Chapter 6:More on the Selection

Structure

Introduction to Programming with C++

Fourth Edition

Introduction to Programming with C++, Fourth Edition 2

Objectives

• Include a nested selection structure in pseudocode and in a flowchart

• Code a nested selection structure in C++• Recognize common logic errors in selection

structures

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Objectives (continued)

• Include the switch form of the selection structure in pseudocode and in a flowchart

• Code the switch form of the selection structure in C++

• Format numeric output in C++

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Nested Selection Structures

• Nested selection structure: – Selection structure within another selection

structure– Used when more than one decision must be

made before the appropriate action can be taken• Primary decision - always made by the outer

selection structure• Secondary decision - always made by the

inner (nested) selection structure

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Nested Selection Structures (continued)

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Nested Selection Structures (continued)

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Logic Errors in Selection Structures

• Logic errors are commonly made as a result of the following mistakes:– Using a logical operator rather than a nested

selection structure

– Reversing the primary and secondary decisions

– Using an unnecessary nested selection structure

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Logic Errors in Selection Structures (continued)

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• Test Data for Desk-Checking– Data for first desk-check

• Status: F• Years: 4

– Data for second desk-check• Status: F• Years: 15

– Data for third desk-check• Status: P• Years: 11

Logic Errors in Selection Structures (continued)

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Results of Desk-Checking the Correct Algorithm

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Using a Logical Operator Rather Than a Nested Selection Structure

• One common error made when writing selection structures is to use a logical operator in the outer selection structure’s condition when a nested selection structure is needed

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Correct Algorithm and an Incorrect Algorithm Containing the First Logic

Error

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Results of Desk-Checking the Incorrect Algorithm

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Reversing the Primary and Secondary Decisions

• Common error: putting the secondary decision in the outer selection structure, and putting the primary decision in the nested selection structure

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Correct Algorithm and an Incorrect Algorithm Containing the Second

Logic Error

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Results of Desk-Checking the Incorrect Algorithm

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Using an Unnecessary Nested Selection Structure

• In most cases, a selection structure containing this error still produces the correct results

• However, it is less efficient than selection structures that are properly structured

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Correct Algorithm and an Inefficient Algorithm Containing the Third

Logic Error

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Results of Desk-Checking the Inefficient Algorithm

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Using the if/else Form to Create Multiple-Path Selection Structures

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Two Versions of the C++ Code for the Grade Problem

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Two Versions of the C++ Code for the Grade Problem (continued)

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Using the switch Form to Create Multiple-Path Selection Structures

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Using the switch Form

• Switch statement:– Begins with switch followed by an open brace– Ends with a closing brace

• Switch clause - keyword switch followed by a selectorExpression enclosed in parentheses

• selectorExpression – – Can contain any combination of variables,

constants, functions, methods, and operators– Must result in a bool, char, short, int, or long

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Using the switch Form (continued)

• Each clause in the switch statement contains a value followed by a colon

• Data type of the value should be compatible with the data type of the selectorExpression

• Break statement - tells the computer to leave (“break out of”) the switch statement at that point

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Formatting Numeric Output

• Use setiosflags stream manipulator to display a program’s numeric output in fixed-point notation only

• Must use #include <iomanip> directive and

using std::ios; and using std::setiosflags;• Use setprecision to control the number of

decimal places displayed

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Formatting Numeric Output (continued)

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Formatting Numeric Output (continued)

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Summary

• Nested selection structure: – Selection structure within another selection structure– Used when more than one decision must be made before

the appropriate action can be taken – Three common logic errors in selection structures

• Logic errors are commonly made as a result of the following mistakes:– Using a logical operator rather than a nested selection

structure

– Reversing the primary and secondary decisions

– Using an unnecessary nested selection structure

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Summary (continued)

• The switch form of the selection structure is often simpler to use if there are many paths from which to choose

• Format output using stream manipulators