Lesson 6

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The Decision Logic Structure The If Instructions Using Straight-through Logic Using Positive Logic Using Negative Logic Logic Conversion Which Decision Logic? Decision Tables

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Decision◦ True/False/Else

Process Assign

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Assign

Process

Decision

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Snoozeor Off?

Alarm goes off!

Sleep or Get up?

Zzzzz!

Snooze

Go Back to Sleep

Get Dressed. . .

Snooze= 9?

< 9

Oh, no!

Allows us to ask questions of our data. Similar to the way we think.

◦ We make decisions every day. Have two parts:

◦ recognition of what action to take (you have to get out of bed) and

◦execution of that action (you actually jump up out of bed)

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The If Statement◦ The Single Selection If/Then◦ The Double If/Then/Else◦ The Multiple If/Then/Else

The Case Statement The Switch Statement

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Straight-through Logic◦ All decisions are processed sequentially, one after

another.◦ Least efficient, but most thorough

Positive Logic◦ Processing flow continues through the module instead of

processing succeeding decisions, once the result is True. Negative Logic

◦ Flow is based on result being False. Nested decisions use Positive or Negative, but not

Straight-through.

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All conditions are tested.

Least efficient, but most exhaustive.

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TF

TF

Uses If/Then/Else instructions

Continues processing based on True results

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Grade >= 90

Grade >= 80LtrG

= “A”

LtrG = “Other”

T

T

F

LtrG= “B”

F

Note that the strings are in quotations marks!

Executes process based on False

Processes another decision when the result is True

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T

T

F

F

Grade < 90

LtrG= “A”

Grade < 80

LtrG = “Other”

LtrG= “B”

Optimum?◦ Evaluate all three

Reality?◦ this almost never happens

The Goal◦ Easiest to understand◦ Requires fewest tests◦ Easiest to maintain

Avoid the trap of always using the same

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SyntaxIf condition Then

statement(s)End If

Tests for one thing only.◦ If the statement results in a

False, it drops out without performing the statements.

Very efficient, but not very flexible

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Grade >= 60

true

false

Display“Passed”

If Grade <= 100 and Grade >= 90 Then

LetterGrade = “A”EndIf

This asks the question: if the value stored in Grade is less than or equal to 100 and is greater than or equal to 90, then LetterGrade takes on the value of the character “A”.

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Condition

Private Sub Swap(X, Y) ‘X & Y passed inDim Temp As Integer ‘local variableIf X > Y Then

Temp = X ‘Copies X into TempX = Y ‘Copies Y into XY = Temp ‘Copies Temp into Y

End IfEnd Sub

VB code

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Allows two questions to be asked Syntax

If condition Thenstatement(s)

Elsestatement(s)

End If

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Grade >= 60

true

Display“Passed”

false

Display“Failed”

If Grade >= 60 ThenLetterGrade = “Passed”

ElseLetterGrade = “Failed”

End If

Notice that there are two exclusive options. Greater than or equal to 60 or Less than. No other option is available.

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If Hours > 40 ThenPay = PayRate * (40 + (1.5 * (Hours – 40)))

ElsePay = PayRate * Hours

End If

Another way to write:Pay = (40 * PayRate) + _

((1.5 * PayRate) * (Hours - 40))

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The If/Then/Else permits multiple questions Always place the most likely to occur first.

◦ Increases efficiency

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If Grade >= 90 ThenLetterGrade = “A”

ElseIf Grade >= 80 ThenLetterGrade = “B”

ElseIf Grade >= 70 ThenLetterGrade = “C”

ElseIf Grade >= 60 ThenLetterGrade = “D”

ElseLetterGrade = “F”

End If

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Grade >= 90

true LetterGrade = “A”

false

Grade >= 80

true LetterGrade = “B”

false

Grade >= 70

true LetterGrade = “C”

false

Grade >= 60

true LetterGrade = “D”

false

LetterGrade = “F”Notice

Indentions

If Grade < 60 ThenLetterGrade = “F”

ElseIf Grade >= 60 ThenLetterGrade = “D”

ElseIf Grade >= 70 ThenLetterGrade = “C”

ElseIf Grade >= 80 ThenLetterGrade = “B”

ElseLetterGrade = “A”

End If

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Would you want me to use this one for your grades?

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nested ifs

Which is easier to read? debug?

Nested Ifs Multiple-Alternative Ifs

if condition then if condition then

if condition then statement1

if condition then else if condition then

statement1 statement2

else else if condition then

else statement3;

else; {next statement}

{next statement}

If Grade < 90 ThenIf Grade < 80 Then

If Grade < 70 ThenIf Grade < 60 Then

LetterGrade = “F”Else LetterGrade = “D”

Else LetterGrade = “C”Else LetterGrade = “B”

Else LetterGrade = “A”End If Ugly, isn’t it?

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Forces each decision to be

tested

Called Logical Opposites

When switching between Positive and Negative Logic, change all

◦< to >=◦<= to >◦> to <=◦>= to <◦= to <>◦<> to =◦And…exchange

Then statements with Else statements

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Is similar to a series of If/Then/Else statements.

Syntax:Select Case testvalue

Case value1statement group 1

Case value2statement group 2

End Select

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Case a actionCase aTrue

False

Case b

False

Case b actionTrue

Case Else actionCase Else

Select Case GradeCase 90..100

LetterGrade = “A”Case 80..89.9

LetterGrade = “B”Case 70..79.9

LetterGrade = “C”Case 60..69.9

LetterGrade = “D”Else

LetterGrade = “F”End Select

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Related to the If/Then/Else structure. Each argument that is passed to Switch is

either a condition or a value. All possible values must be accounted for

because there is no Else.

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Switch a action

Switch aTrue

False

Switch b action

Switch bTrue

False

Switch c actionSwitch c

True

LetterGrade = Switch _ (grade >= 90, “A”, _ grade >= 80, “B”, _ grade >= 70, “C”, _ grade >= 60, “D”, _ grade < 60, “F”)

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Much more concise than even the Case statement, but not in every

language.

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Exist in several different forms Most consist of 4 parts:

◦ The conditions◦ The actions◦ The combinations of True and False for the

conditions◦ The action to be taken or the consequences for

each combination of conditions.

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Filing Status(Condition)

Single or Divorced

Married filing Jointly or Qualifying Widower

Married filing separately

Head of Household

Select tax rate (Action)

a. Singleb. Married jtc. Married spd. Head hshd

X

X

X

X

Compute standard deduction (Action)

a. $4,000b. $2,500c. $1,500

X

X

X

X

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How would you set up a Decision Table

for Numeric Grades associated with Letter Grades?

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Letter Grade (Solution)

Numeric Grade (Condition)

A B C D F

90 <= n X

80 <= n < 90 X

70 <= n < 80 X

60 <= n < 70 X

< 60 X

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Write the calculation module to choose the largest number from a set of three numbers, A, B, and C.

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If A > B

If A > CIf B > C

Largest = C

Largest = B

Largest = C

Largest = A

True

TrueTrue

False

False

False

If A is greater than B Then A is > BIf A is greater than C Then A is > C

Largest equals A A is > B & A > CElse

Largest equals C A is < B & B < CElse

If B is greater than C Then Largest equals B A is < B & B > C

Else Largest equals C A is < B & C > B

End If

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How would you modify the logic to print the results in order, A, B, C?

What are the possible outputs that you would expect to see?A, B, C B, C, A C, B, AA, C, B B, A, C C, A, B

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Largest A B C

Order A B C X

A C B X

B C A X

B A C X

C B A X

C A B X

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In Visual Basic

Note: Again, up this point it didn’t matter what language we use…

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Variable Name

Data Type Module Defined Domain

Scope

A Short

2 bytes

GetInput

Calculate

0 < n < 10 Global

B Short

2 bytes

GetInput

Calculate

0 < n < 10 Global

C Short

2 bytes

GetInput

Calculate

0 < n < 10 Global

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Yes, I know…the variables are not mnemonic!

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Option Explicit On

' Input Values by UserDim A As ShortDim B As ShortDim C As Short

Private Sub txtNum1_TextChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles txtNum1.TextChanged 'converts user input to numeric format A = Val(txtNum1.Text) End Sub

Private Sub txtNum2_TextChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles txtNum2.TextChanged 'converts user input to numeric format B = Val(txtNum2.Text) End Sub

Private Sub txtNum3_TextChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles txtNum3.TextChanged 'converts user input to numeric format C = Val(txtNum3.Text) End Sub

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Private Sub Calculate _(ByVal A, ByVal B, ByVal

C)

Dim Largest As Short If A > B Then If A > C Then Largest = A Else Largest = C End If

ElseIf B > C Then Largest = B Else Largest = C End If lblInstructions.Text = _

("The Largest number is " & _ CStr(Largest))

End Sub

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Private Sub cmdPrintResult_Click()

lblOutput.Text = ("A = " & CStr(A) & _ " B = " & CStr(B) & _

" C = " & CStr(C))

End Sub

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1st step2nd

step 3rd step

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Input Variables to be tested◦ A

any numeric value greater than 0 and less than 10

◦ B any numeric value greater than 0 and less than 10

◦ C any numeric value greater than 0 and less than 10

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Analyze the Problem◦ Being able to restate the problem is one

indicator that you understand the problem Develop the Structure Chart (or flow chart

of its subparts) Develop the Algorithms Develop the Data Dictionary Develop the Decision Table Develop the Code Test the Solution

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