selection / decisions

CIS 115 Lecture 7

There are 3 control structures common to most computer languages that determine the flow, or path of execution, of the code:

Sequential

Selection / Decisions

Repetition / Looping

Visual Basic decision statements If…Then▪ one-way selection structure

If…Then…Else▪ two-way selection structure

If…Then…ElseIf▪ multi-way selection structure

If…Then…ElseIf…Else▪ multi-way selection structure

Select Case▪ multi-way selection structure

If…Then decision structureprovides one choice

Evaluate the condition: True or FalseEx: Is it cold outside?

True – execute codeEx: If yes, wear a coat

False – do not execute code Ex: If no,

Condition

ConditionalCode

True

False

If condition Thenstatement[s]

End If

Syntax explanation: If , Then, and End – Keywords Condition – True/False value, variable,

function call or expression Statement[s] – one or more code

statements to be executed if condition is true

The execution of an If block is controlled by a condition

Must be (or evaluate to) either true or false

Can be a value, variable, or function call (Boolean DataType)

Can be formed by using the six Relational operators and the three Logical operators

A flag is a Boolean variable that signals when some condition exists in the program

Since a Boolean variable is either True or False, it can be used as the condition of an If Note that an operator is not required

(there is alternate syntax that does use operator)

If blnQuotaMet Then lblMessage.Text = “Congratulations you have met your sales quota"

End If

Boolean Functions return a single True or False Value

Since Boolean Functions return either True or False, a Boolean Function Call can be used as the condition of an If Note that an operator is not required

(there is alternate syntax that does use operator)If isNumeric(strInput) Then

intNumber = Val(strInput)End If

Often a condition is formed using a relational operator

A relational operator determines if a specific relationship exists between two values > Greater than < Less than = Equal to <> Not equal to >= Greater than or equal to <= Less than or equal to

Relational operators are binary – meaning they use two operands Either or both relational operator operands may

be values, variables, expressions or function calls

length <= 10 (Is length less than or equal to 10)

len * wid > max + 1 (Is len * wid greater than max + 1)

Val(txtNum.Text) = 0 (Is Val result equal to 0 – not assignment)

Relational operators yield a True or False result

Either or both relational operator operands may be expressions Math operators are evaluated before relational

operators

x+y and a-b are evaluated first Each result is then compared using the > operator

Either or both relational operator operands may be function calls

If x + y > a - b ThenlblMessage.Text = "It is true!"

End If

If Val(txtInput.Text) < getMinValue() ThenlblMessage.Text = "Invalid: Below Minimum"

End If

These operators are used to evaluate boolean values and will yield a boolean result

And Both operands must be true for the overall expression to be

true, otherwise it is false

Or One or both operands must be true for the overall

expression to be true, otherwise it is false

Xor One operand (but not both) must be true for the overall

expression to be true, otherwise it is false

Not Reverses the logical value of an expression

The truth table for the And Operator

Expression 1 Expression 2 Expression 1 And Expression 2

True FalseFalseFalse True

FalseFalse False

FalseTrue True True

If temperature < 20 And minutes > 12 ThenlblMessage.Text = “Temperature is in the danger zone."

End If

AndAlso operator works identically but does not test minutes>12 if temperature<20 is false

If temperature < 20 Or temperature > 100 ThenlblMessage.Text = “Temperature is in the danger zone."

End If

OrElse operator works identically but does not test temperature>100 if temperature<20 is true

The truth table for the Or Operator

Expression 1 Expression 2 Expression 1 Or Expression 2

True False TrueFalse True

TrueTrue True TrueFalse False

False

If total > 1000 Xor average > 120 ThenlblMessage.Text = “You may try again."

End If

The truth table for the Xor Operator

Expression 1 Expression 2 Expression 1 Xor Expression 2

True False TrueFalse True

TrueTrue TrueFalseFalse False

False

If Not temperature > 100 ThenlblMessage.Text = "You are below the max temp."

End If

The truth table for the Not Operator

Expression 1 Not Expression 1

True FalseFalse True

Checking for a value inside a range uses And

Checking for a value outside a range uses Or

Must pay careful attention to differences in resulting range using: < vs <= or > vs >= Check problem requirements for ranges carefully

If x >= 20 And x <= 40 ThenlblMessage.Text = “Value is in the acceptable range."

End If

If x < 20 Or x > 40 ThenlblMessage.Text = “Value is outside the acceptable range."

End If

Logical operators have an order of precedence just as arithmetic operators do

From highest to lowest precedence Not And Or Xor

As with arithmetic operations, parentheses are often used to clarify order of operations

For example, in the statement If x < 0 And y > 100 Or z = 50 x < 0 And y > 100 is evaluated first If the And condition is true, we then evaluate True Or z = 50 If the And condition is false, we then

evaluate False Or z = 50

If the Or condition is to be evaluated first parentheses must be used If x < 0 And (y > 100 Or z = 50)

ParenthesisArithmetic

Exponential Multiplication / Division Integer Division MOD Addition / Subtraction

String ConcatenationRelational Operators (< , > , >= , <= ,

<>)Logical Operators

Not And Or, Xor)

Evaluate the following if a=5, b=7, x=100, y=30 If x > a * 10 And y < b + 20 Evaluating the math operators leaves us with If x > 50 And y < 27 Evaluating the relational operators leaves If True And False Evaluating the logical operators leaves False

Parentheses make order of operations clear If (x > (a * 10)) And (y < (b + 20))

If (intSales > 50000) ThenblnGetsBonus = True

End If-----------------------------------------------------------------If ((blnGetsBonus)Or((intMissedDays < 2)And(intSales > 30000)))

intDaysOff = intDaysOff + 1intEmpRating += 1

End If-----------------------------------------------------------------If (Not(isNumeric(txtInput.text))) Then

txtInput.text = “”MsgBox(“Please enter a number in the textbox”)

End If-----------------------------------------------------------------If (intGrade >= 80)And(intGrade < 90) Then lblMessage.text = “B” -----------------------------------------------------------------If ((Val(txtGrade.text) < 0)Or(Val(txtGrade.text) > 100)) Then

lblMessage.text = “Invalid Grade: Not in the range 0-100”End If

The If…Then construct will execute or ignore a group of statements (do something or do nothing)

The If…Then…Else construct will execute one group of statements or another group (do this or do that)

Condition

Statement(s)If True

True

False

Statement(s)If True

TrueFalse

Statement(s)If False

Condition

If…Then…Else provides two choices

Evaluate condition:True or False

True – execute codein If…Then block

False – execute codein Else Block

One of the two choices must be selectedThey are mutually exclusive

Condition

Statement(s)If True

TrueFalse

Statement(s)If False

If condition Thenstatement[s]1

Elsestatement[s]2

End If

Syntax explanation: If , Then, Else, and End – Keywords Condition – True/False value, variable, function call

or expression Statement[s]1 – executed if condition is True Statement[s]2 – executed if condition is False

If (intSales > 50000) ThenblnGetsDoubleBonus = TruedecBonus = 4000.00

ElsedecBonus = 2000.00

End If-----------------------------------------------------------------If (Not(isNumeric(txtInput.text))) Then

MsgBox(“You did not enter a valid number – program will end”)End

ElseintNumber = Val(txtInput.text)

End If-----------------------------------------------------------------If (intTemp >= 60)And(intTemp < 90)And(VisibRating() > 5) Then

lblMessage.text = “Go - Weather conditions are ideal” Else

lblMessage.text = “Wait - Weather conditions unacceptable”End If

If…Then…ElseIfallows for multiplemutually exclusivechoices

Each of the conditionsis tested in sequence

When a condition istrue, the correspondingcode is executed and theremaining conditions are ignored

C1

C2

C3

Statement(s)1

True

Statement(s)2

True

Statement(s)3

True

False

False

False

If it is very cold ThenWear a coat

Elseif it is chillyWear a light jacket

Elseif it is windyWear a windbreaker

Elseif it is hotWear no jacket

The order of the conditions is vital Wrong order can result in wrong decision What if it’s chilly and windy? If windy is tested before chilly, you’d go out

with a windbreaker when you need a jacket

If condition1 Thenstatement[s]1

ElseIf condition2

statement[s]2

• • •ElseIf conditionn

statement[s]n

End If

Syntax explanation: If , Then, ElseIf, and End –

Keywords Condition1 thru n – True/False

value, variable, functioncall or expression

Statement[s]1 – executed ifcondition1 is True

Statement[s]2 – executed if condition1 is False and if condition2 is True

Statement[s]n – executed if condition1 thru (n-1) is False and if conditionn is True

If sngAvg < 59.5 ThenlblGrade.Text = "F"

ElseIf sngAvg < 69.5 ThenlblGrade.Text = "D"

ElseIf sngAvg < 79.5 ThenlblGrade.Text = "C"

ElseIf sngAvg < 89.5 ThenlblGrade.Text = "B"

ElseIf sngAvg <= 100 ThenlblGrade.Text = "A"

End If

In each example, does the order of the conditions matter?

What happens if the order is reversed in each example?

If radCredCrd.checked ThenCredCrdPayment(decSubTot)

ElseIf radDebCrd.checked ThenDebCrdPayment(decSubTot)

ElseIf radCheck.checked ThenCheckPayment(decSubTot)

End If

If…Then…ElseIf…Else is simply anIf…Then…ElseIfwith an Else at the end

Called a Trailing Else If the initial If and none

of the ElseIf conditionsare True, the trailingElse statement(s) willbe executed

C1

C2

C3

Statement(s)1

True

Statement(s)2

True

Statement(s)3

True

False

False

False

Statement(s)Else

If condition1 Thenstatement[s]1

• • •ElseIf conditionn

statement[s]n

Else

statement[s]Else

End If

Syntax explanation: Same as If..Then…

ElseIfthru statement[s] n

Statement[s]Else – executed if condition1

thru n

(all previous conditions) are False

If sngAvg < 59.5 ThenlblGrade.Text = "F"

ElseIf sngAvg < 69.5 ThenlblGrade.Text = "D"

ElseIf sngAvg < 79.5 ThenlblGrade.Text = "C"

ElseIf sngAvg < 89.5 ThenlblGrade.Text = "B"

ElseIf sngAvg <= 100 ThenlblGrade.Text = "A“

ElselblGrade.Text = "Invalid"

End If

If intCredScr >= 700 ThenstrLoanType = “Prime”intLoanRate = 1

ElseIf intCredScr >= 600 ThenstrLoanType = “Standard”intLoanRate = 2

ElseIf intCredScr >= 500 ThenstrLoanType = “Risk”intLoanRate = 3

ElseIf intCredScr >= 400 ThenstrLoanType = “HiRisk”intLoanRate = 4

ElseMsgBox(“Not Qualified“)End

End If

If Statements Within If StatementsAny type of statement may be used

inside the statement(s) portion of any form of If

This includes other If statementsIf statements within If statements

create a more complex decision structure called a Nested If

A customer qualifies for a special rate loan if: If credit score is higher than 650 and▪ Income is more than 30000 Or Debt is less than 1000

Or If credit score is higher than 700

If intCredScr > 650 ThenIf decIncome > 30000 Then

lblMessage.Text = “qualified"ElseIf DecDebt < 1000

lblMessage.Text = “qualified“Else

lblMessage.Text = “not qualified“End If

ElseIf intCredScr > 700 ThenlblMessage.Text = “qualified"

ElselblMessage.Text = “not qualified“

End If

Similar to If…Then…ElseIf Performs a series of tests Conditionally executes the first true condition

Select Case is different in that: A single test expression may be evaluated The test expression is listed once The possible values of the expression are then

listed with their conditional statementsCase Else may be included and executed

if none of the values match the expression

Select Case Val(txtInput.Text)Case 1

MsgBox("Day 1 is Monday.")Case 2

MsgBox("Day 2 is Tuesday.")Case 3

MsgBox("Day 3 is Wednesday.")Case 4

MsgBox("Day 4 is Thursday.")Case 5

MsgBox("Day 5 is Friday.")Case 6

MsgBox("Day 6 is Saturday.")Case 7

MsgBox("Day 7 is Sunday.")Case Else

MsgBox("The value is invalid.")End Select

Select Case strAnimalCase "Dog“,"Cat"

MsgBox("House Pet")Case "Cow“,"Pig“,"Goat"

MsgBox("Farm Animal")Case "Lion“,"Tiger“,"Bear"

MsgBox("Oh My!")End Select

Select Case intScoreCase Is >= 90

strGrade = “A”Case 80 to 89

strGrade = “B”Case 70 to 79

strGrade = “C”Case 60 to 69

strGrade = “D”Case 0 to 59

strGrade = “F”End Select

Write a program that will prompt the user to input a number. Check for valid input. If the input is invalid (non-numeric) – give an error message via MsgBox and end the Event Procedure. If valid – assign the number to a variable and output the number to the user

Now experiment with validity checking for more restrictive input criteria with numbers (ex: only numbers from 1-100, only integers, only positive integers, etc.) and text (ex: only single characters, only the letters a-d, etc.)

Write a program that inputs 2 values and displays their positive difference. For example, if the first input is 6 and the second input is 9, then the positive difference is 3 (note: 3 is still the answer if the first input is 9 and the second input is 6).

Now add the code to handle invalid (non-numeric) input .

Write a VB application to have the user input via textbox an integer from 1 t0 100,000 (inclusive). Determine if the input is a valid. If invalid, give an error message, clear the textbox, and end the event procedure (discuss). If valid, use a boolean function to determine if the integer is even or odd and use an integer function to determine if the integer is a perfect square (return the root if yes, return -1 if no). Report your results via label.

Write a VB application to have the user input a 2 digit binary number via input box. Determine if the input is a valid 2 digit binary number. If not give a specific error message and terminate the app. If valid, convert the number to a decimal value (try using a function to do this) and report the results via message box. (try with 3 digits)

Lab 6 and Homework 6 Visual Basic – Decisions See handout for details and due date Questions?