Decisions

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Chapter 5
Decisions
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Figure 1 A Decision
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if statement

• if(condition) statement
• if (amount lt balance) balance balance -
  amount
• statement blockif(amount lt balance) double
  newBalance balance - amount balance
  newBalance

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Figure 2 Alternative Conditions
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if/else statement

• if(condition) statementelse statement
• if (amount lt balance) balance balance -
  amountelse balance balance -
  OVERDRAFT_PENALTY

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Relational operators

• lt gt like in math
• lt gt correspond to ³
• ! correspond to ¹
• is not the same as if (x 5) . . .x 5

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Comparing floating-point numbers

• Roundoff errorsdouble r Math.sqrt(2)r r is
  2.0000000000000004, not 2r r 2 is false
• Test if x and y are close enoughx - y e, e
  a small value (e.g. 10-14.)
• Not good enough for very small or large values.
  Use x - y e max(x, y)

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String comparison

• Don't use for strings!if (input "Y") //
  WRONG!!!
• Use equals methodif (input.equals("Y"))
• tests identity, equals tests equal contents
• Case insensitive test ("Y" or "y")if
  (input.equalsIgnoreCase ("Y"))

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Comparing Objects

• tests for identity, equals for identical
  content
• Rectangle a new Rectangle(5, 10, 20,
  30)Rectangle b new Rectangle(5, 10, 20,
  30)
• a ! b, but a.equals(b)
• Caveat equals must be defined for the class (see
  chapter 9)

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Figure 4 Comparing Objects
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Lexicographic Comparison

• s.compareTo(t) lt 0 means s comes before t in the
  dictionary
• "car"comes before "cargo" comes before
  x"cathode".
• All uppercase letters come before lowercase
  "Hello" comes before "car"

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Figure 3 Lexicographic Comparison
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Multiple alternatives

• if (condition1) statement1else if
  (condition2) statement2else if (condition3)
  statement3else statement4
• The first matching condition is executed.Order
  matters.

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Program Richter.java public class Richter
public static void main(String args)
ConsoleReader console new ConsoleReader(System.i
n) System.out.println ("Enter a
magnitude on the Richter scale") double
magnitude console.readDouble()
Earthquake quake new Earthquake(magnitude)
System.out.println(quake.getDescription())
class Earthquake public Earthquake(double
magnitude) richter magnitude
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public String getDescription() String
r if (richter gt 8.0) r "Most
structures fall" else if (richter gt 7.0)
r "Many buildings destroyed"
else if (richter gt 6.0) r "Many
buildings considerably damaged, some collapse"
else if (richter gt 4.5) r "Damage
to poorly constructed buildings" else if
(richter gt 3.5) r "Felt by many
people, no destruction" else if (richter
gt 0) r "Generally not felt by
people" else r "Negative
numbers are not valid" return r
private double richter
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Nested branches

• if (condition1) if (condition1a)
  statement1a else statement1b

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Figure 7 Income Tax Computation
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Program Tax.java public class Tax public
static void main(String args)
ConsoleReader console new ConsoleReader(System.i
n) System.out.println("Please enter your
income") double income
console.readDouble() System.out.println("P
lease enter S for single, " "M for
married") String status
console.readLine() TaxReturn aTaxReturn
new TaxReturn(income, status)
System.out.println("The tax is "
aTaxReturn.getTax())
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class TaxReturn public TaxReturn(double
anIncome, String aStatus) income
anIncome status aStatus
public double getTax() double tax 0
final double RATE1 0.15 final double
RATE2 0.28 final double RATE3 0.31
final double SINGLE_CUTOFF1 21450
final double SINGLE_CUTOFF2 51900
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final double SINGLE_BASE2 3217.50 final double
SINGLE_BASE3 11743.50 final double
MARRIED_CUTOFF1 35800 final double
MARRIED_CUTOFF2 86500 final double
MARRIED_BASE2 5370 final double MARRIED_BASE3
19566 if (status.equalsIgnoreCase("S")) if
(income lt SINGLE_CUTOFF1) tax RATE1
income else if (income lt SINGLE_CUTOFF2)
tax SINGLE_BASE2 RATE2 (income -
SINGLE_CUTOFF1) else tax
SINGLE_BASE3 RATE3 (income -
SINGLE_CUTOFF2)
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else if (income lt
MARRIED_CUTOFF1) tax RATE1
income else if (income lt
MARRIED_CUTOFF2) tax MARRIED_BASE2
RATE2 (income -
MARRIED_CUTOFF1) else tax
MARRIED_BASE3 RATE3 (income -
MARRIED_CUTOFF2) return tax
private double income private String
status
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The boolean type

• George Boole (1815-1864) pioneer in the study of
  logic
• value of expression x lt 10 is true or false.
• boolean type one of these 2 truth values
• equals method has return type xboolean

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Boolean operators

• and or! not
• if (tday bday tmonth bmonth)...
• if (tmonth 4 tmonth 6 tmonth 9
  tmonth 11)...
• if (tmonth gt bmonth (tmonth bmonth tday
  gt bday))...

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Figure 8 Flowcharts for and Combinations
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Truth tables

• A B ABtrue true truetrue false
  falsefalse any false
• A B ABtrue any truefalse true
  truefalse false false
• A !Atrue falsefalse true

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De Morgan's Law

• !(A B)x is the same as !A !B
• !(A B)x is the same as !A !B
• if (!(country.equals("USA") !state.equals("AK)
  !state.equals("HI")))...
• if (!country.equals("USA") !!state.equals("AK)
  !!state.equals("HI"))...

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Boolean Variables

• boolean shipByAir falseif (!country.equals("US
  A")) shipByAir trueelse if
  (state.equals("AK") state.equals("HI"))
  shipByAir trueif (shipByAir) ... else ...
• Boolean variables are sometimes called flags

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Boolean do's and don'ts

• don't if (shipByAir true)...if (shipByAir
  ! false)...
• doif (shipByAir)...
• don'tif (balance lt 0) return true else return
  false
• doreturn balance lt 0

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Useful tips

• Brace layout
• Indentation and tabs
• Copy and paste
• Prepare test cases
• Make a schedule