1. Making Decisions in a Program

2. Objectives
Include the selection structure in pseudocode and in a flowchart
Write an If…Then…Else statement
Write code that uses comparison operators and logical operators
Create a variable having block-scope
Concatenate strings
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3. Objectives (continued)
Use the ControlChars.NewLine constant
Change the case of a string
Determine whether a string contains data
Display a message in a message box
Include a nested selection structure in pseudocode, a flowchart, and code
Include a Case selection structure in pseudocode, a flowchart, and code
Generate random numbers
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4. The Selection Structure
Selection structure (or decision structure):
Used to select a path to take based on the outcome of a decision or comparison
Condition:
The decision to be made
Results in a Boolean (True or False) answer
Four forms of selection structure: If
If/Else
If/ElseIf/Else
Case
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5. Writing Pseudocode for the If
If selection structure: contains one set of instructions to process when the condition is true
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6. Writing Pseudocode for the If/Else Selection Structures
Contains two sets of instructions
One set is processed when the condition is true
The other set is processed when the condition is false
True path: path to follow when condition is true
False path: path to follow when condition is false
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7. Flowcharting the If and If/Else Selection Structures
Selection/repetition symbol:
Diamond shape
Represents both selection and repetition structures
One flowline entering and two flowlines leaving
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8. Flowcharting the If and If/Else Selection Structures (continued)
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9. Coding the If and If/Else Selection Structures
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10. Coding the If and If/Else Selection Structures (continued)
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11. Coding the If and If/Else Selection Structures (continued)
Else clause: an optional part of the If statement
Statement block: set of statements terminated by an Else or End If
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12. Comparison Operators Comparison operators (or relational operators):
Used as part of the condition in an If statement
Most commonly used comparison operators:
Equal to: =
Greater than: >
Greater than or equal to: >=
Less than: <
Less than or equal to: <=
Not equal to: <>
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13. Comparison Operators (continued)
Have no order of precedence
Are evaluated from left to right in an expression
Example: Evaluation Steps
4/2* 5 > 7 + 2
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14. Using Comparison Operators – Swapping Numeric Values
Pseudocode for a procedure that displays highest and lowest of two numbers:
Store If
display
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15. Using Comparison Operators – Swapping Numeric Values (continued)
Block scope: the scope of a variable created within a block
Block-scope variable: can only be used within the statement block in which it was declared
Concatenation operator (&): links two strings
ControlChars.NewLine constant:
Advances the insertion point to the next line
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16. Using Comparison Operators – Swapping Numeric Values (continued)
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17. Using Comparison Operators – Example 2
Pseudocode for a procedure to allow the user to display the sum or difference of two numbers:
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18. Using the ToUpper and ToLower Methods
String comparisons in Visual Basic are case-sensitive
ToUpper method: converts a string to uppercase
Syntax:
String.toUpper()
ToLower method: converts a string to lowercase
ToUpper and ToLower can be used to permanently or temporarily convert a variable’s contents
String.ToLower()
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19. Logical Operators Logical operators (or Boolean operators):
Used to combine one or more conditions
Compound condition: a combination of conditions using logical operator(s)
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20. Logical Operators (continued)
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21. Using the Truth Tables (continued)
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22. Using the Truth Tables (continued)
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23. The String.IsNullOrEmpty Method
String.IsNullOrEmpty method: determine if a control’s Text property or String variable contains data
Syntax:
String.IsNullOrEmpty method(string)
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24. The MessageBox.Show Method
Display message box with text, buttons and an icon
When a message box is displayed, the program waits until the user selects a button
MessageBox.Show returns an integer value indicating which button the user selected
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25. The MessageBox.Show Method (continued)
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26. The MessageBox.Show Method (continued)
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27. The MessageBox.Show Method (continued)
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28. Nested Selection Structures
Nested selection structure: a selection structure that is completely contained within another selection structure
Primary decision: decision made by the outer selection structure
Secondary decision: decision made by the inner selection structure
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29. Nested Selection Structures (continued)
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30. Nested Selection Structures (continued)
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31. Nested Selection Structures (continued)
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32. Nested Selection Structures (continued)
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33. The If/ElseIf/Else Selection Structure
Need a procedure to display a message based on a letter grade:
Letter grade
Message A
Excellent B
Above Average C
Average D
Below Average F
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34. The Case Selection Structure
Used when there are many paths from which to choose
Simpler and clearer than using If/ElseIf/Else
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35. The Case Selection Structure (continued)
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36. The Case Selection Structure (continued)
Case selection structure in a flowchart:
Uses the diamond symbol
One flowline into the diamond, but many flowlines out of the diamond
Case selection structure evaluates an expression to determine which path to take
Case selection structure:
Begins with Select Case
Ends with End Select
Has one Case clause for each possible value
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37. The Case Selection Structure (continued)
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38. Using To and Is in an ExpressionList
TO and IS keywords: specify a range of values in a Case clause’s expression list
TO:
When you know both the upper and lower bounds of the range
IS:
When you know only one end of the range
Used with a comparison operator
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39. Generating Random Integers
Pseudo-random number generator: produces a sequence of numbers that meets certain statistical requirements for randomness
Random.Next method:
Generates a random integer
Can specify a minimum and maximum value
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40. Generating Random Integers (continued)
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41. Summary Selection structure is used for decisions
Four forms of selection structures: If, If/Else, If/ElseIf/Else, and Case
Diamond symbol represents a decision in a flowchart
Expressions with comparison operators will result in an answer of True or False
Variables declared within a selection expression have block-level scope
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42. Summary (continued) Concatenation: linking two strings together
Use logical operators to create compound conditions
String.IsNullOrEmpty method will determine if a string contains data
MessageBox.Show method returns an integer indicating which button was chosen
Selection structures can be nested
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43. Summary (continued)
Use If/ElseIf/Else or Case structures when there are several possible alternative outcomes
Use TO keyword to specify a range of valid values when both the lower and upper bound are known
Use IS keyword with a comparison operator to specify a lower or upper bound but not both
Use the pseudo-random number generator to generate random numbers
Making Decisions in a Program
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