Section 5.2. If Blocks

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5.2. If Blocks

An If block allows a program to decide on a course of action based on whether certain conditions are true or false.

If Block

A block of the form:

If condition Then   action1 Else   action2 End If

causes the program to take action1 if condition is true and action2 if condition is false. Each action consists of one or more Visual Basic statements. After an action is taken, execution continues with the line after the If block. Figure 5.1 contains the pseudocode and flowchart for an If block.

Figure 5.1. Pseudocode and flowchart for an If block.

Example 1. (This item is displayed on pages 201 - 202 in the print version)

The following program finds the larger of two numbers input by the user. The condition is

num1 > num2

and each action consists of a single assignment statement. With the inputs 3 and 7, the condition is false, and so the second action is taken.

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Object	Property	Setting
frmMaximum	Text	Maximum
lblFirstNum	Text	First Number:
txtFirstNum
lblSecondNum	Text	Second Number:
txtSecondNum
btnFindLarger	Text	Find Larger Number
txtResult	ReadOnly	True

Private Sub btnFindLarger_Click(...) Handles btnFindLarger.Click   Dim num1, num2, largerNum As Double   num1 = CDbl (txtFirstNum.Text)   num2 = CDbl (txtSecondNum.Text)   If num1 > num2 Then     largerNum = num1   Else     largerNum = num2   EndIf   txtResult.Text = "The larger number is " & largerNum End Sub

[Run, type 3 and 7 into the text boxes, and press the button.]

Example 2. (This item is displayed on pages 202 - 203 in the print version)

The following program requests the costs and revenue for a company and displays the message "Break even" if the costs and revenue are equal; otherwise, it displays the profit or loss. The action following Else is another If block.

Object	Property	Setting
frmStatus	Text	Profit/Loss
lblCosts	Text	Costs:
txtCosts
lblRev	Text	Revenue:
txtRev
btnShow	Text	Show Financial Status
txtResult	ReadOnly	True

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Private Sub btnShow_Click(...) Handles btnShow.Click   Dim costs, revenue, profit, loss As Double   costs = CDbl (txtCosts.Text)   revenue = CDbl (txtRev.Text)   If costs = revenue Then     txtResult.Text = "Break even"   Else     If costs < revenue Then       profit = revenue - costs       txtResult.Text = "Profit is " & FormatCurrency(profit)     Else       loss = costs - revenue       txtResult.Text = "Loss is " & FormatCurrency(loss)     End If   End If End Sub

[Run, type 9500 and 8000 into the text boxes, and press the button.]

Example 3. (This item is displayed on pages 203 - 204 in the print version)

The If block in the following program has a logical operator in its condition.

Object	Property	Setting
frmQuiz	Text	A Quiz
lblQuestion	Text	How many gallons does a ten-gallon hat hold?
txtAnswer
btnEvaluate	Text	Evaluate Answer
txtSolution	ReadOnly	True

Private Sub btnEvaluate_Click(...) Handles btnEvaluate.Click   'Evaluate answer   Dim answer As Double   answer = CDbl (txtAnswer.Text)   If (answer >= 0.5) And (answer <= 1) Then     txtSolution.Text = "Good, "   Else 
[Page 204]    txtSolution.Text = "No, "   End If   txtSolution.Text &= "it holds about 3/4 of a gallon." End Sub

[Run, type 10 into the text box, and press the button.]

The Else part of an If block can be omitted. This important type of If block appears in the next example.

Example 4. (This item is displayed on pages 204 - 205 in the print version)

The following program offers assistance to the user before presenting a quotation.

Object	Property	Setting
frmQuotation	Text	Quotation
lblQuestion	Text	Do you know what the game of skittles is (Y/N)?
mtxtAnswer	Mask	L
btnDisplay	Text	Display Quotation
txtQuote	ReadOnly	True

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim message As String   message = "Skittles is an old form of bowling in which a wooden " & _              "disk is used to knock down nine pins arranged in a square."   If mtxtAnswer.Text.ToUpper = "N" Then     MsgBox(message, 0, "")   End If   txtQuote.Text =  "Life ain't all beer and skittles." & _                   " - Du Maurier (1894)" End Sub

[Run, type "N" into the masked text box, and press the button.]

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[Press OK.]

Note: Rerun the program, type "Y" into the masked text box, press the button, and observe that the description of the game is skipped.

ElseIf Clauses

An extension of the If block allows for more than two possible alternatives with the inclusion of ElseIf clauses. A typical block of this type is

If condition1 Then   action1 ElseIf condition2 Then   action2 ElseIf condition3 Then   action3 Else   action4 End If

This block searches for the first true condition, carries out its action, and then skips to the statement following End If. If none of the conditions is true, then Else's action is carried out. Execution then continues with the statement following the block. In general, an If block can contain any number of ElseIf clauses. As before, the Else clause is optional.

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Example 5.

The following program redoes Example 1 so that if the two numbers are equal, the program so reports:

Private Sub btnFindLarger_Click(...) Handles btnFindLarger.Click   Dim num1, num2 As Double   num1 = CDbl (txtFirstNum.Text)   num2 = CDbl (txtSecondNum.Text)   If (num1 > num2) Then      txtResult.Text = "The larger number is " & num1   ElseIf (num2 > num1) Then      txtResult.Text = "The larger number is " & num2   Else      txtResult.Text = "The two numbers are equal."   End If End Sub

[Run, type 7 into both text boxes, and press the button.]

If blocks allow us to define functions whose values are not determined by a simple formula. The function in Example 6 uses an If block.

Example 6. (This item is displayed on pages 206 - 207 in the print version)

The Social Security or FICA tax has two componentsthe Social Security benefits tax, which in 2005 is 6.2 percent on the first $90,000 of earnings for the year, and the Medicare tax, which is 1.45 percent of earnings. The following program calculates an employee's FICA tax for the current pay period.

Object	Property	Setting
frm FICA	Text	FICA Taxes
lblToDate	Text	Total earnings for this year prior to the current pay period:
txtToDate
lblCurrent	Text	Earnings for the current pay period:
txtCurrent
btnCalculate	Text	Calculate FICA Taxes
lblTax	Text	FICA taxes for the current pay period:
txtTax	ReadOnly	True

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Private Sub btnCalculate_Click(...) Handles btnCalculate.Click   Dim ficaTaxes As Double   ficaTaxes = CalcFICA(CDbl (txtToDate.Text), CDbl (txtCurrent.Text))   txtTax.Text = FormatCurrency(ficaTaxes) End Sub Function CalcFICA(ByVal ytdEarnings As Double, _                    ByVal curEarnings As Double) As Double   'Calculate Social Security benefits tax and Medicare tax   'for a single pay period in 2005   Dim socialSecurityBenTax, medicareTax As Double   If (ytdEarnings + curEarnings) <= 90000 Then     socialSecurityBenTax = 0.062 * curEarnings   ElseIf ytdEarnings < 90000 Then     socialSecurityBenTax = 0.062 * (90000 - ytdEarnings)   End If   medicareTax = 0.0145 * curEarnings   Return socialSecurityBenTax + medicareTax End Function

[Run, type 12345.67 and 543.21 into the top two text boxes, and press the button. The following is displayed in txtTax.]

$41.56

Comments

Constructs in which an If block is contained inside another If block are referred to as nested If blocks.
Care should be taken to make If blocks easy to understand. For instance, in Figure 5.2, the block on the left is difficult to follow and should be replaced by the clearer block on the right.
Figure 5.2. A confusing If block and an improvement.
If cond1 Then If cond2 Then action End If End If
If cond1 And cond2 Then action End If
In Appendix C, the section "Stepping through Programs Containing Decision Structures: Chapter 5" uses the Visual Basic debugging tools to trace the flow through an If block.
Some programs call for selecting among many possibilities. Although such tasks can be accomplished with complicated If blocks, the Select Case block (discussed in the next section) is often a better alternative.

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Practice Problems 5.2

Suppose the user is asked to input a number into txtNumber for which the square root is to be taken. Fill in the If block so that the lines of code that follow will display either the message "Number can't be negative." or the square root of the number.

Private Sub btnSqrt_Click(...) Handles btnSqrt.Click   'Check reasonableness of data   Dim num As Double   num = CDbl(txtNumber.Text)   If   End If End Sub

Improve the block

If a < b Then   If c < 5 Then     txtBox.Text = "hello"   End If End If

Exercises 5.2

In Exercises 1 through 12, determine the output displayed in the text box when the button is clicked.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim num As Double = 4   If num <= 9 Then     txtOutput.Text = "Less than ten."   Else     If num = 4 Then       txtOutput.Text = "Equal to four."     End If   End If End Sub

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim gpa As Double = 3.49   txtOutput.Clear()   If gpa >= 3.5 Then     txtOutput.Text = "Honors "   End If  txtOutput.Text &= "Student" End Sub

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Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim a As Double = 5   txtOutput.Clear()   If (3 * a - 4) < 9 Then     txtOutput.Text = "Remember, "   End If   txtOutput.Text &= "tomorrow is another day." End Sub

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim change As Double = 356    'Amount of change in cents   If change >= 100 Then     txtOutput.Text = "Your change contains "  & _                      Int(change / 100) & " dollars."   Else     txtOutput.Text = "Your change contains no dollars."   End If End Sub

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim a as Double = 2   Dim b As Double = 3   Dim c As Double = 5   If a * b < c Then     b = 7   Else     b = c * a   End If   txtOutput.Text = CStr(b) End Sub

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim a, b As Double   a = CDbl(InputBox("Enter a number."))   b = CDbl(InputBox("Enter another number."))   If a > b Then     a += 1   Else     b += 1   End If   txtOutput.Text = a & "  "& b End Sub

(Assume the responses are 7 and 11.)

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   'Cost of phone call from New York to London   Dim length As Double   InputLength(length)   DisplayCost(length) End Sub 
[Page 210]Function Cost(ByVal length As Double) As Double   If length < 1 Then     Return .46   Else     Return .46 + (length - 1) * .36   End If End Function Sub DisplayCost(ByVal length As Double)   'Display the cost of a call   txtBox.Text = "Cost of call: "& FormatCurrency(Cost(length)) End Sub Sub InputLength(ByRef length As Double)   'Request the length of a phone call   length = CDbl(InputBox("Duration of the call in minutes?")) End Sub

(Assume the response is 31.)

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim letter As String   letter = InputBox("Enter A, B, or C.")   If letter = "A" Then     DisplayAmessage()   ElseIf letter = "B" Then     DisplayBmessage()   ElseIf letter = "C" Then     DisplayCmessage()   Else     txtOutput.Text = "Not a valid letter."   End If End Sub Sub DisplayAmessage()   txtOutput.Text = "A, my name is Alice." End Sub Sub DisplayBmessage()   txtOutput.Text = "To be, or not to be." End Sub Sub DisplayCmessage()   txtOutput.Text = "Oh, say, can you see." End Sub

(Assume the response is B.)

 Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim vowels As Integer    'Number of vowels   ExamineLetter(vowels) 
[Page 211]  ExamineLetter(vowels)   ExamineLetter(vowels)   txtOutput.Text = "The number of vowels is "& vowels End Sub Sub ExamineLetter(ByRef vowels As Integer)   Dim ltr As String   ltr = InputBox("Enter a letter.")   ltr = ltr.ToUpper   If (ltr = "A") Or (ltr = "E") Or (ltr = "I") Or _     (ltr = "O") Or (ltr = "U") Then     vowels += 1   End If End Sub

(Assume the three responses are U, b, and a.)

10.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim a As Double = 5   If (a > 2) And ((a = 3) Or (a < 7)) Then     txtOutput.Text = "Hi"   End If End Sub

11.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim num As Double = 5   If num < 0 Then     txtOutput.Text = "neg"   Else     If num = 0 Then       txtOutput.Text = "zero"     Else       txtOutput.Text = "positive"     End If   End If End Sub

12.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim msg As String, age As Integer   msg = "You are eligible to vote"   age = CInt(InputBox("Enter your age."))   If age >= 18 Then     txtOutput.Text = msg   Else     txtOutput.Text = msg & " in "& (18 - age) &  " years."   End If End Sub

(Assume the response is 16.)

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In Exercises 13 through 20, identify the errors.

13.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim num As Double = 0.5   If (1 < num < 3) Then     txtOutput.Text = "Number is between 1 and 3."   End If End Sub

14.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim num As Double = 6   If num > 5 And < 9 Then     txtOutput.Text = "Yes"   Else     txtOutput.Text = "No"   End If End Sub

15.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   If (2 <> 3)     txtOutput.Text = "Numbers are not equal"   End If End Sub

16.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim major As String   major = "Computer Science"   If major = "Business" Or "Computer Science" Then     txtOutput.Text = "Yes"   End If End Sub

17.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   Dim numName As String, num As Double   numName = "Seven"   num = CDbl(InputBox("Enter a number."))   If num < numName Then     txtOutput.Text = "Less than"   Else     txtOutput.Text = "Greater than"   End If End Sub

18.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   'Change switch from "on" to "off", or from "off" to "on"   Dim switch As String   switch = InputBox("Enter On or Off.")   If switch = "Off" Then     switch = "On"   End If 
[Page 213]  If switch = "On" Then     switch = "Off"   End If   txtOutput.Text = switch End Sub

19.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   'Display "OK" if either j or k equals 4   Dim j As Double = 2   Dim k As Double = 3   If j Or k = 4 Then     txtOutput.Text = "OK"   End If End Sub

20.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click   'Is your program correct?   Dim query, answer1, answer2 As String   query = "Are you sure everything in your program is correct?"   answer1 = InputBox(query)   answer1 = answer1.ToUpper.Substring(0, 1)   If answer1 = "N" Then     txtOutput.Text = "Don't patch bad code, rewrite it."   Else     query = "Does your program run correctly?"     answer2 = InputBox(query)     answer2 = answer2.ToUpper.Substring(0, 1)     If answer2 = "Y" Then       txtOutput.Text = "Congratulations."     Else       txtOutput.Text = "Something you are sure about is wrong."     End If End Sub

In Exercises 21 through 26, simplify the code.

21.

If (a = 2) Then   a = 3 + a Else   a = 5 End If

22.

If Not (answer <> "y") Then   txtOutput.Text = "YES" Else   If (answer = "y") Or (answer = "Y") Then   txtOutput.Text = "YES"   End If End If

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23.

If (j = 7) Then   b = 1 Else   If (j <> 7) Then     b = 2   End If End If

24.

If (a < b) Then   If (b < c) Then     txtOutput.Text = b & " is between "& a & " and "& c   End If End If

25.

message = "Is Alaska bigger than Texas and California combined?" answer = InputBox(message) If (answer.Substring(0, 1) = "Y") Then   answer = "YES" End If If (answer.Substring(0, 1) = "y") Then   answer = "YES" End If If (answer = "YES") Then   txtOutput.Text = "Correct" Else   txtOutput.Text = "Wrong" End If

26.

message = "How tall (in feet) is the Statue of Liberty?" feet = CDbl(InputBox(message)) If (feet <= 141) Then   lstOutput.Items.Add("Nope") End If If (feet > 141) Then   If (feet < 161) Then     lstOutput.Items.Add("Close")   Else     lstOutput.Items.Add("Nope")   End If End If sltOutput.Items.Add("The statue is 151 feet from base to torch.")

27.

Write a program to determine how much to tip the server in a restaurant. The tip should be 15 percent of the check, with a minimum of $1.

28.

A bagel shop charges 75 cents per bagel for orders of less than a half-dozen bagels and charges 60 cents per bagel for orders of a half-dozen or more bagels. Write a program that requests the number of bagels ordered and displays the total cost. (Test the program for orders of four bagels and a dozen bagels.)

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29.

A computer store sells diskettes at 25 cents each for small orders or at 20 cents each for orders of 100 diskettes or more. Write a program that requests the number of diskettes ordered and displays the total cost. (Test the program for purchases of 5 and 200 diskettes.)

30.

A copy center charges 5 cents per copy for the first 100 copies and 3 cents per copy for each additional copy. Write a program that requests the number of copies as input and displays the total cost. (Test the program with the quantities 25 and 125.)

31.

Write a quiz program to ask "Who was the first Ronald McDonald?" The program should display "Correct." if the answer is "Willard Scott" and "Nice try" for any other answer.

32.

Suppose a program has a button with the caption "Quit." Suppose also that the Name property of this button is btnQuit. Write a btnQuit_Click event procedure that gives the user a second chance before ending the program. The procedure should use an input box to request that the user confirm that the program should be terminated, and then end the program only if the user responds in the affirmative.

33.

Write a program to handle a savings-account withdrawal. The program should request the current balance and the amount of the withdrawal as input and then display the new balance. If the withdrawal is greater than the original balance, the program should display "Withdrawal denied." If the new balance is less than $150, the message "Balance below $150" should be displayed.

34.

Write a program that requests three scores as input and displays the average of the two highest scores. The input and output should be handled by Sub procedures, and the average should be determined by a user-defined function.

35.

A grocery store sells apples for 79 cents per pound. Write a cashier's program that requests the number of pounds and the amount of cash tendered as input and displays the change from the transaction. If the cash is not enough, the message "I need $x.xx more." should be displayed, where $x.xx is the difference between the cash and the total cost. (Test the program with six pounds and $5, and four pounds and $3.)

36.

Federal law requires hourly employees be paid "time-and-a-half" for work in excess of 40 hours in a week. For example, if a person's hourly wage is $8 and he works 60 hours in a week, his gross pay should be

(40 x 8) + (1.5 x 8 x (60 - 40)) = $560

Write a program that requests as input the number of hours a person works in a given week and his hourly wage, and then displays his gross pay.

37.

Write a program that requests a word (with lowercase letters) as input and translates the word into pig latin. The rules for translating a word into pig latin are as follows:

If the word begins with a consonant, move the first letter to the end of the word and add ay. For instance, chip becomes hipcay.
If the word begins with a vowel, add way to the end of the word. For instance, else becomes elseway.

38.

The current calendar, called the Gregorian calendar, was introduced in 1582. Every year divisible by four was declared to be a leap year, with the exception of the years ending in 00 (that is, those divisible by 100) and not divisible by 400. For instance, the years 1600 and 2000 are leap years, but 1700, 1800, and 1900 are not. Write a program that requests a year as input and states whether it is a leap year. (Test the program on the years 1994, 1995, 1900, and 2000.)

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39.

Create a form with a text box and two buttons captioned Bogart and Raines. When Bogart is first pressed, the sentence "I came to Casablanca for the waters." is displayed in the text box. The next time Bogart is pressed, the sentence "I was misinformed." is displayed. When Raines is pressed, the sentence "But we're in the middle of the desert." is displayed. Run the program and then press Bogart, Raines, and Bogart to obtain a dialogue.

40.

Write a program that allows the user to use a button to toggle the color of the text in a text box between black and red.

41.

Write a program that allows the user ten tries to answer the question, "Which U.S. President was born on July 4?" After three incorrect guesses, the program should display the hint, "He once said, 'If you don't say anything, you won't be called upon to repeat it."' in a message box. After seven incorrect guesses, the program should give the hint, "His nickname was 'Silent Cal."' The number of guesses should be displayed in a text box. Note: Calvin Coolidge was born on July 4, 1872.

42.

Write a program that reads a test score from a text box each time a button is clicked and then displays the two highest scores whenever a second button is clicked. Use two class-level variables to track the two highest scores.

43.

The flowchart in Figure 5.3 (on the next page) calculates New Jersey state income tax. Write a program corresponding to the flowchart. (Test the program with taxable incomes of $15,000, $30,000, and $60,000.)

Figure 5.3. Flowchart for New Jersey state income tax program (This item is displayed on page 217 in the print version)

44.

Write a program to play "Hide and Seek" with the name of our programming language. When the button is pressed, the name should disappear and the caption on the button should change to "Show Name of Language." The next time the button is pressed, the name should reappear and the caption should revert to "Hide Name of Language," and so on.

Object	Property	Setting
frmHideSeek	Text	Hide and Seek
lblLanguage	Text	VB 2005
	Font.Size	26
btnDisplay	Text	Hide Name of Language

Solutions to Practice Problems 5.2

If (num < 0) Then    MsgBox("Number can't be negative.", 0, "Input Error")    txtNumber.Clear()    txtNumber.Focus() Else    txtSquareRoot.Text = CStr(Math.Sqrt(num)) End If

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The word "hello" will be displayed when (a < b)is true and (c < 5)is also true. That is, it will be displayed when both of these two conditions are true. The clearest way to write the block is

If (a < b) And (c < 5) Then   txtBox.Text = "hello" End If