if Statement if (amount <= balance) balance = balance - amount;

if/else Statement if (amount <= balance) balance = balance - amount; else balance = balanceOVERDRAFT_PENALTY;

Block Statement if (amount <= balance) { double newBalance = balance - amount; balance = newBalance; } Note: block allows more than one ‘statement’ to be combined, to form a new ‘statement’

Equality Testing vs. Assignment  The = = operator tests for equality: if (x = = 0).. // if x equals zero  The = operator assigns a value to a variable: x = 0; // assign 0 to x  Don't confuse them.  Java = = is the same as mathematical =

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);

 It prints: sqrt(2)squared minus 2 is not 0 but E-16  Don't use == to compare floating-point numbers

Comparing Floating-Point Numbers  Two numbers are close to another if |x - y| <= ε  ε is a small number such as  Not good enough if x, y very large or very small. Then use |x - y| / max(|x|, |y|) <= ε  But if one of the numbers is zero, don't divide by max(|x |, |y|)

String Comparison  Don't use = = for strings! if (input = = "Y") // WRONG!!!  Use equals method: if (input.equals("Y"))  = = tests identity, equals tests equal contents  Case insensitive test ("Y" or "y") if (input.equalsIgnoreCase("Y"))

Lexicographic Comparison  s.compareTo(t) < 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"

Lexicographic Comparison

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)

Object Comparison

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 button: String input = JOptionPane.showInputDialog("..."); if (input != null) {... }  null is not the same as the empty string ""

Multiple Alternatives if (condition1) statement1; else if (condition2) statement2; else if (condition3) statement3; else statement4; The first matching condition is executed. Order matters.

Multiple Alternatives  Order matters: if (condition1) Statement1; else if (condition2) Statement2; else if (condition3) Statement3; else // option – executes of all conditions fail Statement4;

Nested Branches Branch inside another branch if (condition1) { if (condition1a) statement1a; else statement1b; } else statement2;

The boolean Type  George Boole ( ): pioneer in the study of logic  value of expression amount < 1000 is true or false.  boolean type: set of 2 values, true and false

Predicate Method  return type boolean  Example: public boolean isOverdrawn() { return balance < 0; }  Use in conditions if (harrysChecking.isOverdrawn())...  Useful predicate methods in Character class: isDigit isLetter isUpperCase isLowerCase  if (Character.isUpperCase(ch))...

Boolean Operators  ! not  && and (short circuited)  || or (short circuited)

Truth Tables

Boolean Variables  private boolean married;  Set to truth value: married = input.equals("M");  Use in conditions: if (married)... else... if (!married)...  Also called flag  Don't test Boolean variables against truth values-- sign of cluelessness: if (married == true) // DON'T if (married == false) // DON'T if (married != false) // NO!!!

Investment with Compound Interest Invest $10,000, 5% interest, compounded annually When will the balance be at least $20,000?

while Statement while (condition) statement; //repeats the statement while the condition is true while (balance < targetBalance){ year++; double interest = balance * rate / 100; balance = balance + interest; }

Flowchart for while Loop

Common Error: Infinite Loop while (year < 20) { balance = balance + balance * rate / 100; } while (year > 0) { year++; // oops, meant -- } Loops run forever--must kill program

Common Error: Off-by-1 Error year = 0; while (balance < 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 < or <=?

Avoiding Off-by-1 Error  Run through a simple example: target balance = $20,000, interest rate 50% after one year: balance = $15,000 after two years: balance = $22,500 Therefore: year must start at 0  interest rate 100% after one year: balance = $20,000 loop should stop Therefore: must use <  Think, don't compile and try at random

do Statement Executes loop body at least once: do statement while (condition); Example: Validate input double value; do{ String input = JOptionPane.showInputDialog( "Please enter a positive number"); value = Integer.parseInt(input); } while (input <= 0);

Flowchart for do Loop

for (initialization; condition; update) statement  Example: for (int i = 1; i <= n; i++){ double interest = balance * rate / 100; balance = balance + interest; }  Equivalent to: initialization; while (condition){ statement; update; } for Statement

Flowchart for for Loop

Common Errors: Semicolons A semicolon that shouldn't be there sum = 0; for (i = 1; i <= 10; i++); sum = sum + i; System.out.println(sum);

Common Errors: Semicolons A semicolon that shouldn't be there sum = 0; i = 1; while ( i <= 10) ; { sum = sum + i; i = i + 1; } System.out.println(sum);

Nested Loops  Create triangle pattern [] [][] [][][] [][][][]  Loop through rows for (int i = 1; i <= n; i++) { // make triangle row }

 Make triangle row is another loop for (int j = 1; j <= i; j++) r = r + "[]"; r = r + "\n";  Put loops together => Nested loops for (int i = 1; i <= width; i++) { // make triangle row for (int j = 1; j <= i; j++) r = r + "[]"; r = r + "\n"; }

Reading Input Values General pattern: boolean done = false; while (!done) { String input = read input; if (end of input indicated) done = true; else { process input } } "Loop and a half"

Another option… General pattern: while (end of input != read input ){ process input }

String Tokenization  StringTokenizer breaks up string into tokens  By default, white space separates tokens  " " breaks into three tokens: "4.3", "7", "-2" StringTokenizer tokenizer = new StringTokenizer(a String); while (tokenizer.hasMoreTokens()) { String token = tokenizer.nextToken(); process token }

Traversing Characters in String s.charAt(i) is the ith character of the string s for (int i = 0; i < s.length(); i++) { char ch = s.charAt(i); process ch }