1. Control Structures
special statements whose purpose is to change the order of execution in a program.
Sequential structures – Statements executed sequentially
Selection structures
Repetition structures (loop)
decision.ppt

2. Selection Given hours worked and pay rate, calculate total pay
How about if you work overtime?
How do you indicate if your work overtime?
How about double overtime?
decision.ppt

3. Selection Single-selection structure Double-selection structure
The if structure
selects or ignores a single action.
Double-selection structure
The if/else structure
selects between two different actions.
Multiple-selection structure
The switch structure
selects one of many possible actions (to be discussed later)
decision.ppt

4. What is truth?
0 is false
anything other than 0 is true
decision.ppt

5. Relational Operators < Less than <= less than or equal to >
greater than
>=
Greater than or equal to ==
equals !=
not equal to
decision.ppt

6. examples: int x = 4; int y = 6 ;
 EXPRESSION                 VALUE  x < y         true  x + 2 < y             false  x != y                true  x + 3 >= y            true  y == x                false  y == x + 2             true
'r' < 't' true
decision.ppt

7. Beware: == vs = Hard to find bug = assignment == test for equality
x = 5;
if (x == 5)
if (x=5)
Has two unexpected results
Assigns x a 5
Evaluates to true
Hard to find bug
decision.ppt

8. Java Operator Precedence
Description
operator
Unary
++, --, +, -
Multiplicative
*, /, %
Additive
+, -
Relational
<, >, <=, >=
Equality
==, !=
Assignment
=, +=, -=, *=, /=, %=
decision.ppt

9. Comparing strings str1.compareTo(str2)
Compares two string lexicographically
A negative integer value if str1 < str2
0 if str1 is equivalent to str2
A positive integer if str1 > str2
compareTo
public int compareTo(String anotherString)
The comparison is based on the Unicode value of each character in the strings. The character sequence represented by this String object is compared lexicographically to the character sequence represented by the argument string. The result is a negative integer if this String object lexicographically precedes the argument string. The result is a positive integer if this String object lexicographically follows the argument string. The result is zero if the strings are equal; compareTo returns 0 exactly when the equals(Object) method would return true.
This is the definition of lexicographic ordering. If two strings are different, then either they have different characters at some index that is a valid index for both strings, or their lengths are different, or both. If they have different characters at one or more index positions, let k be the smallest such index; then the string whose character at position k has the smaller value, as determined by using the < operator, lexicographically precedes the other string. In this case, compareTo returns the difference of the two character values at position k in the two string -- that is, the value:
this.charAt(k)-anotherString.charAt(k)
If there is no index position at which they differ, then the shorter string lexicographically precedes the longer string. In this case, compareTo returns the difference of the lengths of the strings -- that is, the value:
this.length()-anotherString.length()
Specified by:
compareTo in interface Comparable<String>
Parameters:
anotherString - the String to be compared.
Returns:
the value 0 if the argument string is equal to this string; a value less than 0 if this string is lexicographically less than the string argument; and a value greater than 0 if this string is lexicographically greater than the string argument.
decision.ppt

10. Example >String str1 = "Hello"; >String str2 = "Hi";
>System.out.println(str1.compareTo(str2)); -4
> System.out.println(str2.compareTo("Hi"));
> System.out.println(str2.compareTo("Big")); 6
decision.ppt

11. equals – tests objects for equality
str1.equals(str2)
str1.equalsIgnoreCase(str2);
if (s.equalsIgnoreCase("yes"))
decision.ppt

12. Logical Operators Operator Meaning Syntax && AND exp1 && exp2 || OR!
NOT
!exp1
decision.ppt

13. && - Logical AND P 
  Q 
P && Q
true
false
decision.ppt

14. || - Logical OR P Q
P || Q
true
false
decision.ppt

15. ! Not P !P
true
false
decision.ppt

16. Equivalent Expression
DeMorgan's Law:
    not(P and Q) is equivalent to (not P) or (not Q),  !(P && Q) = (!P) || (!Q)      not(P or Q) is equivalent to (not P) and (not Q),  !(P || Q) = (!P) && (!Q)
Expression
Equivalent Expression
!(P == Q)
P != Q
!(P == Q | | P == R)
P != Q && P != R
!(P == Q && P == R)
P != Q | | P != R
!(P == Q && R > S)
P !=Q | | R <=S
Note: left expression is equivalent to right expression with! added and the relational and logical operators reversed
decision.ppt

17. Java Operator Precedence
Description
operator
Unary
++, --, +, -, !
Multiplicative
*, /, %
Additive
+, -
Relational
<, >, <=, >=
Equality
==, !=
Assignment
=, +=, -=, *=, /=, %=
Logical And
&&
Logical Or || =
decision.ppt

18. Examples:
Do not depend on operator precedence while using expressions containing side-effects
x>3 && x++
5 > 2 && 4 > 7 false
5 > 2 || 4 < 7 true
!(4 < 7) false
if ((sex == 'F') && (cityCode == 18) && (gpa >= 3.8))
if ((zipCode.equals("48002")) ||
(zipCode.equals("48003")) ||
(zipCode.equals( "48004"))
!(hours > 40)  same as: hours <=  40     
decision.ppt

19. Short-Circuit Evaluation
evaluation proceeds from left to right
the computer stops evaluating sub-expressions as soon as the truth value of the entire expression can be determined
Exercise caution when the operands following the first operand contain side effects (see notes)
Rule of thumb:
first true in OR => Entire expression is true
first false in AND => Entire expression is false
The conditional AND and OR operators (&& and ||, respectively) exhibit short-circuit behavior. That is, the second operand is evaluated only when the result cannot be deduced solely by evaluating the first operand.
Exercise caution when the operands following the first operand contain side effects. In the following code, the value of i is incremented only when i >= 0.
int i = // initialize to user supplied value if ((i >= 0) && ((i++) <= Integer.MAX_VALUE)) { // ... }
Although the behavior is well defined, it is unclear whether i gets incremented.
For more information see:
EXP07-J. Be aware of the short-circuit behavior of the conditional AND and OR operators
decision.ppt

20. Short-Circuit Examples:
int age = 25; int weight = 150;
(age > 50) && (weight > 140)  // false
Evaluation can stop after determining that (age > 50) is false, it is already determined that the entire expression will be false.
(weight >= 150)||(age > 40)  // true
Evaluation can stop after determining that (weight >= 50) is true, it is already determined that the entire expression will be true.
(age < 50) && (weight > 140)  // true
After determining that (age < 50) is true, we have to determine the truth value of the second part.
(age < 20) || (weight < 140)  // false
After determining that (age < 20) is false, we have to determine the truth value of the second part.
decision.ppt

21. Exercises Write an expression for the following:
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
For what value(s) of x would this condition be true?
(x < 1) && (x > 10)
For what value(s) of y would this condition be true?
(y >= 1) && (y <= 10)
decision.ppt

22. Example x > y + 2 => x > (y + 2) x + 3 <= y * 10
a <= b > c < d >= e will be executed as:
( ) > c
( ) < d
( ) >= e
( )
decision.ppt

23. Relational Operators with Floating Point Types
Do not compare floating-point numbers for equality; approximate values, they are rarely exactly equal.
test for near equality
compute the difference between the 2 numbers and check if the result is less than some max difference.
Example:
float x = 1.0; float y = 3.0; float z; z = x/y; if (x == z * y)... // x will not be 1.0, but if (abs(x - z * y) < )... // close enough
decision.ppt

24. Compound Logical Expressions
if (a < b < c)
syntax error
In this expression, c is compared with 1 or 0 (the result of evaluating a < b
int  total = 50; if (80 < total < 90)      System.out.println("Final grade is B");
According to the precedence chart, the expression
(80 < total < 90) means (80 < total) < 90 
because "<" is left associative, (80 < total ) is false
(false < 90) is invalid.
if ((80 < total) && (total < 90)) //correction
decision.ppt

25. if Syntax if ( Expression ) Statement
NOTE: Statement can be a single statement, a null statement, or a block (compound statement).
decision.ppt

26. if statement is a selection
of whether or not to execute a statement (which can be a single statement or an entire block)
TRUE
expression
FALSE
statement
decision.ppt

27. if Single statement EX1: if (number % 2 == 0)
System.out.println("Even number!");
EX2: if (i)
{ //defensive programming
System.out.println(i is nonzero"; }
decision.ppt

28. compound statement Multiple statements where one is allowed
if (state == MD) {
amt = amt + (amt * .05);
cout <<amt; }
decision.ppt

29. if else syntax if (Expression )
StatementA
else
StatementB
NOTE: StatementA and StatementB each can be a single statement, a null statement, or a compound statement.
decision.ppt

30. 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
decision.ppt

31. Example: if (i >= 0) { System.out.println("positive"); } else
System.out.println("negative");
if (i >= 0)
System.out.println("positive");
else
System.out.println("negative");
braces may be omitted if there is only one clause:
Recommended - leaving the braces in case another statement is added.
decision.ppt

32. => Watch out for null statement
if (x > 0); //does nothing
if (x > 0); //hard to find bug System.out.println("Positive");
decision.ppt

33. Nested IF Statements
The body of an if-clause or else-clause can contain another if or if-else statement
this is called a nested control structure or a nested if
In general, any problem that involves a multiway branch (more than 2 alternative solutions) can use nested if statements.
decision.ppt

34. System.out.println("Positive"); else // nested if else if (x < 0)
System.out.println("Negative");
else
System.out.println("Zero");
decision.ppt

35. change indentation (remove whitespace)
if (x > 0)
System.out.println("Positive");
else if (x < 0) System.out.println("Negative");
else
System.out.println("Zero");
decision.ppt

36. 3 way choice version 1:separate ifs if (x > 0) posCount++;
negCount++;
if (x==0)
zeroCount++;
How many branches are tested, best case, worst case?
decision.ppt

37. 3 way choice Version 2: Nested ifs if (x > 0) posCount++; else
negCount++;
zeroCount++;
How many branches are tested, best case, worst case?
decision.ppt

38. 3 way choice
Version 3: Nested ifs, special indentation (preferred version)
if (x > 0)
posCount++;
else if (x < 0)
negCount++;
else
zeroCount++;
How many branches are tested, best case, worst case?
decision.ppt

39. else if if (cond) statement else if (cond) else
evaluated in order, if any expression is true, that statement and that statement only is executed.
only difference is whitespace
preferred method - shows we are making a choice
decision.ppt

40. Example: if (temp <= 32) System.out.println("freeze");
System.out.println("boil");
if ((temp >= 32) and (temp <= 212))
System.out.println("liquid");
if (temp <= 32)
System.out.println("freeze";
else if (temp >= 212) System.out.println("boil");
else
System.out.println("liquid");
decision.ppt

41. Situation Construct Basic form
You want to execute any combination of control statements
Sequentials ifs
if (<test1>)
<statement1>;
if (<test2>)
<statement2>;
if (<testn>)
<statementn>;
You want to execute zero or one of the control statements
Nested ifs ending in test
else if (<test2>)
else if (<testn>)
You want to execute exactly one of the controlled statements
Nested ifs ending in else
else
decision.ppt

42. Dangling else: else matches with most recent unmatched if
if (n > 0)
if (a > b)
z = a;
else
z = b;
if (n > 0)
if (a > b)
z = a;
else
z = b;
autoformatting in IDEs is a good way to catch
many of these kinds of errors
decision.ppt

43. Dangling else if ( num > 0 ) if ( num < 10 )
System.out.println("aaa") ;
else
System.out.println("bbb") ;
decision.ppt

44. Overtime example if (hours <= 40) totalPay = hours * payRate;
else if (hours <= 60) {
regPay = 40 * payRate;
otPay = (hours – 40) * payRate * 1.5;
totalPay = regPay + otPay; }
else
otPay = 20 * payRate * 1.5;
doubleOtPay =(hours – 60)* 2 * payRate;
totalPay = regPay + otPay + doubleOtPay;
decision.ppt

45. Overtime example if ((hours < 0 || (hours > 168))
System.out.println("Invalid");
else if (hours <= 40)
totalPay = hours * payRate;
else if (hours <= 60) {
regPay = 40 * payRate;
otPay = (hours – 40) * payRate * 1.5;
totalPay = regPay + otPay; }
else
otPay = 20 * payRate * 1.5;
doubleOtPay =(hours – 60)* 2 * payRate;
totalPay = regPay + otPay + doubleOtPay;
decision.ppt

46. Grade Example // Get grade System.out.println( "Enter the grade:");
int grade;
// Get grade
System.out.println( "Enter the grade:");
grade = console.nextInt();
System.out.println( "Your grade of " + grade + " is ");
// Print letter grade
if (grade >= 90)
System.out.println( "A." );
else if (grade > 80)
System.out.println( "B." );
else if (grade >= 70)
System.out.println( "C." );
else if (grade >= 60)
System.out.println( "D." );
else
System.out.println( "F." )
decision.ppt

47. ?: expression1 ? expression2:expression3 max = (a>=b) ? a : b;
if (a >= b)
max = a;
else
max = b;
decision.ppt

48. switch switch (expression) { case value1: statements break;…
default: }
decision.ppt

49. switch switch works with:
byte, short, char, and int primitive data types.
enumerated types
the String class,
and a few special classes that wrap certain primitive types: Character, Byte, Short, Int
All switch statements can be re-written as if..else
Not all if..else can be rewritten as switch
An if-then-else statement can test expressions based on ranges of values or conditions,
whereas a switch statement tests expressions based only on a single integer, enumerated value, or String object
decision.ppt

50. break
Each break statement terminates the enclosing switch statement. Control flow continues with the first statement following the switch block. The break statements are necessary because without them, statements in switch blocks fall through: All statements after the matching case label are executed in sequence, regardless of the expression of subsequent case labels, until a break statement is encountered.
decision.ppt

51. public static void main (String [] args) {
import java.util.*;
public class Switch {
public static void main (String [] args) {
Scanner console = new Scanner(System.in);
int yearLevel;
System.out.print("Enter year Level: ");
yearLevel = console.nextInt();
switch (yearLevel) {
case 1:
System.out.println("You are a freshman!");
break;
case 2:
System.out.println("You are a sophomore!");
case 3:
System.out.println("You are a junior!");
case 4:
System.out.println("You are a Senior!");
default:
System.out.println("Invalid entry!"); }
decision.ppt

52. import java.util.*;
public class SwitchDemo {
public static void main(String[] args) {
Scanner console = new Scanner(System.in);
System.out.print("Enter month: ");
int month = console.nextInt();
switch (month) {
case 1: System.out.println("January"); break;
case 2: System.out.println("February"); break;
case 3: System.out.println("March"); break;
case 4: System.out.println("April"); break;
case 5: System.out.println("May"); break;
case 6: System.out.println("June"); break;
case 7: System.out.println("July"); break;
case 8: System.out.println("August"); break;
case 9: System.out.println("September"); break;
case 10: System.out.println("October"); break;
case 11: System.out.println("November"); break;
case 12: System.out.println("December"); break;
default: System.out.println("Invalid month.");break; }
decision.ppt