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Week 2- Selection / Decisions HNDIT23073- Rapid Application Development.
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Transcript of Week 2- Selection / Decisions HNDIT23073- Rapid Application Development.
Week 2- Selection / Decisions
HNDIT23073- Rapid Application
Development
Control Structures
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
VB Decisions• 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• If…Then decision structure
provides 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
Condition
ConditionalCode
True
False
If…Then Statement SyntaxIf 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
Conditions
• 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
Boolean Variables• 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• 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) ThenintNumber = Val(strInput)
End If
Relational Operators
• 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 (cont.)• 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
Relational Operators (cont.)• 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
Logical OperatorsThese 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 And OperatorThe truth table for the And Operator
Expression 1 Expression 2 Expression 1 And Expression 2
True False FalseFalse 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
The Or Operator
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
The Xor Operator
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 True FalseFalse False False
The Not Operator
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
Example: Checking Numerical Ranges• 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
Precedence of Logical Operators
• 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
Relational & Logical Operators Combined
• 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)
All Operators Precedence• Parenthesis• Arithmetic– Exponential– Multiplication / Division– Integer Division– MOD– Addition / Subtraction
• String Concatenation• Relational Operators (< , > , >= , <= , <>)• Logical Operators – Not– And– Or, Xor)
Arithmetic, Relational, & Logical Operators Combined
• 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…Then ExamplesIf (intSales > 50000) Then
blnGetsBonus = TrueEnd 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
If…Then vs If…Then…ElseThe 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 Decision Structure• If…Then…Else
provides two choices• Evaluate condition:
True or False• True – execute code
in If…Then block• False – execute code
in Else Block • One of the two choices must be selected• They are mutually exclusive
Condition
Statement(s)If True
TrueFalse
Statement(s)If False
If…Then…Else SyntaxIf condition Then
statement[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…Then…Else ExamplesIf (intSales > 50000) Then
blnGetsDoubleBonus = 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…ElseIf Decision Structure• If…Then…ElseIf
allows 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…Then…ElseIf ConditionsIf it is very cold Then
Wear a coatElseif it is chilly
Wear a light jacketElseif it is windy
Wear a windbreakerElseif 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
If…Then…ElseIf SyntaxIf condition1 Then
statement[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, function– call or expression Statement[s]1 – executed 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…Then…ElseIf ExamplesIf 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
• 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 (Trailing Else)• 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…Then…ElseIf …Else SyntaxIf condition1 Then
statement[s]1
• • •
ElseIf conditionn
statement[s]n
Else
statement[s]Else
End If
Syntax explanation: Same as If..Then…ElseIf
thru statement[s] n
Statement[s]Else – executed if condition1 thru n
(all previous conditions) are False
If…Then…ElseIf…Else ExamplesIf 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 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
Nested 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
Nested If Example• 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
Select Case Statement• 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 Statement ExamplesSelect Case Val(txtInput.Text)
Case 1MsgBox("Day 1 is Monday.")
Case 2MsgBox("Day 2 is Tuesday.")
Case 3MsgBox("Day 3 is Wednesday.")
Case 4MsgBox("Day 4 is Thursday.")
Case 5MsgBox("Day 5 is Friday.")
Case 6MsgBox("Day 6 is Saturday.")
Case 7MsgBox("Day 7 is Sunday.")
Case ElseMsgBox("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
Example Decision ProblemsWrite 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.)
Example Decision ProblemsWrite 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 .
Example Decision ProblemsWrite a VB application to have the user input via textbox an integer from 1 to 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)
Reference
• http://www.cis.usouthal.edu,2014/03/10