1. Problem Solving with Decisions
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Problem Solving with Decisions
Lesson 6
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2. Overview The Decision Logic Structure The If Instructions
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Overview
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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3. Flowchart Symbols Decision Process Assign Decision Process Assign
True/False/Else
Process
Assign
Decision
Process
Assign
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4. Decisions, Decisions… Zzzzz! Oh, no! Alarm goes off! Snooze Snooze
or Off?
Snooze
= 9?
Snooze
< 9
Sleep or
Get up?
Go Back to Sleep
Get
Dressed. . .
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5. Decision Logic Structure
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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6. Common Forms The If Statement The Case Statement The Switch 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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7. Three Types of Decisions
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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8. Straight-through F T F T All conditions are tested.
Least efficient, but most exhaustive. F T F T
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9. Positive Logic T F T F Uses If/Then/Else instructions
Continues processing based on True results T
Grade >= 90 F
LtrG = “A”
Grade >= 80 F T
LtrG = “Other”
LtrG = “B”
Note that the strings are in quotations marks!
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10. Negative Logic Executes process based on False
Processes another decision when the result is True
Grade < 90 T F
Grade < 80
LtrG = “A” F T
LtrG = “B”
LtrG = “Other”
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11. So, Which Do You Use? Optimum? Reality? The Goal
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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12. If Statements
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13. The Single Selection IF
Syntax
If 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
Grade
>= 60
true
false
Display
“Passed”
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14. Example
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”.
Condition
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15. A Classic Example
Private Sub Swap(X, Y) ‘X & Y passed in Dim Temp As Integer ‘local variable If X > Y Then Temp = X ‘Copies X into Temp X = Y ‘Copies Y into X Y = Temp ‘Copies Temp into Y End If End Sub VB code
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16. The Double Selection If
Allows two questions to be asked
Syntax
If condition Then statement(s)
Else
statement(s)
End If
Grade
>= 60
true
Display
“Passed”
false
“Failed”
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17. An Example
If Grade >= 60 Then LetterGrade = “Passed” Else LetterGrade = “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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18. An Another Example
If Hours > 40 Then Pay = PayRate * (40 + (1.5 * (Hours – 40))) Else Pay = PayRate * Hours End If Another way to write: Pay = (40 * PayRate) + _ ((1.5 * PayRate) * (Hours - 40))
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19. The Multiple Selection If
The If/Then/Else permits multiple questions
Always place the most likely to occur first.
Increases efficiency
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20. Example Notice Indentions
Grade
>= 90
true
LetterGrade = “A”
false
If Grade >= 90 Then LetterGrade = “A” ElseIf Grade >= 80 Then LetterGrade = “B” ElseIf Grade >= 70 Then LetterGrade = “C” ElseIf Grade >= 60 Then LetterGrade = “D” Else LetterGrade = “F” End If
Grade
>= 80
true
LetterGrade = “B”
false
Grade
>= 70
true
LetterGrade = “C”
false
Grade
>= 60
true
LetterGrade = “D”
false
LetterGrade = “F”
Notice Indentions
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21. Would you want me to use this one for your grades?
How About This One?
If Grade < 60 Then LetterGrade = “F” ElseIf Grade >= 60 Then LetterGrade = “D” ElseIf Grade >= 70 Then LetterGrade = “C” ElseIf Grade >= 80 Then LetterGrade = “B” Else LetterGrade = “A” End If
Would you want me to use this one for your grades?
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22. Nested Vs. Multiple if condition then if condition then
Nested Ifs Multiple-Alternative Ifs
if condition then if condition then
if condition then statement1
if condition then else if condition then
statement statement2
else else if condition then
else statement3;
else; {next statement}
{next statement}
nested ifs
Which is easier to read? debug?
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23. Forces each decision to be tested
Nested If’s
Forces each decision to be tested
If Grade < 90 Then If Grade < 80 Then If Grade < 70 Then If 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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24. Logical Operators & Opposites
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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25. The Case & The Switch
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26. The Case Statement Is similar to a series of If/Then/Else statements.
Syntax:
Select Case testvalue
Case value1
statement group 1
Case value2
statement group 2
End Select
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27. Case Flow Chart True Case a Case a action False True Case b
Case b action
False
Case Else
Case Else action
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28. Case Example Select Case Grade Case 90..100 LetterGrade = “A”
LetterGrade = “B”
Case
LetterGrade = “C”
Case
LetterGrade = “D”
Else
LetterGrade = “F”
End Select
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29. The Switch Statement 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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30. Switch Flow Chart Switch a True Switch a action False Switch b True
Switch b action
False
Switch c
Switch c action
True
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31. Switch Example
LetterGrade = Switch _ (grade >= 90, “A”, _ grade >= 80, “B”, _ grade >= 70, “C”, _ grade >= 60, “D”, _ grade < 60, “F”)
Much more concise than even the Case statement, but not in every language.
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32. Decision Tables
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33. Decision Tables 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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34. Decision Table Example
Filing Status (Condition)
Single or Divorced
Married filing Jointly or Qualifying Widower
Married filing separately
Head of Household
Select tax rate (Action)
a. Single
b. Married jt
c. Married sp
d. Head hshd X
Compute standard deduction (Action)
a. $4,000
b. $2,500
c. $1,500
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35. Another Decision Table Example
How would you set up a Decision Table for Numeric Grades associated with Letter Grades?
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36. Decision Table Solution
Letter Grade (Solution)  Numeric Grade (Condition)  A B C D F
90 <= n X
80 <= n < 90
70 <= n < 80
60 <= n < 70
< 60
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37. Let’s work a problem…
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38. The Problem
Write the calculation module to choose the largest number from a set of three numbers, A, B, and C.
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39. The Flow Chart False If A > B True False If B > C True False
If A > C
True
Largest
= C
Largest
= B
Largest
= C
Largest
= A
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40. Pseudocode Solution
If A is greater than B Then A is > B If A is greater than C Then A is > C Largest equals A A is > B & A > C Else Largest equals C A is < B & B < C If B is greater than C Then Largest equals B A is < B & B > C Largest equals C A is < B & C > B End If
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41. Modify?
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, A
A, C, B B, A, C C, A, B
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42. Decision Table Solution
Largest  A B C
Order 
A B C X
A C B
B C A
B A C
C B A
C A B
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43. Coding the Problem In Visual Basic
Note: Again, up this point it didn’t matter what language we use…
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44. Yes, I know…the variables are not mnemonic!
Data Dictionary
Variable Name
Data Type
Module
Defined Domain
Scope A
Short
2 bytes
GetInput
Calculate
0 < n < 10
Global B C
Yes, I know…the variables are not mnemonic!
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45. VB Solution – Declare Variables
Option Explicit On
' Input Values by User
Dim A As Short
Dim B As Short
Dim C As Short
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46. VB Solution - Get Input
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 B = Val(txtNum2.Text) Private Sub txtNum3_TextChanged(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles txtNum3.TextChanged C = Val(txtNum3.Text)
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47. VB Solution – Calculate Result
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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48. VB Solution - Produce Output
Private Sub cmdPrintResult_Click() lblOutput.Text = ("A = " & CStr(A) & _ " B = " & CStr(B) & _ " C = " & CStr(C)) End Sub
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49. VB Solution – the Form
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50. Getting the Input
1st step
2nd step
3rd step
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51. Calculating the Solution
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52. Testing the Solution Input Variables to be tested A B C
any numeric value greater than 0 and less than 10 B C
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53. Summary - Reminder 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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54. Next?
Problem Solving
with Iteration
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