Chapter 6: More on the Selection Structure

1
Chapter 6More on the Selection Structure

• 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

3
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

4
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

5
Nested Selection Structures (continued)
6
Nested Selection Structures (continued)
7
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

8
Logic Errors in Selection Structures (continued)
9
Logic Errors in Selection Structures (continued)

• 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

10
Results of Desk-Checking the Correct Algorithm
11
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 structures condition when a
  nested selection structure is needed

12
Correct Algorithm and an Incorrect Algorithm
Containing the First Logic Error
13
Results of Desk-Checking the Incorrect Algorithm
14
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

15
Correct Algorithm and an Incorrect Algorithm
Containing the Second Logic Error
16
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

18
Correct Algorithm and an Inefficient Algorithm
Containing the Third Logic Error
19
Results of Desk-Checking the Inefficient Algorithm
20
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
22
Two Versions of the C Code for the Grade
Problem (continued)
23
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

25
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

26
Formatting Numeric Output

• Use setiosflags stream manipulator to display a
  programs numeric output in fixed-point notation
  only
• Must use include ltiomanipgt directive and
• using stdios and using stdsetiosflags
• Use setprecision to control the number of decimal
  places displayed

27
Formatting Numeric Output (continued)
28
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