if Statement

1
if Statement

• if (amount lt balance)
• balance balance - amount

2
if/else Statement

• if (amount lt balance) balance balance -
  amount
• else balance balanceOVERDRAFT_PENALTY

3
Block Statement

• if (amount lt balance)
• double newBalance balance - amount
  balance newBalance
• Note block allows more than one statement
• to be combined, to form a new statement

4
Equality Testing vs. Assignment

• The operator tests for equalityif (x 0)
  . . // if x equals zero
• The operator assigns a value to a variablex
  0 // assign 0 to x
• Don't confuse them.
• Java is the same as mathematical

5
Comparing Floating-Point Numbers

• Consider this code
• double r Math.sqrt(2) double d r r -2
  if (d 0) System.out.println(
  "sqrt(2)squared minus 2 is 0") else
  System.out.println( "sqrt(2)squared minus 2
  is not 0 but " d)

6

• It printssqrt(2)squared minus 2 is not 0 but
  4.440892098500626E-16
• Don't use to compare floating-point numbers

7
Comparing Floating-Point Numbers

• Two numbers are close to another if x - y lt e
• e is a small number such as 10-14
• Not good enough if x, y very large or very small.
  Then usex - y / max(x, y) lt e
• But if one of the numbers is zero, don't divide
  by max(x , y)

8
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"))

9
Lexicographic Comparison

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

10
Lexicographic Comparison
11
Object Comparison

• tests for identical object
• equals for identical object content
• Rectangle cerealBox new
  Rectangle(5, 10, 20, 30)Rectangle oatmealBox
  new Rectangle(5, 10, 20,
  30)Rectangle r cerealBox
• cerealBox ! oatmealBox, but
  cerealBox.equals(oatmealBox)
• cerealBox r
• Caveat equals must be defined for the
  class (chap 11)

12
Object Comparison
13
The null Reference

• null reference refers to no object at all
• Can be used in tests if (account null)
  . . .
• Use , not equals, to test for null
• showInputDialog returns null if the user hit the
  cancel buttonString input JOptionPane.showInpu
  tDialog("...")if (input ! null) ...
• null is not the same as the empty string ""

14
Multiple Alternatives

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

15
Multiple Alternatives

• Order matters
• if (condition1)
• Statement1
• else if (condition2)
• Statement2
• else if (condition3)
• Statement3
• else // option executes of all conditions
  fail
• Statement4

16
Nested Branches

• Branch inside another branch
• if (condition1) if (condition1a)
  statement1a else statement1belse
  statement2

17
The boolean Type

• George Boole (1815-1864) pioneer in the study of
  logic
• value of expression amount lt 1000 is true or
  false.
• boolean type set of 2 values, true and false

18
Predicate Method

• return type boolean
• Examplepublic boolean isOverdrawn() return
  balance lt 0
• Use in conditionsif (harrysChecking.isOverdrawn()
  ) ...
• Useful predicate methods in Character
  classisDigitisLetterisUpperCaseisLowerCase
• if (Character.isUpperCase(ch)) ...

19
Boolean Operators

• ! not
• and (short circuited)
• or (short circuited)

20
Truth Tables
21
Boolean Variables

• private boolean married
• Set to truth valuemarried input.equals("M")
• Use in conditionsif (married) ... else ...if
  (!married) ...
• Also called flag
• Don't test Boolean variables against truth
  values--sign of cluelessnessif (married
  true) // DON'Tif (married false) // DON'Tif
  (married ! false) // NO!!!

22
Investment with Compound Interest
Invest 10,000, 5 interest, compounded annually
When will the balance be at least 20,000?
23
while Statement

• while (condition) statement//repeats
  the statement while the condition is truewhile
  (balance lt targetBalance) year double
  interest balance rate / 100 balance
  balance interest

24
Flowchart for while Loop
25
Common Error Infinite Loop

• while (year lt 20)
• balance balance balance rate / 100
• while (year gt 0)
• year // oops, meant --
• Loops run forever--must kill program

26
Common Error Off-by-1 Error

• year 0while (balance lt targetBalance)
• year double interest balance
  rate / 100 balance balance interest
  System.out.println("Reached target after "
  year " years.")
• Should year start at 0 or 1?
• Should the test be lt or lt?

27
Avoiding Off-by-1 Error

• Run through a simple exampletarget balance
  20,000, interest rate 50after one year
  balance 15,000after two years balance
  22,500Therefore year must start at 0
• interest rate 100after one year balance
  20,000loop should stopTherefore must use lt
• Think, don't compile and try at random

28
do Statement

• Executes loop body at least once
• do statement
• while (condition)
• Example Validate input
• double valuedo String input
  JOptionPane.showInputDialog( "Please enter
  a positive number") value
  Integer.parseInt(input) while (input lt 0)

29
Flowchart for do Loop
30

for Statement

• for (initialization condition update)
  statement
• Example for (int i 1 i lt n i) double
  interest balance rate / 100 balance
  balance interest
• Equivalent to
• initialization
• while (condition)
• statement
• update

31
Flowchart for for Loop
32
Common Errors Semicolons

• A semicolon that shouldn't be there
• sum 0for (i 1 i lt 10 i) sum
  sum iSystem.out.println(sum)

33
Common Errors Semicolons

• A semicolon that shouldn't be there
• sum 0 i 1
• while ( i lt 10)
• sum sum i
• i i 1
• System.out.println(sum)

34
Nested Loops

• Create triangle pattern
• Loop through rowsfor (int i 1 i lt n i)
  // make triangle row

35

• Make triangle row is another loop
• for (int j 1 j lt i j) r r ""r
  r "\n"
• Put loops together gt Nested loops
• for (int i 1 i lt width i)
• // make triangle row
• for (int j 1 j lt i j)
• r r ""
• r r "\n"

36
Reading Input Values

• General pattern
• boolean done falsewhile (!done) String
  input read input if (end of input
  indicated) done true else
  process input
• "Loop and a half"

37
Another option

• General pattern
• while (end of input ! read input )
  process input

38
String Tokenization

• StringTokenizer breaks up string into tokens
• By default, white space separates tokens
• "4.3 7 -2" breaks into three tokens"4.3", "7",
  "-2"
• StringTokenizer tokenizer new
  StringTokenizer(a String)while
  (tokenizer.hasMoreTokens()) String token
  tokenizer.nextToken() process token

39
Traversing Characters in String

• s.charAt(i) is the ith character of the string s
• for (int i 0 i lt s.length() i) char ch
  s.charAt(i) process ch