Chapter 6 - Decisions

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Chapter 6 - Decisions
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Chapter Goals

• To be able to implement decisions using if
  statements
• To understand how to group statements into blocks
  
• To learn how to compare integers, floating-point
  numbers, strings, and objects
• To recognize the correct ordering of decisions in
  multiple branches
• To program conditions using boolean operators and
  variables

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Decisions

• So far, our examples havent made any decisions
• The same code gets executed no matter what
• Assignment 1
• Path
• BankAccount
• This is called sequential execution
• Is this how real applications work?

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Decisions

• The real world requires decisions
• Is it time to feed the cat? Can a book be
  checked out?
• Essential feature of nontrivial programs is to
  make decisions based on input
• The answers to these decision require different
  actions
• Introduces the need for control statements

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If statements

• Conditional statements are represented with
  if-then-else statements
• Scanner stdin new Scanner(System.in)
• int testScore stdin.nextInt()
• if (testScore lt 70)
• System.out.println(You are below the mean)
• else
• System.out.println(You are above the mean)

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If Statements Syntax

• if (ltboolean expressiongt)
• ltthen blockgt
• else
• ltelse blockgt

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Example

• withdraw() method we implemented allowed user to
  withdraw as much money as they wanted
• Is this realistic?
• What decision should withdraw() make?
• What should the decision be based on?

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Example

• public void withdraw(double amount)
• if (amount lt balance)
• balance - amount

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Example

• Most banks also charge an overdraft penalty, how
  would we go about changing our example to
  implement this?

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Statements

• The decision we made only resulted in one
  instruction whichever way we took
• Most decisions lead to a sequence of instructions
  (multiple statements).
• In order to show that the entire sequence of
  instructions belong to the then-block (or
  else-block), we group them with curly braces
• A sequence of instructions surrounded by curly
  braces is called a block

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Example

• if (amount lt balance)
• System.out.println(Legal Withdrawal)
• balance balance amount
• else
• balance balance OVERDRAFT_PENALTY

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Statements

• Block statement a group of statements enclosed
  within curly braces
• Methods
• Classes
• if statements, switch statements, loops
• Simple statement
• Basic statement such as balance balance
  amount
• Compound statement
• Statements that have multiple paths of execution
• If statements, loops

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If Statements

• If you have a single statement, not required
• if (testScore lt 70)
• System.out.println(You are below the mean)
• else
• System.out.println(You are above the mean)
• But convention to use them anyways
• Helpful for adding temporary output to check if a
  branch is executed
• Makes nested if-else statements easier to read

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If Statements Syntax

• if(condition) statement
• if (condition) statement1
• else statement2
• statement must be a statement - simple, compound,
  or block

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Style

• Use indentation to indicate nesting levels
• public class BankAccount
• public void withdraw()
• if (amount lt balance)
• balance - amount

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Die Example

• // Declare and create Die objects
• Die die1 new Die()
• Die die2 new Die()
• // Roll die objects
• die1.roll() die2.roll()
• // Save results in new variables
• int roll1 die1.getTop()
• int roll2 die2.getTop()

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Die Example

• // Test for doubles
• if ( roll1 roll2 )
• System.out.println( "Doubles" )
• // rest of program

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Die Example
roll1 roll2
true
System.out.println( doubles! )
false
... continue with rest of program
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Die Example

• // Test for doubles
• if ( roll1 roll2 )
• System.out.println( "Doubles" )
• else
• System.out.println(
• "Sorry, no doubles" )
• // rest of program

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Die Example
roll1 roll2
true
System.out.println( doubles! )
false
System.out.println( sorry ... )
... continue with rest of program
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Selection Operator

• Selection Operator
• condition ? value1 value2
• If the condition is true, value1 is returned.
• If the condition is false, value2 is returned.
• Example Absolute Value
• y (x gt 0) ? x -x

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Booleans

• boolean is a primitive data type that holds one
  of two values
• true
• false
• Any time a decision needs to be made, we make
  that decision using a boolean expression, which
  evaluates to a boolean value
• Note a mathematical expression returns a
  numerical value

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Boolean Expressions

• What operators are used for boolean expressions?
• lt less than
• lt less than or equal to
• gt greater than
• gt greater than or equal to
• equal to
• ! not equal to

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Boolean Expressions

• Boolean expressions are just like mathematical
  expressions, but they return true or false (not
  int, double, etc)
• Examples
• 3 8 lt 5 5
• 6 4 gt 0
• testScore lt 80
• testScore 2 gt 350

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If Statements

• When the computer reaches an if statement, it
  first evaluates the boolean expression
• If that expression is true, the ltthen blockgt is
  executed
• Otherwise, the ltelse blockgt is executed
• An if statement is called a branching statement
  because it branches to a block of code

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Comparisons

• Comparing whole numbers is trivial
• int a 5
• if (a 5)
• System.out.println(Wow this is easy!)
• else
• System.out.println(Java really sucks)

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Comparisons

• Comparing real numbers is more difficult because
  we have to take into account roundoff errors.
• double r Math.sqrt(2)
• double d r r 2
• if (d ! 0)
• System.out.println(Your equation is incorrect,
  the result is d)
• Your equation is incorrect, the result is
  4.440892098500626E-16

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Comparisons

• This is a problem, since logically the equation
  makes sense
• Lesson Floating point numbers may not be exact
• Solution Use range of values that are close
  enough
• Should be close to 0

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Comparisons

• To avoid roundoff errors, don't use or ! to
  compare floating-point numbers
• To compare floating-point numbers test whether
  they are close enough x - y e
• final double EPSILON 1E-14if (Math.abs(x -
  y) lt EPSILON)// x is approximately equal to y
• e is a small number such as 10-14

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Comparisons

• We can do comparisons with characters as well
• They work according to ASCII values
• char choice 'Q'
• if (choice lt 'Z') System.out.println("valid"
  ) else
• System.out.println("invalid")
• What is the output?

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Side Note Characters

• Remember that we said a char is one letter,
  digit, or symbol that the computer thinks of as a
  number.
• We can cast chars to int and vice versa
• Examples
• (int) 'D' -gt 68
• (char) 106 -gt 'j'
• (char) ('D' 1) -gt 'E'

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Comparisons

• Boolean operators can be used on objects, but do
  they mean the same thing?
• They probably dont do what you expect
• Primitive types vs. reference types (objects)
• To test two Strings, use equals() method

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String Comparisons

• Don't use for strings!if (input "Y") //
  WRONG!!!
• Use equals methodif (input.equals("Y"))
• string1.equals(string2)
• tests references, .equals() tests contents

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String Comparisons

• String s1 "Robby", s2
• s2 s1

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String Comparisons

• What is the result of
• s1 s2

Why?
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String Comparisons

• String s1 "Robby", s2
• s2 "Robby"

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String Comparisons

• What is the result of
• str1 str2

Why?
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String Comparisons

• String s1 "Robby", s2
• s2 new String( s1 )

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String Comparisons

• What is the result of
• s1 s2

Why?
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String Comparisons

• What is the result of
• s1.equals(s2)

Why?
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String Comparisons

• equalsIgnoreCase can be used to compare strings
  while ignoring case
• Case insensitive test ("Y" or "y")if
  (input.equalsIgnoreCase("Y"))
• string1.equalsIgnoreCase(string2) // returns
  boolean
• compareTo returns a number that tells which
  string comes before the other in the dictionary
• 0 indicates that the strings are the same

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String Comparisons

• string1.compareTo(string2)
• Returns lt 0 if string1 comes first
• Returns 0 if they are equal
• Returns gt 1 if string2 comes first
• Example "car" comes before "cargo"
• All uppercase letters come before lowercase
  "Hello" comes before "car"

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Comparisons

• tests for identity, equals() for identical
  content
• Most classes have an equals() method defined
• Rectangle box1 new Rectangle(5, 10, 20, 30)
• Rectangle box2 box1
• Rectangle box3 new Rectangle(5, 10, 20, 30)
• box1 box3 is false
• box1.equals(box3)is true
• box1 box2 is true
• Note equals() must be defined for the class for
  this to work

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Null

• Reference variables store a reference (address)
  for an actual object
• What do they store when they havent been set to
  refer to an object?
• null is a Java reserved word to designate that no
  object has been set for that variable

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Null

• Can be used in tests
• if (middleInitial null)System.out.println(firs
  tName " " lastName)
• elseSystem.out.println(firstName " "
  middleInitial ". " lastName)
• Use , not equals, to test for null
• null is not the same as the empty string ""

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Multiple Alternatives

• if (score gt 90)
• System.out.println("Great!")
• else if (score gt 80)
• System.out.println("Good")
• else if (testScore gt 70)
• System.out.println("OK")
• else if (testScore gt 60)
• System.out.println("Passing")
• else if (testScore gt 50)
• System.out.println("Hmm...")
• else if (testScore gt 0)
• System.out.println("Study!")
• else
• System.out.println("Invalid Score")

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Multiple alternatives

• First condition that is true is the ONLY
  statement executed
• Therefore, order matters
• if (testScore gt 90)
• System.out.println(Great!)
• else if (testScore gt 80)
• System.out.println(Good)
• else if (testScore gt 70)
• System.out.println(OK)

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Multiple Alternatives

• Wrong order here causes problems!
• if (testScore gt 0)
• System.out.println(Study!)
• else if (testScore gt 50)
• System.out.println(Hmm...)
• else if (testScore gt 60)
• System.out.println(Passing)

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Multiple Alternatives

• if (testScore gt 90)
• System.out.println(Great!)
• if (testScore gt 70)
• System.out.println(OK)
• if (testScore gt 50)
• System.out.println(Hmm...)
• These aren't exclusive statements anymore
• Else matters!!!

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Switch Statements

• switch ( ltvariable namegt )
• ltcase label 1gt ltcase body 1gt
• ...
• ltcase label ngt ltcase body ngt
• Can only be used on integers, characters, or
  enumerated constants
• Good for testing multiple values for one
  expression

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Switch Statements
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Switch Statements

• The break statement causes execution to skip the
  remaining portion of the switch statement and
  resume execution following the switch statement.
• The break statement is necessary to execute
  statements in one and only one case.

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Switch Statements

• switch( c )
• case y System.out.println(Yes)
• break
• case n System.out.println(No)
• break
• default System.out.println(Invalid
  entry)

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Nested Decisions

• then and else blocks can contain as many
  statements as needed
• These blocks can also have another if statement
• An if statement inside of another if statement is
  called a nested-if statement

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Nested Decisions

• if (testScore gt 70)
• if (studentAge lt 10)
• System.out.println(You did a great job)
• else
• System.out.println(You did pass)
• //test score gt70 and age gt10
• else //test score lt 70
• System.out.println(You did not pass)

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Tax Schedule Example
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Tax Schedule Example

• Compute taxes due, given filing status and income
  figure
• branch on the filing status
• for each filing status, branch on income level
• The two-level decision process is reflected in
  two levels of if statements
• We say that the income test is nested inside the
  test for filing status

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• if (status SINGLE)
• if (income lt SINGLE_BRACKET1)
• tax RATE1 income
• else if (income lt SINGLE_BRACKET2)
• tax RATE1 SINGLE_BRACKET1
• RATE2 (income - SINGLE_BRACKET1)
• else
• tax RATE1 SINGLE_BRACKET1
• RATE2 (SINGLE_BRACKET2 -SINGLE_BRACKET1)
• RATE3 (income - SINGLE_BRACKET2)

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• else
• if (income lt MARRIED_BRACKET1)
• tax RATE1 income
• else if (income lt MARRIED_BRACKET2)
• tax RATE1 MARRIED_BRACKET1
• RATE2 (income - MARRIED_BRACKET1)
• else
• tax RATE1 MARRIED_BRACKET1
• RATE2 (MARRIED_BRACKET2 - MARRIED_BRACKET1)
  
• RATE3 (income - MARRIED_BRACKET2)

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Dangling Else

• There is a good reason we always use curly braces
  in our programs
• Very common error can be made
• if (testScore gt60)
• if(testScore lt 80)
• System.out.println(You are in safe range)
• else // Pitfall!
• System.out.println(You may be in trouble)

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Dangling Else

• Which if statement does the else apply to?
• Always the closest unclosed if
• if (testScore gt60)
• if(testScore lt 80)
• System.out.println(You are in safe range)
• else
• System.out.println(You may be in trouble)

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Using Boolean Expressions

• Recall that the last primitive that we really
  haven't said much about is the boolean data type
• double temp 100
• boolean isHot temp gt 60
• System.out.println(Is it hot? Ans isHot)

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Predicate Methods

• A predicate method returns a boolean valuepublic
  boolean isOverdrawn()   return balance lt 0
  
• Use to test conditions just like another other
  method
• if (harrysChecking.isOverdrawn()) . . .

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Predicate Methods

• Many predicate methods
• String class equals()
• Character class isUpperCase(), isDigit(),
  isLetter(), isLetter(), isLowerCase()
• All of these are public static methods
• Convention is to prefix "is" or "has"
• Much like accessors use get and mutators use
  set

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Example

• Scanner class has a hasNextInt() method which
  allows you to test if there is an int before you
  get it (and cause program to crash)
• Scanner in new Scanner(System.in)
• int score
• System.out.println(Enter Score)
• if(in.hasNextInt())
• score in.nextInt()

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Boolean Operators

• What if we want one expression to test multiple
  conditions?
• Example Does a student have a score above 50
  and below 75?
• Need boolean operators

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Boolean Operators

• Can use a logical operator to test simultaneously
• AND true if both are true, false
  otherwise
• OR true if either is true, false
  otherwise
• ! NOT true of the expression is false,
• false if it is true

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Boolean Operators
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Boolean Operators

• Example
• if (testScore gt 50 testScore lt 75)
• System.out.println(In range C to D range)
• Example
• char c
• if (!(c !))
• System.out.print(c)

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• Many ways to write one expression
• if (age lt 0)
• System.out.print(valid)
• else
• System.out.print(invalid)
• -------------------------------------
• if (!(age gt 0))
• System.out.print(valid)
• else
• System.out.print(invalid)
• ----------------------------------
• if (age gt 0)
• System.out.print(invalid)
• else
• System.out.print(valid)

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Order of Operations

• Like before, boolean operators are evaluated left
  to right
• Example Suppose y has the value 0
• x / y gt z y 0 // Run time error, divide by
  // zero
• y 0 x / y gt z // Legal

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Precedence Table
Precedence Group Operator Associativity
9 subexpression ( ) inner to outer left to right
8 unary ! - right to left
7 multiplicative / left to right
6 additive - left to right
5 relational lt lt gt gt left to right
4 equality ! left to right
3 AND left to right
2 OR left to right
1 assignment right to left
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Example

• In math, we represent the range of x as
• 80 x lt 90
• How do we represent this as a boolean expression?
  (Be careful)

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Boolean Expressions

• Check validity of data
• Divide by zero, out or range errors
• Classify data
• Test above mean, below mean
• Flags
• Store system settings or user preferences
• Long messages, noise off, debug mode

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Style

• Equivalent statements given a boolean isRaining
• if (isRaining true)
• if (isRaining) //Preferred
• Use good identifier names
• isMoving vs. motionStatus
• isCheckedOut vs. bookStatus
• MOST COMMON ERROR!
• if (x 5)