visual c++ programming: concepts and projects chapter 4a selection (concepts)

VISUAL C++ PROGRAMMING: CONCEPTS AND PROJECTS

Chapter 4ASelection (Concepts)

Objectives

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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

Objectives (continued)

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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

Control Structures

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Control structures 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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Sequential Control Structures

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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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Selection Control Structures

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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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Relational Operators

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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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Using if Statements to Provide a Single Alternative

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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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Using if…else Statements to Provide Two Alternatives

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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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Logical Operators

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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.

Logical Operators

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Operators with Boolean operands Include operators for “and”, “or” and

“not” Logical operators

Not (!) And (&&) Or (||)

The not Operator (!)

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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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The and Operator (&&)

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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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The and Operator (&&) (continued)

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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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Determining When to Use the and operator (&&) and the or operator (||)

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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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The or Operator (||)

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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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Nested Control Structures

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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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Nested Control Structures (continued)

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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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Multiple Alternative Selection

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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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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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Summary

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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

Summary (continued)

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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

Summary (continued)

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Boolean expressions often use relational operators• >, >=, <, <=, ==, !=

Complex Boolean expressions can be evaluated using logical operators and (&&) and or (||)