" < "cat" Answer: True (because alphabetically, "car" precedes "cat") n "Dog" < "dog" Answer: True (because Uppercase precedes lowercase) n ("Y" "X") And ( < ) Answer: False

10 Determine whether the following conditions are true or false? n (0 = 14) Or (6 ^ <= 4 / 2 + 8) Answer: False n Not (6 = 7) And ( 44 > 33) Answer: True In the next two cases, suppose that n = 4 n (2 < n) And (n < 6) Answer: True n (2 < n) Or (n = 6) Answer: true

11 Determine whether the following conditions are true or false? In the next cases, assume a = 2 and b = 3 n 3 * a = 2 * b Answer: True n (5 – a) * b < 7 Answer: False n b <= 3 Answer: True n a ^ b = b ^ a Answer: false

12 Determine whether the following conditions are true or false? In the next cases, assume a = 2 and b = 3 n a ^ (5 – 2) > 7 Answer: True n (a < b) Or (b < a) Answer: True n (a * a < b) Or Not (a * a < a) Answer: True n Not (a < b) Or Not (a < (b + a)) Answer: False

13 Determine the output displayed in the picture box when the command button is clicked. Private Sub cmdDisplay_Click() Dim a As Single a = 5 If 3 * a – 4 < 9 Then picOutput.Print "Remember, " End If picOutput.Print "Tomorrow is another day." End Sub Answer: Tomorrow is another day.

14 Determine the output displayed in the picture box when the command button is clicked. Private Sub cmdDisplay_Click() Dim Gpa As Single Gpa = 3.49 If Gpa >= 3.5 Then picOutput.Print "Honors"; End If picOutput.Print "Student" End Sub Answer: Student

15 Determine the output displayed in the picture box when the command button is clicked. Private Sub cmdDisplay_Click() Dim a As Single a = 5 If (a > 2) And (a = 3 Or a <7) Then picOutput.Print "Hi" End If End Sub Answer: Hi

16 Do Loops (using While) n Used to repeat a sequence of statements while a condition is True Do While Condition Statement(s) Loop Syntax 1 Private Sub cmdDisplay_Click() Dim Num As Integer ' Display the numbers from 1 to 10 Num = 1 Do While Num <= 10 picNumbers.Print Num; Num = Num + 1 Loop End Sub Example Do Statement(s) Loop While Condition Syntax 2 n Condition involve: – Relational operators (, >=, ….) – Logical operators (And, Or, Not) Same rules than Condition in IF Blocks (seen before Spring Break)

17 Do Loops (using Until) n Used to repeat a sequence of statements until a condition becomes True Do Statement(s) Loop Until condition Syntax 4 Private Sub cmdDisplay_Click() Dim Num As Integer ' Display the numbers from 1 to 10 Num = 1 Do Until Num > 10 picNumbers.Print Num; Num = Num + 1 Loop End Sub Example Do Until conditon Statement(s) Loop Syntax 3 n Note: To get the same result using While and Until, Conditions should be opposites. For instance, While Num 10

18 Do Loops n Used to display all or selected items from a file – Using EOF function Private Sub cmdDisplay_Click() Dim Name As String, Phone As String picPhones.Cls Open " A:\Phone.txt " For Input As #1 Do While Not EOF(1) Input #1, Name, Phone picPhones.Print Name, Phone Loop Close #1 End Sub Example " Bert ", " " " Ernie ", " " " Grover ", " " " Oscar ", " " Phone.txt

19 Replace each statement containing Until with an equivalent statement containing a While, and vice versa. n Loop Until AccountBalance >= 1200 Answer: Loop While AccountBalance < 1200 n Do While Amount = 100 Answer: Do Until Amount 100 n Do Until Name = "Bob" Answer: Do While Name "Bob" n Do While (a > 1) And (a < 3) Answer: Do Until (a = 3) n Do Until (Answer "Y") Or (n >= 7) Answer: Do While (Answer = "Y") And (n < 7)

20 Identify the errors n Private Sub cmdDisplay_Click() Dim Num As Integer ' Display the numbers from 1 to 10 Do While Num >= 10 Num = 1 picOutut.Print Num; Num = Num + 1 Loop End Sub Answer: Program never stops. Statement Num = 1 should appear before Do Loop n Private Sub cmdDisplay_Click() Dim q As Single q = 1 Do While q > 0 q = 3 * q - 1 picOutput.Print q; Loop End Sub Answer: Program never stops because q = 2 within the loop all time.

21 Identify the errors n Private Sub cmdDisplay_Click() Dim Num As Single Open “DATA.TXT” For Input As #1 Do While (Not EOF (1)) And (Num > 0) Input #1, Num picOutut.Print Num; Close #1 End Sub Note: Assume that DATA.TXT contains: 7, 6, 0, -1, 2 Answer: Loop statement is missing. Also, loop cannot be entered because the (default) value of Num is 0 n Private Sub cmdDisplay_Click() Dim Num As Integer ' Display numbers Do While 1 < Num < 5 Num = 1 picOutut.Print Num; Num = Num + 1 Loop End Sub Answer: Invalid condition. Condition should be (1 < Num) And (Num < 5)

22 Simple program using a Do loop n The world population reached 6 billion people in 1999 and was growing at the rate of 1.4 percent each year. Assuming that the population will continue to grow at the same rate, write a program to determine when the population will exceed 10 billion. Private Sub cmdCalculate_Click( ) Dim Year As Integer, Population As Single Year = 1999 Population = 6 Do Until Population > 10 Population = Population + (0.014 * Population) Year = Year + 1 Loop picOutput.Print " The answer is: " ; Year End Sub

23 For …Next n Repeats a group of statements a specified number of times n For…Next are used when we know exactly how many times a loop should be executed. Private Sub cmdDisplay_Click() Dim i As integer picTable.Cls For i = 1 To 5 picTable.Print i; i^2 Next i End Sub Example For counter = start To end Statement(s) Next [counter] Where: counter is a numeric variable used as a loop counter start is the initial value of counter end is the final value of counter Private Sub cmdDisplay_Click() Dim i As integer picTable.Cls i = 1 Do While i <= 5 picTable.Print i; i^2 i = i + 1 Loop End Sub Same example using Do loop

24 Select Case Blocks n Very efficient decision-making structures: – Simplifies choosing among several actions n Avoid using Nested IF and ElseIf statements n Actions based on the value of an expression called selector Select Case Selector Case ValueList1 Action1 Case ValueList2 Action2 : Case Else Action of last resort End Select Syntax Age = Val(txtAge.Text) Select Case Age Case 5 Category = "5-year old" Case 13 To 19 Category = "Teenager" Case Is > 65 Category = "Senior" Case Else Category = "Everyone else" End Select Example

25 Select Case Statements n Selectors can be: – Numeric or string variables. Example: Select Case FirstName – Expressions. Example: Select Case Left(EmployeeName, 1) Select Case Selector Case ValueList1 Action1 Case ValueList2 Action2 : Case Else Action of last resort End Select Syntax n ValueList items must be separated by commas n ValueList can be: – A constant – An expression – An inequality sign preceded by Is and followed by a constant, a variable, or an expression – A range expressed in the form a To b. x = 2 : y = 3 Num = Val(txtNumber.Text) Select Case Num Case y – x, x picOutput.Print "Buckle my shoes" End Select Grade = Val(txtBox.Text) Select Case Grade Case Is >= 90 picOutput.Print "A" End Select Grade = Val(txtBox.Text) Select Case Grade Case 60 To 89 picOutput.Print "Pass" End Select

26 Problem 1: Assigning letter grades (To be done in class) n Write a Click event procedure associated to the command button cmdShow that asks the user to type in an exam score and: – Assigns a letter grade based on the scale in the table below. The variable to be assigned the letter grade as value should be named Grade – Displays the score typed by the user and the letter grade in picBox. ScoreLetter grade A 80-89B 70-79C 60-69D 0-59F txtScore cmdShow picBox

27 Problem 1: Assigning letter grades Private Sub cmdShow_Click() Dim Score As Single Score = Val(txtScore.Text) Select Case Score Case 90 To 100 Grade = "A" Case 80 To 89 Grade = "B" Case 70 To 79 Grade = "C" Case 60 To 69 Grade = "D" Case 0 To 59 Grade = "F" End Select picBox.Print Score; Grade End Sub ScoreLetter grade A 80-89B 70-79C 60-69D 0-59F txtScore cmdShow picBox

28 Problem 2: IRS n IRS informants are paid cash rewards based on the value of the money recovered. n Write a Click event procedure associated to the command button cmdReward that asks the user to type in the Amount recovered and: – Determine the Reward based on the following rule: 10% for up to $75000 recovered, 15% for a recovered amount greater than but less than $100000; and 18% for more than $ – Displays the calculated reward in picReward. txtAmount cmdShow picReward

29 Problem 2: IRS Private Sub cmdShowReward_Click() Dim Amount As Single Amount = Val(txtAmount.Text) Select Case Amount Case Is <= Reward = 0.1 * Amount Case Is <= Reward = 0.15 * Amount Case Is > Reward = 0.18 * Amount End Select picReward.Print "The reward is "; Reward End Sub txtAmount cmdShow picReward

30 Identify the errors. Private Sub cmdDisplay_Click() Dim num As Single num = 2 Select Case num picOutput.Print "Two" End Select End Sub Answer: Case statement is missing.

31 Identify the errors. Private Sub cmdDisplay_Click() Dim num As Single num = 5 Select Case num Case 5, Is <>5 picOutput.Print "Five" Case Is > 5 picOutput.Print "Greater than 5" End Sub Answer: - First Case statement is executed every time, no matter the value of num - Should have an End Select.

32 Suppose the selector of a Select Case statement, word, evaluates to a string value. Determine whether the Case clause is valid. n Case Left("abc", 1) Answer: Valid n Case word "No" Answer: Invalid. Should be Case Is "No" n Case 0 To 9 Answer: Invalid (because 0 To 9 doesn’t refer to a string value) n Case "Hello", Is < "goodbye" Answer: Valid n Case "un" & "til" Answer: Valid (because "un" & "til" will give until)