Post on 26-Mar-2015
Chapter 4Decision Making
Lecture Slides to Accompany
An Introduction to Computer Science Using Java (2nd Edition)
by
S.N. Kamin, D. Mickunas, E. Reingold
Chapter Preview
In this chapter we will:• discuss the use of decision making in
computer programming• describe the use of the Java if and switch
statements• describe the use of Boolean expressions in
Java if statements• discuss the use of integer selector variables
in Java switch statements
if Statement
• Ensures that a statement is executed only when a condition is true
• Conditions typically involve comparison of variables or quantities for equality or inequality
• Example: if (age >= 18)
out.println(“You are eligible to vote.”);
Relational Operators
< < less than
> > greater than
<= less than or equal to
>= greater than or equal to
= == equal to
!= not equal to
if Statement with Optional else
• An if statement may have an optional else clause that will only be executed when the condition is false
• Example:
if (wages <= 57600)
tax = 0.124 * wages;
else
tax = 0.124 * 57600;
Compound Alternatives
• To execute more than one statement conditionally, you may use { } to define a compound statement for either (or both) conditional alternatives
• Example:
if (firstNumber <= secondNumber) {
quotient = secondNumber / firstNumber;
remainder = secondNumber % firstNumber;
}
else {
quotient = firstNumber / secondNumber;
remainder = firstNumber % secondNumber;
}
Cascading if-else Statements
• Example:
if (condition-1)
statement-1;
else
if (condition-2)
statement-2;
else
statement-3;
• Another format:
if (condition-1)
statement-1;
else if (condition-2)
statement-2;
else
statement-3;
Dangling else
• Code written:
if (condition-1)
if (condition-2)
statement-1;
else
statement-2;
• Interpreted as:
if (condition-1)
if (condition-2)
statement-1;
else
statement-2;
Correcting the Problem
• Code written:
if (condition-1) {
if (condition-2)
statement-1;
}
else
statement-2;
• Interpreted as:
if (not condition-1)
statement-2;
else if (condition-2)
statement-1;
Boolean Operators• Logical “and” (conjunction)
– true only when both expressions are true(MINIMUM_WAGE <= wages) && (wages <= MAXIMUM_WAGE)
• Logical inclusive “or” (disjunction)– true when either or both expressions are true(MINIMUM_WAGE < wages) || (MINIMUM_WAGE == wages)
• Logical exclusive “or”– true when exactly one of the expressions is true(MINIMUM_WAGE < wages) ^ (MINIMUM_WAGE == wages)
• Logical “not” (negation)(MINIMUM_WAGE != wages)
Boolean Operator Truth Table
A B A && B A || B A^B !A
True True True True False False
True False False True True False
False True False True True True
False False False False False True
Operator Precedence Rules
1. ! - (unary)
2. * / %
3. + -
4. < <= > >=
5. == !=
6. ^
7. &&
8. ||
Complicated Boolean Expressions
boolean isLeapYear = ((year % 4) == 0)
&& ((year % 100) != 0)
|| ((year % 400) == 0);
// Assume all months have 31 days
dayNumber = (month - 1) * 31 + day;
// Correct for months beyond February
if (month > 2) {
dayNumber = dayNumber - ((4 * month +23) / 10);
if (isLeapYear)
// Correct for leap year
dayNumber = dayNumber + 1;
}
Comparing Strings
• Comparison is done by comparing strings character-by-character left to right, the first character that differs dictates which string is smaller lexicographically
• What if one string is a prefix of the other?– The prefix is smaller than the longer string
• How do upper- and lower-case characters compare?– If a case-sensitive comparison is used lower-case
is always less than upper case• How do special characters (e.g. %) compare?
– Decided by the character ASCII representation
Java String Class Comparison Methods
Example Explanation
int compareTo(String a) Compare two stringslexigraphically, returns<0, ==0, >0 values
int compareToIgnoreCase(String a) Compare two stringslexigraphically, ignoring caseconsiderations
boolean equals(String a) Returns true is the stringsare equal and falseotherwise
Boolan equalsIgnoreCase(String a) Returns true is the stringsare equal, ignoring case, andfalse otherwise
switch Statement• Used to accomplish multi-way branching based on
the value of an integer selector variable• Example:
switch (numberOfPassengers) { case 0: out.println(“The Harley”);
break;
case 1: out.println(“The Dune Buggy”);
break;
default: out.println(“The Humvee”);
}
Why is break used in switch statements?
• Consider the code fragment below
int i = 1; switch (i) {
case 0: out.println(“0”);
case 1: out.println(“1”);
case 2: out.println(“2”);
case 3: out.println(“3”);
}
out.println( );
• Without breaks the output is: 123
Symbolic Constants in switch Statementsfinal int SUNDAY = 1, MONDAY = 2, TUESDAY = 3, WEDNESDAY = 4, THURSDAY = 5, FRIDAY = 6, SATURDAY = 7;ind d;...switch (d) { case SUNDAY: out.print(“Sunday”); break; case MONDAY: out.print(“Monday”); break; case TUESDAY: out.print(“Tuesday”); break; case WEDNESDAY: out.print(“Wednesday”); break; case THURSDAY: out.print(“Thursday”); break; case FRIDAY: out.print(“Friday”); break; case SUNDAY: out.print(“Sunday”); break;}
Multiple case Labels in switch Statements
switch (d) { case MONDAY: case WEDNESDAY: case FRIDAY: out.println(“C.S. meets at 9:00 today”); out.println(“Math meets at 10:00 today”); break; case TUESDAY: case THURSDAY: out.println(“English meets at 9:00 today”); out.println(“Chemistry meets at 10:00 today”); break; case SUNDAY: case Saturday out.println(“Enjoy the weekend”);}
Comparing switch and if statements
• switch statement
switch (expression) {
case value-1: statement-1; break;
case value-2: statement-2; break;
…
case value-i: statement-i; break;
default: statement-(i+1);
}
• if equivalent
switchValue = expression;
if (switchValue == value-1)
statement-1;
else if (switchValue == value-2)
statement-2;
…
else if (switchValue == value-i)
statement-i;
else
statement-(i+1);
Building Classes with Multiple Methods
public class Classname {
// Author, date, explanation
declarations of public variables
public void methodName ( parameters ) {
declarations of “local” variables
executable statements with relevant comments
}
public void methodName ( parameters ) {
declarations of “local” variables
executable statements with relevant comments
}
…
}