The Selection Structure

1
The Selection Structure

• Tutorial 6

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Objectives

• Use the selection structure in a program
• Write pseudocode for the selection structure
• Create a flowchart for the selection structure
• Code the if and if/else forms of the selection
  structure
• Write code that uses comparison operators and
  logical operators
• Change the contents of a String variable to
  uppercase or lowercase
• Compare strings using the CompareTo() method
• Return a floating-point number when dividing two
  integers

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Concept Lesson
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Using the Selection Structure

• Selection structure/decision structure allows
  program to make a decision or comparison and then
  select one of two paths, depending on the result
  of the comparison
• Condition
• Specifies the decision you are making
• Phrased so that it results in either a true or
  false answer

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Selection Structures You Might Use Today
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Including the Selection Structure in Pseudocode

• True path - the instructions following the
  condition
• False path
• When using else, false path includes instructions
  between else and end if
• When else is not used, processing continues after
  the end if

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Including the Selection Structure in Pseudocode
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Drawing a Flowchart of a Selection Structure

• Flowcharts - use standardized symbols to show
  steps the computer must take to accomplish
  programs goal
• Diamond symbol (selection/repetition symbol)
  used to represent both selection and repetition

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Drawing a Flowchart of a Selection Structure
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Coding the Selection Structure

• Items in square brackets () in the syntax are
  optional
• You create a statement block by enclosing the
  statements in a set of braces ()
• Although it is not required to do so, it is a
  good programming practice to use a comment, such
  as //end if, to mark the end of each if statement

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Syntax of the C if statement
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Coding the Selection Structure
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Comparison Operators

• Comparison (relational) operators - used to make
  comparisons in a C program
• Precedence numbers - indicate the order in which
  the computer performs the comparisons in a C
  expression
• Parentheses - used to override the order of
  precedence

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Comparison Operators
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Comparison Operators
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Comparison Operators
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Comparison Operators
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Swapping Values

• Study closely the four instructions that swap the
  two values
• int temp 0 creates and initializes a local
  variable named temp
• temp first assigns the value contained in the
  first variable to the temp variable
• first second assigns the value contained in
  the second variable to the first variable
• second temp assigns the value contained in the
  temp variable to the second variable

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Illustration of the Swapping Concept
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Logical Operators

• Logical/Boolean operators - And and Or
• Allow you to combine two or more conditions into
  one compound condition
• And logical operator - all of the conditions must
  be true for the compound condition to be true
• Or logical operator - only one of the conditions
  must be true for the compound condition to be true

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Truth Tables for the And and Or Logical Operators
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Logical Operators
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Logical Operators
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Using Logical Operators in a Program

• Data validation - the process of verifying that
  the input data is within the expected range

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Comparing Strings

• String comparisons are case-sensitive
• Yes is not the same as the string YES or
  yes
• ToUpper() method - converts a string to uppercase
• ToLower() method - converts a string to lowercase
• Member access operator (-gt) - used to access the
  members of a class
• CompareTo() method used to compare strings

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Comparing Strings
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Using the ToUpper() and CompareTo() Methods in a
Program

• ToUpper() method converts the contents of the
  variable to uppercase
• CompareTo() method - tells the computer to
  compare the contents of the variable to each of
  the three state IDs

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Comparing Strings
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Application Lesson
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Analyzing, Planning, and Desk-checking
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Desk-checking

• Data for first desk-check
• Total calories 150
• Grams of fat 6
• Data for second desk-check
• Total calories 105
• Grams of fat 2
• Data for third desk-check
• Total calories 100
• Grams of fat -3

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Completed Desk-check Tables
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Coding the main() Function

• main() function - requires four memory locations
  to store the values of its input and output items
• Use the int data type for variables that store
• total calories, grams of fat, and fat calories
  (these variables need to store whole numbers
  only)
• Use the double data type for the variable that
  stores
• fat percentage

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Coding the main() Function
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Coding the calcFatInfo() Function
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Data and Completed Desk-check Table for the
Program
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Completing the Health Club Program

• To open the partially completed C program
• Start Microsoft Visual Studio .NET. If necessary,
  close the Start Page window
• Click File on the menu bar, and then click Open
  Solution. The Open Solution dialog box opens
• Locate and then open the CppNet\Tut06 \T6App
  Solution folder
• Click T6App Solution (T6App Solution.sln) in the
  list of filenames, and then click the Open button
• If the T6App.cpp source file is not displayed,
  right-click T6App.cpp in the Solution Explorer
  window, and then click Open

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Summary

• Selection structure
• As pseudocode
• In a flowchart
• As an if and if/else statement
• Use comparison operators and logical operators
• String operations
• Uppercase to lowercase
• Compare strings using the CompareTo() method
• Return a floating-point number when dividing two
  integers