1. Visual C++ Programming: Concepts and Projects
Chapter 4A
Selection (Concepts)

2. Objectives
Discover what control structures are and why they are important
Learn the difference between sequential control structures and selection control structures
Compare values using relational operators
Use an if statement to select single alternative
Use an if...else statement to select one of two alternatives
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3. Objectives (continued)
Develop an understanding of logical operators and their use in complex expressions
Create nested control structures
Become familiar with multiple selection and the switch statement
Visual C++ Programming

4. Control Structures Control structures Three types
Are the fundamental building blocks of all programs
Control the flow of instruction execution
Three types
Sequential
Every statement is executed in sequence
Selection
Allows you to skip statements
Repetition
Allows you to repeat statements
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6. Sequential Control Structures
Linear in nature
Each statement is performed in order
No statement is skipped
No statement is repeated
The simplest programs tend to be sequential in nature
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9. Selection Control Structures
Provide alternative course of action
Single alternative selection
Double alternative selection
Multiple alternative selection
A Boolean expression is evaluated and used to select a course of action
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11. Relational Operators A Boolean expression often compares two variables
The relationship between two variables is evaluated using relational operators
The equal to (==) and not equal to (!=) operators have lowest precedence
All relational operators are binary (have two operands)
All relational operators are left-to-right associative
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14. Using if Statements to Provide a Single Alternative
Syntax: keyword if followed by a Boolean expression)
If a Boolean expression is true then one or more statements are executed
If only one task is to be executed it can be listed after the Boolean expression
If more than one task is to be executed they must be enclosed in curly brackets { }
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17. Using if…else Statements to Provide Two Alternatives
The keyword else is used to separate the two alternatives
If the Boolean expression is true then the statements in the first alternative are selected
If the Boolean expression is false then the statements in the second alternative are selected
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20. Logical Operators Operators with Boolean operands
Include operators for “and”, “or” and “not”
Examples from everyday life. You may have an exam on Monday but want to go to a concert in another city this weekend.
If it is true that I have an exam, then I will not try to get concert tickets
If it is not true that I have an exam, then I will try to get tickets.
Visual C++ Programming

21. Logical Operators Operators with Boolean operands
Include operators for “and”, “or” and “not”
Logical operators
Not (!)
And (&&)
Or (||)
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22. The not Operator (!) Reverses a Boolean value
Has highest precedence among logical operators
Uses the ! Operator
Example: !(score >= 60)
Assume that score is 45. Then, the relational expression score >= 60 is false, ! reverses the evaluation to true
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25. The and Operator (&&)
Used with two Boolean operands – often relational expressions
Lower precedence than not (!)
Example:
if ((score > 0) && (score <= 100))
The operands may both be true
The left operand may be true and the right operand false
The right operand may be true and the left operand false
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28. The and Operator (&&) (continued)
If either the left or right operands are false then the entire expression evaluates to false
The only way and expression evaluates to true using the and operator (&&) is if both operands are true
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30. Determining When to Use the and operator (&&) and the or operator (||)
Consider a program with
two TextBoxes that must contain integers
and a ComboBox control to indicate which arithmetic operation to perform
Possible errors of omission
No data in both TextBoxes
Data in one TextBox but not the other
No operation selected from the ComboBox
Example:
if ((txtLeft->Text == “”) && (txtRight->Text == “”))
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35. The or Operator (||)
Unlike the and operator (&&) if either the left or right operands are true then the entire expression evaluates to true
The only way and expression evaluates to false using the or operator (||) is if both operands are false
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38. Nested Control Structures
Nested control structures are ones that are placed inside of another
Often used to implement multiple alternative selection
A double alternative (if…else) statement within one alternative of another if…else statement
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40. Nested Control Structures (continued)
Example: a ComboBox can be evaluated to determine whether a selection has been made.
Valid ComboBox choices have index values starting at 0
If no selection has been made the SelectedIndex property of a ComboBox is set to -1 by default
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43. Multiple Alternative Selection
Can be implemented with nested if…else statements
The statements work like a filter
Whichever Boolean expression evaluates to true first controls that path of execution that the program has
The if…else if statement is made to accommodate multiple selection without nesting
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47. The switch statement also implements multiple selection
Keyword switch is followed by an integral value
The integral value is used to determine which case will be executed
Each case has statements that will be executed
Control will transfer out only it a break statement or the end of the switch statement is encountered
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50. Summary
Control structures are the building blocks of every computer program
Types of control structures
Sequential – linear, no statement repeated or skipped
Selection – allows one or more statements to be skipped under specific conditions
Repetition – the topic of Chapter 5
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51. Summary (continued) Types of selection structures
Single alternative (if statement)
Double alternative (if…else statement)
Multiple alternative
Nested if…else statements
Multiple alternative if (if…elseif statement)
Switch statement
All if statements evaluate a Boolean expression
Visual C++ Programming

52. Summary (continued) Boolean expressions often use relational operators
>, >=, <, <=, ==, !=
Complex Boolean expressions can be evaluated using logical operators and (&&) and or (||)
Visual C++ Programming