Logical Expressions

1
Chapter 4

• Logical Expressions If-Else

2
Overview

• More on Data Type bool
• Using Relational Logical Operators to Construct
  Evaluate Logical Expressions
• If-Else Statements
• If Statements
• Nested If Statements for Multi-way Branching
• Testing a C Program

3
Boolean expressions

• Boolean expressions (expressions of type bool)
  are constructed by using relational and logical
  operators
• 6 Relational Operators
• lt lt gt gt !
• 3 Logical Operators
• !

4
Relational operators

• Relational operators and variables of any type
  are combined to form Boolean expressions

5
Evaluating relational expressions

• int x 4, y 6
• EXPRESSION VALUE
• x lt y true
• x 2 lt y false
• x ! y true
• x 3 gt y true
• y x false
• y x2 true
• y x 3 7 (not Boolean)

6
Assignment expressions

• In C, it is possible to use the assignment
  operator inside a larger expression
• The assignment sub-expression returns the value
  and type of the variable after assignment

include ltiostreamgt //what is the output? using
namespace std int main() int i0 cout ltlt
(i int(3.14159)) ltlt endl cout ltlt i ltlt endl
//what is the output? return 0
7
Comparing Strings

• Two objects of type string (or a string object
  and a string literal) can be compared using the
  relational operators
• A character-by-character comparison is made using
  the ASCII code of each character
• If all the characters are equal, then the 2
  strings are equal
• Otherwise, the string with the first character of
  smaller ASCII value is the lesser string

8
Comparison Operator Examples

• string myState string yourState
• myState Texas yourState Maryland
• EXPRESSION VALUE
• myState yourState false
• myState gt yourState true
• myState Texas true
• myState lt texas true

9

• Operator Meaning Associativity
• !, , - NOT, unary , unary - Right
• , / , Multiplication, Division, Modulus
  Left
• , - Addition, Subtraction Left
• lt Less than Left
• lt Less than or equal to Left
• gt Greater than Left
• gt Greater than or equal to Left
• , ! Is equal to, Is not equal to Left
• logical AND Left
• logical OR Left
• Assignment Right

10

• LOGICAL
• EXPRESSION MEANING DESCRIPTION
• ! p NOT p ! p is false if p is
  true.
• ! p is true if p is false.
• p q p AND q p q is true
  if
• both p and q are true.
• It is false otherwise.
• p q p OR q p q is false if
  both
• p and q are false. It is true
  otherwise.

11
Logical Expression Examples

• int age bool isSenior, hasFever
• float temperature
• age 20 temperature 102.0
• isSenior (age gt 55)//isSenior is false
• hasFever (temperature gt 98.6)
• // hasFever is true
• EXPRESSION VALUE
• isSenior hasFever false
• isSenior hasFever true
• !isSenior true
• !hasFever false

12
What is the value?

• int age, height
• age 25
• height 70
• EXPRESSION VALUE
• !(age lt 10) ?
• !(height gt 60) ?

13
Short-Circuit Evaluation

• C uses short circuit evaluation of logical
  expressions
• This means logical expressions are evaluated left
  to right and evaluation stops as soon as the
  final truth value can be determined

14
Short-Circuit Example

• int age, height
• age 25
• height 70
• EXPRESSION
• (age gt 50) (height gt 60) //false
• Evaluation can stop now because result of is
  only true when both sides are true
• It is already determined that the expression will
  be false

15
What happens?

• int age, height
• age 25
• height 70
• EXPRESSION
• ! (height gt 60) (age gt 50)
• Does this part need to be evaluated?

16
Write an expression for each

• taxRate is over 25 and income is less than
  20000
• temperature is less than or equal to 75 or
  humidity is less than 70
• age is over 21 and age is less than 60
• age is 21 or 22

17
Some Answers

• (taxRate gt .25) (income lt 20000)
• (temperature lt 75) (humidity lt .70)
• (age gt 21) (age lt 60)
• (age 21) (age 22)

18
Short-Circuit Benefits

• one Boolean expression can be placed first to
  guard a potentially unsafe operation in a
  second Boolean expression
• time is saved in evaluation of complex
  expressions using operators and

19
Short Circuit Example

• int number bool test
• float x
• test (number ! 0) (x lt 1/number)
• is evaluated first and has value false
• The entire expression will have a value of false
  
• Due to short-circuiting the sub-expression to
  the right of is not evaluated
• This avoids a possible division-by-zero error

20
WARNING about Expressions in C

• Boolean expression means an expression whose
  value is true or false
• an expression is any valid combination of
  operators and operands
• each expression has a value
• this can lead to UNEXPECTED RESULTS
• construct your expressions CAREFULLY
• use of parentheses is encouraged
• otherwise, use the precedence chart to determine
  order

21
Comparing float (or double) Values

• do not compare float values for equality, compare
  them for near-equality.

float myNumber float yourNumber cin gtgt
myNumber cin gtgt yourNumber test
(fabs(myNumber - yourNumber) lt 0.00001) //NOT
test (myNumber yourNumber)
22
Control Structures

• Control structures alter the order in which
  statements in a program are executed
• By default, each statement is executed exactly
  once
• Statements are executed sequentially
• There are 2 general types of control structures
• Selection, also called branching
• Repetition, also called looping

23
C Control Structures

• Selection
• if
• if . . . else
• switch
• Repetition
• for loop
• while loop
• do . . . while loop

24
Control Structures

• use logical expressions which may include
• 6 Relational Operators
• lt lt gt gt !
• 3 Logical Operators
• !

25
What can go wrong here?

• float average
• float total
• int howMany
• .
• .
• .
• average total/howMany

26
Improved Version

• float average,
• float total
• int howMany
• if (howMany gt 0)
• average total/howMany
• cout ltlt average
• else
• cout ltlt No prices were entered

27
If-Else Syntax

• if ( Expression )
• StatementA
• else
• StatementB
• NOTE StatementA and StatementB each can be a
  single statement, a null statement, or a block.

28
if ... else Provides Two-way Selection

• between executing one of 2 clauses
• the if clause or the else clause

TRUE
FALSE
expression
if clause
else clause
29
Use of blocks recommended

• if ( condition )
• //begin block
• //one or more statements go here
• //end block
• else
• //begin block
• //one or more statements go here
• //end block

30
If-Else with blocks

• int carDoors, driverAge
• float premium, monthlyPayment
• if ((carDoors 4)(driverAge gt 24))
• premium 650.00
• cout ltlt "LOW RISK "
• else
• premium 1200.00
• cout ltlt "HIGH RISK"
• monthlyPayment premium / 12.0 5.00

31
What happens without braces?

• if ((carDoors 4) (driverAge gt 24))
• premium 650.00
• cout ltlt "LOW RISK"
• else
• premium 1200.00
• cout ltlt "HIGH RISK"
• monthlyPayment premium / 12.0 5.00
• COMPILE ERROR OCCURS
• The if clause is the statement following the if

32
Braces define blocks of code

• Braces can only be omitted when each clause is a
  single statement
• if ( lastInitial lt 45 )
• volume 1
• else
• volume 2
• cout ltlt "Look it up in volume "
• ltlt volume ltlt "of NYC phone book"

33
If-Then-Else for a mail order

• Assign value .25 to discountRate and assign value
  10.00 to shipCost if purchase is over 100.00
• Otherwise, assign value .15 to discountRate and
  assign value 5.00 to shipCost
• Either way, calculate totalBill

34
These braces cannot be omitted

• if (purchase gt 100.00)
• discountRate 0.25
• shipCost 10.00
• else
• discountRate .15
• shipCost 5.00
• totalBill purchase (1- discountRate)
• shipCost

35
If statement is a selection

• of whether or not to execute a statement
• which can be a single statement or a block

TRUE
expression
FALSE
statement
36
Terminating your program

• int number
• cout ltlt "Enter a non-zero number"
• cin gtgt number
• if (number 0)
• cout ltlt "Bad input. Program terminated"
• return 1
• // otherwise continue processing

37
These are equivalent. Why?

• if (number 0) if (! number)
• . .
• . .
• . .
• . .
• Each expression is only true when number has
  value 0.

38
Write If or If-Else for each

• If taxCode is 7, increase price by adding taxRate
  times price to it
• If code has value 1, read values for income and
  taxRate from cin, and calculate and display
  taxDue as their product
• If A is strictly between 0 and 5, set B equal to
  1/A, otherwise set B equal to A

39
Possible Answers

• if (taxCode 7)
• price price taxRate price
• if ( code 1)
• cin gtgt income gtgt taxRate
• taxDue income taxRate
• cout ltlt taxDue
• if (( A gt 0)(A lt 5))
• B 1/A
• else
• B A

40
What output? and Why?

• int age
• age 30
• if ( age lt 18 )
• cout ltlt Do you drive?
• cout ltlt Too young to vote

41
What output? and Why?

• int code
• code 0
• if ( ! code )
• cout ltlt Yesterday
• else
• cout ltlt Tomorrow

42
What output? and Why?

• int number
• number 0
• if (number 0)
• cout ltlt "Zero value"
• else
• cout ltlt "Non-zero value"

43
Chained If

• The if clause else clause of an if...else
  statement can contain any kind of statement
• including another ifelse statement
• This is called multi-way branching

44
Chained if Statements

• if (Expression1)
• Statement_1
• else if (Expression2)
• Statement_2
• . . .
• else if (ExpressionN)
• Statement_N
• else
• Statement_N_plus_1
• EXACTLY 1 of these statements will be executed.

45
Chained if Statements

• Each Expression is evaluated in sequence, until
  some Expression is found that is true
• Only the specific Statement following that
  particular true Expression is executed
• If no Expression is true, the Statement following
  the final else is executed
• The final else and final Statement are optional
• If omitted, when no Expression is true no
  Statement is executed

46
Multi-way Branching

• if (creditsEarned gt 90)
• cout ltlt "SENIOR STATUS"
• else if (creditsEarned gt 60)
• cout ltlt "JUNIOR STATUS"
• else if (creditsEarned gt 30)
• cout ltlt "SOPHOMORE STATUS"
• else
• cout ltlt "FRESHMAN STATUS"

47
Writing Chained if Statements

• Display one word to describe the int value of
  number as Positive, Negative, or Zero
• Your city classifies a pollution index
• less than 35 as Pleasant
• 35 through 60 as Unpleasant
• and above 60 as Health Hazard
• Display the correct description of the pollution
  index value

48
One Answer

• if (number gt 0)
• cout ltlt "Positive"
• else if (number lt 0)
• cout ltlt "Negative"
• else
• cout ltlt "Zero"

49
Other Answer

• if ( index lt 35 )
• cout ltlt Pleasant
• else if ( index lt 60 )
• cout ltlt Unpleasant
• else
• cout ltlt Health Hazard

50
Write a void Function

• called DisplayMessage (which you can call from
  main) to describe the pollution index value it
  receives as an argument
• Write a driver program to test this function
• Your city describes a pollution index
• less than 35 as Pleasant
• 35 through 60 as Unpleasant
• above 60 as Health Hazard

51

• void DisplayMessage(int index)
• if (index lt 35)
• cout ltlt "Pleasant"
• else if (index lt 60)
• cout ltlt "Unpleasant"
• else
• cout ltlt "Health Hazard"

52
The Driver Program

• include ltiostreamgt
• using namespace std
• void DisplayMessage (int) //prototype
• int main ()
• int pollutionIndex //declare variable
• cout ltlt "Enter air pollution index"
• cin gtgt pollutionIndex
• DisplayMessage(pollutionIndex) //call
• return 0

53
Nested Decisions
if(isHungry(Lisa) isHungry(Ed))
if(isHere(Boss) isHungry(Boss))
eat(FOOD_FANCY) shiftAppts(12.0,pm,2.0,pm)
else if(isHere(Jim)
isHere(Lori)) eat(FOOD_CAFETERIA)
else
if(getTime() gt MID_DAY)
eat(FOOD_BROWNBAG) else
cancelLunch()

• Sometimes you will need more than two parts.
• Use nested if/else statements(can get very
  messycareful indenting helps)