Boolean Algebra, Selection If Statement

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Boolean Algebra, Selection If Statement
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Boolean Algebra

• Logical expressions have the one of two values
  true or false
• A rectangle has three sides
• The instructor has a pleasant smile
• The three basic logical operations are AND, OR,
  NOT

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Boolean Algebra

• Three key logical operators in C
• - logical and
• - logical or
• ! - logical not

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Boolean Algebra

• Truth tables
• Lists all combinations of operand values and the
  result of the operation for each combination
• Example truth table for (logical and)

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Boolean Algebra

• Truth table for (logical or)

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Boolean Algebra

• Truth table for ! (logical not)

P ! P
False True True False
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Boolean Algebra

• Can create complex logical expressions by
  combining simple logical expressions
• Example
• ! (P Q)
• A truth table can be used to determine when a
  logical expression is true

P Q P Q ! (P Q)
False False False True False
True False True True False False True True
True True False
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A Boolean Type

• C contains a type named bool
• Type bool has two symbolic constants
• true
• false
• Boolean operators
• - logical and
• - logical or
• ! - logical not
• Warning
• and are also operators so be careful what you
  type

if ( voltage lt 48 milliamp lt 10 )
if ( gpa gt 3.5 class_rank lt 25 )
if ( ! done )
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A Boolean Type

• Example logical expressions
• bool P truebool Q falsebool R
  truebool S (P Q)bool T ((!Q)
  R)bool U !(R (!Q))

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Relational Operators

• The condition in the if statement is based on the
  comparison of two items and is usually expressed
  with on of the following symbols

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Relational Operators

• Equality operators
• equal to
• ! not equal to
• Examples
• int i 32
• int k 45
• bool q (i k)
• bool r (i ! k)

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Relational Operators

• Ordering operators
• Examples
• int i 5
• int k 12
• bool p (i lt 10)
• bool q (k gt i)
• bool r (i gt k)
• bool s (k lt 12)

lt gt gt (?) lt (?)
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Operator Precedence Expanded

• precedence of operators (from highest to lowest)
• ()
• Unary -
• /
• -
• gt lt gt gt !

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If Statements
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Selection IF Statements

• The selection structure represents the decision
  making abilities of the computer.
• Provide
• Ability to control whether a statement list is
  executed
• If statement
• if
• if-else
• if-else-if

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The Basic If Statement

• Syntax
• if (Expression)
• Action
• If the Expression is true then execute Action
• Action is either a single statement or a group of
  statements within braces

Expression
true
false
Action
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The Basic If Statement Example
if (Value lt 0) Value -Value
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The If-Else Statement

• Syntax
• if (Expression)
• Action1 else Action2
• If Expression is true then executeAction1
  otherwise execute Action2
• if (v 0)
• cout ltlt "v is 0"
• else cout ltlt "v is not 0"

Expression
false
true
Action1
Action2
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Example Finding the Max

• int Value1
• int Value2
• int Max
• cout ltlt "Enter two integers "
• cin gtgt Value1 gtgt Value2
• if (Value1 lt Value2)
• Max Value2
• else
• Max Value1
• cout ltlt "Maximum of inputs is " ltlt Max ltlt endl

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Example Finding the Max
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The If-Else-If Statement

• It is often the case that depending upon the
  value of an expression we want to perform a
  particular action
• Two major ways of accomplishing this choice
• if-else-if statement
• if-else statements glued together
• Switch statement (next week)
• An advanced construct

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Example Finding Vowels

• if ((ch 'a') (ch 'A)) cout ltlt ch ltlt
  " is a vowel" ltlt endlelse if ((ch 'e')
  (ch 'E")) cout ltlt " ch ltlt " is a vowel" ltlt
  endlelse if ((ch 'i') (ch 'I")) cout
  ltlt ch ltlt " is a vowel" ltlt endl else if ((ch
  'o') (ch 'O")) cout ltlt ch ltlt " is a vowel"
  ltlt endl else if ((ch 'u') (ch
  'U")) cout ltlt ch ltlt " is a vowel" ltlt endl
• else cout ltlt ch ltlt " is not a vowel" ltlt endl