Decisions with If Statements

Chapter 4

Chapter 4 2

Structures

Sequence Structure (so far) Each statement executed in sequence w/o

branching into other directions. Example:

Const dblRATE As Double = 0.15 Dim dblSubTotal As Double Dim dblDiscount As Double dblSubTotal = val(txtSubtotal.Text) dblDiscount = dblSubTotal * dblRATE lblDiscount.Text = FormatCurrency(dblDiscount)

Chapter 4 3

Structures, cont.

Decision Structure Algorithms often need to execute statements

only when a particular condition exists. Programmers write code that provides more

than one path of execution—based on particular circumstance.

Create a decision structure or tree to illustrate the flow.

Chapter 4 4

Logical Expressions

Alternative actions based on conditions of the situation.

Examples: Billing program causing overdue-payment

notice Payroll program to pay overtime Bonus based on annual performance

evaluation

Chapter 4 5

Condition, Action if True, Action if False

Condition Action if True Action if False

I am really, really hungry.

Eat at Tucano’s. Don’t eat anything.

It is 12:55 p.m. on Tuesday, February 10.

Go to BU 271 for your favorite VB class (of course).

Continue eating at Tucano’s.

Chapter 4 6

Flowchart

Condition to Test

Process ifCondition is True

Process ifCondition is False

True False

Chapter 4 7

Flowchart

Condition to Test

Process ifCondition is True

Process ifCondition is False

True False

Diamond:Yes/no question

Rectangle:Process that is conditionallyexecuted

Chapter 4 8

Example Flowchart

I am hungry.

Drive to Tucano’s Go to comp. lab.

True False

Eat!Do ISYS 1200Assignment.

Chapter 4 9

Pizza Flowchart

Create flowchart for #1 on p. 257.

Chapter 4 10

Executes one group of statements if the condition is true and another group of statements if the condition is false.

If condition Thenstatementblock1statementblock1

Elsestatementblock2statementblock2

End If Syntax Rules:

Start with If Include Then on same line as If End process with End If

If…Then…Else Statement

Only executed if condition is true

Only executed if condition is false

Chapter 4 11

Relational Operators Form Condition A testing expression that evaluates

as True or False. Relational operators determines

whether a specific relationship exists between two values. (See Tables 4.1 and 4.2).

Examples: Const intWEEK40 as Single =

40 Dim sngHours as Single sngHours > intWEEK40 sngHours <= intWEEK40 sngHours <> intWEEK40

Symbol Name

< Less than

<= Less than or equal to

> Greater than

>= Greater than or equal to

= Equal to

<> Not equal

Chapter 4 12

Relational Expression

Relational operators are binary. Require two operands dblLength > dblWidth intAge <= intMINOR

Relational expressions are evaluated as true or false only; they are not assignment statements. If x = y Then Determines if x and y are equal. If both operands are equal, then expression is

true.

Chapter 4 13

<=, >=, and <>

> or < must precede = No space between symbols Examples:

X >= Y X <= Y X <>Y

Chapter 4 14

Statement Block Processed if Condition is True Follows “Then” keyword. Completes series of statements until “Else” or “End If”

statement, whichever comes first. Const intMINRATE As Integer = 5

Dim intRating As IntegerintRating = val(txtValue.Text)If intRating > intMINRATE Then 'condition

dblBonus = dblSales * intHIGHRATE 'True processElse

dblBonus = dblSales * intLOWRATE

End If Processes True statement block code

and skips over the Else statement block.

Chapter 4 15

Statement Block Processed if Condition is False Follows the keyword “Else.” Completes series of statements until “End If” statement. Const intMINRATE As Integer = 5

Dim intRating As Integer intRating = val(txtValue.Text)If intRating > intMINRATE Then 'condition

dblBonus =dblSales * intHIGHRATE 'True process

ElsedblBonus = dblSales * intLOWRATE 'False process

End If Skips “True” statement block since false and processes

statement block after keyword “Else.”

Chapter 4 16

Standard Conventions

While many formats work, use the standard convention of indenting statement blocks to enhance readability (see p. 189).

If condition Thentrue process

Elsefalse process

End If

Chapter 4 17

Multiple Process Lines

If condition is true, continues processing all lines between “Then” and “Else.”

If condition is false, processes all lines between “Else” and “End If.”

If dblSales > 50000 ThenblnGetsBonus = TruedblCommissionRate = 0.12intDaysOff = intDaysOff + 1

End If

Chapter 4 18

Relational Operators and Math Operators Can combine within same expression. Math operators performed first. If intAge + intYears = 80 Then

lblMessage.Text = “You can retire.”End If

If intAge + intYears > 30+25 ThenlblMessage.Text = “You get a raise.”

End If

Chapter 4 19

Val () Function

If Val(txtInput.Text) > 100 ThenintBonus = 1000

End If Intrinsic Val function gets numeric value of

text box Text property. The returned value is compared to 100. If the returned value is > 100, the assignment

statement executes.

Chapter 4 20

Checkpoint

Do exercises on pp. 191-192

Chapter 4 21

Checkpoint (p. 195)

X = 0If x < 1 Then

y = 99Else

y = 0End If

X = 100If x <=1 Then

y = 99Else

y = 0End If

Expression is trueY = 99

Expression is falseY = 0

Chapter 4 22

Practice Exercise w/ Event Procedure

Get revenue and cost from a user. If the revenue is greater than the cost, calculate the profit and display it in a label with a string of text. Otherwise, calculate the loss and display the loss amount with a description in the label.

Chapter 4 23

Practice Solution

Dim dblRevenue as Currency ‘dim other vars also dblRevenue = val(txtRevenue.Text) dblCost = val(txtCost.Text) If dblRevenue > dblCost Then

dblProfit = dblRevenue – dblCostlblResult.Text = "The company had _

a profit of " & Profit & "."Else

dblLoss = dblCost – dblRevenuelblResult.Text = "The company had _

a loss of " & Loss & ".“End If

Chapter 4 24

Practice Examples

Tutorial 4-2 (pp. 193-195) Programming Challenge 1 (p. 258)

Chapter 4 25

Boolean Variable (p. 118 & pp. 190-191)

Stores True or False only.

Stores results of logical expression

Uses Boolean contents to complete decision process in If…Then…Else block

Programmers use Booleans to test conditions or set flags.

A flag is a Boolean variable that signals when some condition exists.

Chapter 4 26

Boolean Example

When flag is false, the condition does not exist. When flag is true, the condition exists.

Note: You do not need to use = True to compare the variable to true.

Chapter 4 27

Compound ConditionsUsing Logical Operators (pp. 206-211) Evaluates multiple conditions. Uses logical operators to connect 2 or more

relational expressions into one expression. And Or Xor Not

Chapter 4 28

And Operator

Connects 2 expressions. Requires that both expressions evaluate to

true. Identify job candidates with > 5 years AND

knows at least 3 programming languages If intYears > 5 And intLang >=3 Then If BOTH are true, follow THEN statement. If Years = 5 and Language = 3, follow ELSE

statement.

Chapter 4 29

And Truth TableJob Candidates

Experience and Language

Both conditions must be true to evaluate as True.

Test 1 (over 5 years experience)

True False

Test 2

Languages 3+

True True False

False False False

Chapter 4 30

Create And Example

Create the following: Experience (label caption) txtExperience (empty text box) Languages (label caption) txtLanguages (empty text box) Check Qualifications (button)

Chapter 4 31

And Logical Operator Example

Compare variable value to constant value using relational operator >. Use parentheses around each comparison (not required by assists with

readability). Type And logical operator to require that both relational expressions

(conditions) are met. Compare variable value to constant value using relational operator >=. Run with 5 & 3 (both conditions not met). Run with 6 & 3 (both conditions met).

Chapter 4 32

Boolean Example

Declare Boolean variable.

Sets Boolean flag to true ifboth conditions are met.

If trigger is true, do something.

Chapter 4 33

Or Operator

Combines 2 expressions. Requires that only one expression is true—

doesn’t matter which expression is true. Checks condition to left of Or operator.

If that condition is true, the whole expression is evaluated as true.

The program does not check the expression on the right side of the Or operator (to avoid wasting CPU time to check 2nd condition).

If condition on left side of Or operator is false, program must check 2nd condition.

Chapter 4 34

Or Truth Table

At least one of the expressions must be true; doesn’t matter which one.

Test 1

(over 5 years)

True False

Test 2

(3+ languages)

True True True

False True False

Chapter 4 35

Or Example

If Temp < 20 Or Temp > 100 ThenlblResult.Text = “Warning”

End If Doesn’t matter if temp is 19 or 101—either

one is true and will produce warning.

Chapter 4 36

Or Example Change And to Or

Run with 6 and 1 6 is > than 5 Program does not even check 2nd condition

Run with 4 and 3 4 is not > 5 (false condition) Program evaluates 2nd condition 3 >= 3 (true) Thus: entire expression is true

Chapter 4 37

Xor Operator

Exclusive Or Creates an expression that is true when one

(not both) of the subexpressions is true. If intYears > 5 Xor intNumJobs > 3 intYears = 6; intNumJobs = 3 True intYears = 5; intNumJobs = 4 True intYears = 6; intNumJobs = 4 False

Chapter 4 38

Xor Truth Table

ONLY one condition must be true; doesn’t matter which one; if both conditions are true, the entire expression is false

Test 1

(over 5 years)

True False

Test 2

(3+ languages)

True False True

False True False

Chapter 4 39

Not Truth Table

Opposite

Reverses the logical value of an expression: makes a true expression false and a false expression true.

Test 1 (only 1)

True False

False True

Chapter 4 40

Not Example

Reverses the truth If Not Age >= 21 Then

lblNote.Text = "You are underage _ and can’t buy alcoholic beverages. _Drink Wild Cherry Pepsi!"

ElselblNote.Text = "You are legal. Drink _whatever you want—in moderation."

End If

Chapter 4 41

Business Computer Proficiency

Students must earn 80% on each test. If Not dblScore >= dblPROFICIENT Then

lblOutput.Text = “Take the test again.”Else

lblOutput.Text = “Congratulations!”End If

If dblScore < dblPROFICIENT ThenlblOutput.Text = “Take the test again.”

ElselblOutput.Text = “Congratulations!”

End If Same results; note the difference in relational operators.

Chapter 4 42

Input Validation (p. 235)

Checking to verify that appropriate values have been entered in a text box. Data entered. Data is numeric. Specific value is entered. Check range of values. Positive value entered (instead of negative).

Necessary before performing calculations to avoid run-time errors.

Chapter 4 43

Validate for No Input (p. 213)

Make sure text box is not empty upon click command button.

Does not detect string of spaces because spaces are characters like “ABC.”

If txtYears.Text = "" Then'display error message'set focus'exit the event procedure

End If

Chapter 4 44

IsNumeric() Validation (p. 216-217)

Evaluates argument in parentheses. Returns True or False based on if argument is valid

number. Example:

IsNumeric(“12a”) returns false IsNumeric(txtAge.Text) returns true if numeric IsNumeric(txtAge.Text) returns false if not

numeric

Helps avoid Type Mismatch error at run time. Evaluates user input, can display MessageBox (error

message) if not valid number.

Chapter 4 45

IsNumeric() Example

If IsNumeric(txtQuantity.Text) ThenintQuantity = Val(txtQuantity.Text)dblAmountDue = intQuantity * dblPRICE

ElseMessageBox("Nonumeric data entered.", _

"Invalid Data“)'Select text in text box for user to retype.

End If Add input validation for numeric to existing

program example to test for numeric years & number of languages.

Chapter 4 46

Validate for Correct Range

If Val(txtHours.Text) < 50 And Val(txtHours.Text) > 10 Then'perform code

Else'display MessageBox error'select text box to re-enter dataExit Sub ‘exits rest of even procedure to avoid

‘performing calculationsEnd If

Chapter 4 47

Validating for Numeric and w/in a Certain Range Use Not IsNumeric as the first condition to

determine if the text box contents are not numeric values. Returns True if the text box does not contain numbers. Also used to return error message if text box I empty.

(Spaces count as characters though.) Create condition to test for range or to avoid negative

value as the second condition. Provide error message, set focus, and exit the event

procedure if data entry does not validate. Omit Else statement and continue statements after

End If to continue processing.

Chapter 4 48

Selecting Text Contents Upon Receiving Focus (p. 312) Select entire contents of text box upon

receiving focus. See Save As dialog box in PowerPoint.

Notice that entire contents of the File name text box are selected. White letters on a blue background

Chapter 4 49

Selecting Existing Text Box Contents

txtTextBoxName.SelectionStart = 0 Specifies text box name. SelectionStart property holds position of the

first selected character. In this case it starts with the first character (0 = first character).

txtTextBoxName.SelectionLength Holds the # of characters that are selected. If use txtName.SelectionLength = 5, then 5

characters are selected.

Chapter 4 50

Selecting Text Box Contents

txtName.Text.Length Gets the length of the text box content. Applies to other selection statements as well (see

string examples on p. 217). txtName.SelectionStart = 0

txtName.SelectionLength = txtName.Text.Length The above two statements identify & select the

entire text box contents (see p. 312). txtName.SelectAll

The above statement also selects the entire text box contents.

Use in a GotFocus event procedure.

Chapter 4 51

Nested If (p. 203)

If statement within another If statement. If condition in the main If expression is true,

then process an internal If statement. If condition in the main If expression is false,

then process a different internal If statement. Examine nested If statement in Tutorial 5

(pp. 203-205)

Chapter 4 52

Nested If

If condition ThenIf nestedcondition Then

perform actionsElse

perform other actionsEnd If

Elseperform action

End If

Chapter 4 53

Example

5% commission on sales under $10,000 10% commission on sales between

$10,000 and $20,000 15% commission on sales over $20,000 Create 3 controls

txtSales btnCalculate lblCommission

What do you need to declare? Write event procedure.

Chapter 4 54

Declarations & Variable Assignment

Before continuing to the next slide, think about how to structurethe nested If statement.

Use a flowchart if necessary.

Chapter 4 55

Nested If

Chapter 4 56

If…Then…ElseIf…Else (p. 196)

Another way to do nested If to create decision-making structures.

Sequence is critical because the order in which the rules are consulted.

Chapter 4 57

If…ElseIf…Else Example

Compare examples•2 lines shorter than nested Ifs (2 slides back)•Can only be ONE Else statement here (compared to 2 on nested If)•No ElseIf can follow an Else•Else must be last set of code; it catches any value that falls through the cracks.

•Edit program with this approach.•Review Tutorial 4-3 on pp. 196-202.

Chapter 4 58

More MessageBox Information

Multiple buttons, such as Yes and No

Determine action based on which button user clicks.

DialogResult.Abort DialogResult.OK

DialogResult.Retry DialogResult.Cancel

DialogResult.Ignore DialogResult.Yes

DialogResult.No

Chapter 4 59

Determine MessageBox Button Clicked Declare integer variable to store MessageBox

button.

Dim intResult as Integer intResult = MessageBox.Show(“Do you want to continue?”, _

“Please Confirm”, MessageBoxButtons.YesNo) If intResult = DialogResult.Yes Then

‘Perform an action hereElseIf intResult = DialogResult.No Then

‘Perform a different action hereEnd If