Transcript Procedures - USA - School of Computing

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 structure
provides one choice
Evaluate the condition:
True or False
Ex: Is it cold outside?

True – execute code
Ex: If yes, wear a coat

False – do not execute code
Ex: If no,
True
Condition
False
Conditional
Code
If condition Then
statement[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
len * wid > max + 1
Val(txtNum.Text) = 0

(Is length less than or equal to 10)
(Is len * wid greater than max + 1)
(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
If x + y > a - b Then
lblMessage.Text = "It is true!"
End If
 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 Val(txtInput.Text) < getMinValue() Then
lblMessage.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
True
False
False
True
False
True
False
True
Expression 1 And Expression 2
False
False
False
True
If temperature < 20 And minutes > 12 Then
lblMessage.Text = “Temperature is in the danger zone."
End If
AndAlso operator works identically but does not test minutes>12 if
temperature<20 is false
The truth table for the Or Operator
Expression 1
Expression 2
True
False
True
False
False
True
True
False
Expression 1 Or Expression 2
True
True
True
False
If temperature < 20 Or temperature > 100 Then
lblMessage.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 Xor Operator
Expression 1
Expression 2
True
False
True
False
False
True
True
False
Expression 1 Xor Expression 2
True
True
False
False
If total > 1000 Xor average > 120 Then
lblMessage.Text = “You may try again."
End If
The truth table for the Not Operator
Expression 1
Not Expression 1
True
False
False
True
If Not temperature > 100 Then
lblMessage.Text = "You are below the max temp."
End If

Checking for a value inside a range uses And
If x >= 20 And x <= 40 Then
lblMessage.Text = “Value is in the acceptable range."
End If

Checking for a value outside a range uses Or
If x < 20 Or x > 40 Then
lblMessage.Text = “Value is outside the acceptable range."
End If

Must pay careful attention to differences in
resulting range using: < vs <= or > vs >=
 Check problem requirements for ranges carefully


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)
Parenthesis
 Arithmetic






Exponential
Multiplication / Division
Integer Division
MOD
Addition / Subtraction
String Concatenation
Relational 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) Then
blnGetsBonus = True
End If
----------------------------------------------------------------If ((blnGetsBonus)Or((intMissedDays < 2)And(intSales > 30000)))
intDaysOff = intDaysOff + 1
intEmpRating += 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)
False
True
Condition
Statement(s)
If False
Statement(s)
If True
True
Condition
False
Statement(s)
If True
The If…Then…Else
construct will execute one
group of statements or
another group (do this or do
that)






If…Then…Else
False
provides two choices
Condition
Evaluate condition:
True or False
True – execute code Statement(s)
If False
in If…Then block
False – execute code
in Else Block
One of the two choices must be selected
They are mutually exclusive
True
Statement(s)
If True
If condition Then
statement[s]1
Else
statement[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) Then
blnGetsDoubleBonus = True
decBonus = 4000.00
Else
decBonus = 2000.00
End If
----------------------------------------------------------------If (Not(isNumeric(txtInput.text))) Then
MsgBox(“You did not enter a valid number – program will end”)
End
Else
intNumber = 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…ElseIf
True
C
allows for multiple
False
mutually exclusive
True
C
choices
Each of the conditions False
True
is tested in sequence
C
False
When a condition is
true, the corresponding
code is executed and the
remaining conditions are ignored
1
Statement(s)1
2
Statement(s)2
3
Statement(s)3
If it is very cold Then
Wear a coat
Elseif it is chilly
Wear a light jacket
Elseif it is windy
Wear a windbreaker
Elseif it is hot
Wear 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

Syntax explanation:
 If , Then, ElseIf, and End –




If condition1 Then
statement[s]1
ElseIf condition2
statement[s]2
Keywords
Condition1 thru n – True/False
•••
value, variable, function
ElseIf conditionn
call or expression
statement[s]n
Statement[s]1 – executed if
End If
condition1 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 Then
lblGrade.Text = "F"
ElseIf sngAvg < 69.5 Then
lblGrade.Text = "D"
ElseIf sngAvg < 79.5 Then
lblGrade.Text = "C"
ElseIf sngAvg < 89.5 Then
lblGrade.Text = "B"
ElseIf sngAvg <= 100 Then
lblGrade.Text = "A"
End If


If radCredCrd.checked Then
CredCrdPayment(decSubTot)
ElseIf radDebCrd.checked Then
DebCrdPayment(decSubTot)
ElseIf radCheck.checked Then
CheckPayment(decSubTot)
End If
In each example, does the order of the conditions matter?
What happens if the order is reversed in each example?



If…Then…ElseIf…
Else is simply an
If…Then…ElseIf
with an Else at the end
Called a Trailing Else
If the initial If and none
of the ElseIf conditions
are True, the trailing
Else statement(s) will
be executed
C1
True
Statement(s)1
False
True
C2
Statement(s)2
False
True
C3
False
Statement(s)Else
Statement(s)3

Syntax explanation:
 Same as If..Then…ElseIf
If condition1 Then
statement[s]1
•••
thru statement[s] n
ElseIf conditionn
 Statement[s]Else –
statement[s]n
executed if condition1 thru n
Else
(all previous conditions)
statement[s]Else
are False
End If
If sngAvg < 59.5 Then
lblGrade.Text = "F"
ElseIf sngAvg < 69.5 Then
lblGrade.Text = "D"
ElseIf sngAvg < 79.5 Then
lblGrade.Text = "C"
ElseIf sngAvg < 89.5 Then
lblGrade.Text = "B"
ElseIf sngAvg <= 100 Then
lblGrade.Text = "A“
Else
lblGrade.Text = "Invalid"
End If
If intCredScr >= 700 Then
strLoanType = “Prime”
intLoanRate = 1
ElseIf intCredScr >= 600 Then
strLoanType = “Standard”
intLoanRate = 2
ElseIf intCredScr >= 500 Then
strLoanType = “Risk”
intLoanRate = 3
ElseIf intCredScr >= 400 Then
strLoanType = “HiRisk”
intLoanRate = 4
Else
MsgBox(“Not Qualified“)
End
End If




If Statements Within If Statements
Any type of statement may be used inside
the statement(s) portion of any form of If
This includes other If statements
If 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 Then
If decIncome > 30000 Then
lblMessage.Text = “qualified"
ElseIf DecDebt < 1000
lblMessage.Text = “qualified“
Else
lblMessage.Text = “not qualified“
End If
ElseIf intCredScr > 700 Then
lblMessage.Text = “qualified"
Else
lblMessage.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 statements

Case 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 strAnimal
Case "Dog“,"Cat"
MsgBox("House Pet")
Case "Cow“,"Pig“,"Goat"
MsgBox("Farm Animal")
Case "Lion“,"Tiger“,"Bear"
MsgBox("Oh My!")
End Select
Select Case intScore
Case 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 (nonnumeric) – 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?