03 Selections

Motivations
If you assigned a negative value for radius in ComputeAreaWithConsoleInput.java, the program would print an invalid result. If the radius is negative, you don't want the program to compute the area. How can you deal with this situation?

Objectives
To declare boolean variables and write Boolean expressions using relational operators (§3.2).
To implement selection control using one-way if statements (§3.3).
To implement selection control using two-way if-else statements (§3.4).
To implement selection control using nested if and multi-way if statements (§3.5).
To avoid common errors and pitfalls in if statements (§3.6).
To generate random numbers using the Math.random() method (§3.7).
To program using selection statements for a variety of examples (SubtractionQuiz, BMI, ComputeTax) (§§3.7–3.9).
To combine conditions using logical operators (&&, ||, and !) (§3.10).
To program using selection statements with combined conditions (LeapYear, Lottery) (§§3.11–3.12).
To implement selection control using switch statements (§3.13).
To write expressions using the conditional expression (§3.14).
To examine the rules governing operator precedence and associativity (§3.15).
To apply common techniques to debug errors (§3.16).

The boolean Type and Operators
The boolean data type declares a variable with the value either true or false.
Often in a program you need to compare two values, such as whether i is greater than j. Java provides six comparison operators (also known as relational operators) that can be used to compare two values. The result of the comparison is a Boolean value: true or false.
boolean b = (1 > 2);

Relational Operators

Problem: Addition Question
AdditionQuiz
This example creates a program to let a first grader practice additions. The program randomly generates two single-digit integers number1 and number2 and displays a question such as "What is 7 + 9 = " to the student. After the student types the answer, the program displays a message to indicate whether the answer is true or false.
Animation

if Statements (One Condition)
if (boolean-expression) {
statement(s);
}
if (radius >= 0) { area = radius * radius * PI;
System.out.println("The area"
+ " for the circle of radius "
+ radius + " is " + area);
}
An if statement is a construct that enables a program to specify alternative paths of execution.

Note

Simple if Demo
SimpleIfDemo
Write a program that prompts the user to enter an integer. If the number is a multiple of 5, print HiFive. If the number is divisible by 2, print HiEven.
Animation

İf-else Statement (2 conditions)
if (boolean-expression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}
An if-else statement decides the execution path based on whether the condition is true or false.

if-else Example
if (radius >= 0) {
area = radius * radius * 3.14159;
System.out.println("The area for the “
+ “circle of radius " + radius +
" is " + area);
}
else {
System.out.println("Negative input");
}

if Statements (more than 2 conditions)

if Statement (more than 2 conditions)

Trace if-else statement
if (score >= 90.0)
System.out.print("A");
else if (score >= 80.0)
System.out.print("B");
else if (score >= 70.0)
System.out.print("C");
else if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
Suppose score is 70.0
The condition is false

Trace if-else statement
if (score >= 90.0)
System.out.print("A");
else if (score >= 80.0)
System.out.print("B");
else if (score >= 70.0)
System.out.print("C");
else if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
Suppose score is 70.0
The condition is false

Trace if-else statement
if (score >= 90.0)
System.out.print("A");
else if (score >= 80.0)
System.out.print("B");
else if (score >= 70.0)
System.out.print("C");
else if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
Suppose score is 70.0
The condition is true

Trace if-else statement
if (score >= 90.0)
System.out.print("A");
else if (score >= 80.0)
System.out.print("B");
else if (score >= 70.0)
System.out.print("C");
else if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
Suppose score is 70.0
grade is C

Trace if-else statement
if (score >= 90.0)
System.out.print("A");
else if (score >= 80.0)
System.out.print("B");
else if (score >= 70.0)
System.out.print("C");
else if (score >= 60.0)
System.out.print("D");
else
System.out.print("F");
Suppose score is 70.0
Exit the if statement

Note
The else clause matches the most recent if clause in the same block.

Note, cont.
Nothing is printed from the preceding statement. To force the else clause to match the first if clause, you must add a pair of braces:
int i = 1;
int j = 2;
int k = 3;
if (i > j) {
if (i > k)
System.out.println("A");
}
else
System.out.println("B");
This statement prints B.
Without block anything is printed.

Common Errors
Adding a semicolon at the end of an if clause is a common mistake.
if (radius >= 0);
{
area = radius*radius*PI;
System.out.println(
"The area for the circle of radius " +
radius + " is " + area);
}
This mistake is hard to find, because it is not a compilation error or a runtime error, it is a logic error.
This error often occurs when you use the next-line block style.
Wrong

TIP

CAUTION

Problem: Subtraction Question
This example creates a program to teach a first grade child how to learn subtractions. The program randomly generates two single-digit integers number1 and number2 with number1 >= number2 and displays a question such as “What is 9 – 2 = ?” to the student. After the student types the answer, the program displays whether the answer is correct.
SubtractionQuiz

Problem: Body Mass Index
Body Mass Index (BMI) is a measure of health on weight. It can be calculated by taking your weight in kilograms and dividing by the square of your height in meters. The interpretation of BMI for people 16 years or older is as follows:
ComputeAndInterpretBMI

Problem: Computing Taxes
The US federal personal income tax is calculated based on the filing status and taxable income. There are four filing statuses: single filers, married filing jointly, married filing separately, and head of household. The tax rates for 2009 are shown below.

Problem: Computing Taxes, cont.
if (status == 0) {
// Compute tax for single filers
}
else if (status == 1) {
// Compute tax for married file jointly
// or qualifying widow(er)
}
else if (status == 2) {
// Compute tax for married file separately
}
else if (status == 3) {
// Compute tax for head of household
}
else {
// Display wrong status
}
ComputeTax

Logical Operators
The logical operators !, &&, ||, and ^ can be used to create a compound Boolean expression.

Truth Table for Operator !

Truth Table for Operator &&

Truth Table for Operator ||

Truth Table for Operator ^

Examples
Here is a program that checks whether a number is divisible by 2 and 3, whether a number is divisible by 2 or 3, and whether a number is divisible by 2 or 3 but not both:
TestBooleanOperators

Examples
System.out.println("Is " + number + " divisible by 2 and 3? " +
((number % 2 == 0) && (number % 3 == 0)));
System.out.println("Is " + number + " divisible by 2 or 3? " +
((number % 2 == 0) || (number % 3 == 0)));
System.out.println("Is " + number +
" divisible by 2 or 3, but not both? " +
((number % 2 == 0) ^ (number % 3 == 0)));

The & and | Operators
If x is 1, what is x after this expression?
(x > 1) & (x++ < 10)
If x is 1, what is x after this expression?
(1 > x) && ( 1 > x++)
How about (1 == x) | (10 > x++)?
(1 == x) || (10 > x++)?

Problem: Determining Leap Year?
LeapYear
This program first prompts the user to enter a year as an int value and checks if it is a leap year.
A year is a leap year if it is divisible by 4 but not by 100, or it is divisible by 400.
(year % 4 == 0 && year%100 != 0) || (year % 400 == 0)

Problem: Lottery
Write a program that randomly generates a lottery of a two-digit number, prompts the user to enter a two-digit number, and determines whether the user wins according to the following rule:
Lottery
If the user input matches the lottery in exact order, the award is $10,000.
If the user input matches the lottery, the award is $3,000.
If one digit in the user input matches a digit in the lottery, the award is $1,000.

switch Statements
switch (status) {
case 0: compute taxes for single filers;
break; case 1: compute taxes for married file jointly;
break;
case 2: compute taxes for married file separately;
break;
case 3: compute taxes for head of household;
break;
default: System.out.println("Errors: invalid status");
System.exit(1);
}

switch Statement Flow Chart

switch Statement Rules
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2; break;
…
case valueN: statement(s)N;
break;
default: statement(s)-for-default;
}

switch Statement Rules
The keyword break is optional, but it should be used at the end of each case in order to terminate the remainder of the switch statement. If the break statement is not present, the next case statement will be executed.
switch (switch-expression) {
case value1: statement(s)1;
break;
case value2: statement(s)2;
break;
…
case valueN: statement(s)N;
break;
default: statement(s)-for-default;
}
When the value in a case statement matches the value of the switch-expression, the statements starting from this case are executed until either a break statement or the end of the switch statement is reached.

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Suppose day is 2:

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Match case 2

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Fall through case 3

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Fall through case 4

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Fall through case 5

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Encounter break

Trace switch statement
switch (day) {
case 1:
case 2:
case 3:
case 4:
case 5: System.out.println("Weekday"); break;
case 0:
case 6: System.out.println("Weekend");
}
Exit the statement

Problem: Chinese Zodiac
Write a program that prompts the user to enter a year and displays the animal for the year.
ChineseZodiac

Conditional Operator (?)
Syntax of conditional operator:
(boolean-expression) ? exp1 : exp2
if (num % 2 == 0)
System.out.println(num + “is even”);
else
System.out.println(num + “is odd”);
System.out.println(
(num % 2 == 0)? num + “is even” :
num + “is odd”);

Conditional Expressions
if (x > 0)
y = 1
else
y = -1;
is equivalent to
y = (x > 0) ? 1 : -1;

Operator Precedence
var++, var--
+, - (Unary plus and minus), ++var,--var
(type) Casting
! (Not)
*, /, % (Multiplication, division, and remainder)
+, - (Binary addition and subtraction)
<, <=, >, >= (Relational operators)
==, !=; (Equality)
^ (Exclusive OR)
&& (Conditional AND) Short-circuit AND
|| (Conditional OR) Short-circuit OR
=, +=, -=, *=, /=, %= (Assignment operator)

Operator Precedence and Associativity
The expression in the parentheses is evaluated first. (Parentheses can be nested, in which case the expression in the inner parentheses is executed first.) When evaluating an expression without parentheses, the operators are applied according to the precedence rule and the associativity rule.
If operators with the same precedence are next to each other, their associativity determines the order of evaluation. All binary operators except assignment operators are left-associative.

Operator Associativity
When two operators with the same precedence are evaluated, the associativity of the operators determines the order of evaluation. All binary operators except assignment operators are left-associative.
a – b + c – d is equivalent to ((a – b) + c) – d
Assignment operators are right-associative. Therefore, the expression
a = b += c = 5 is equivalent to a = (b += (c = 5))

Example
Applying the operator precedence and associativity rule, the expression 3 + 4 * 4 > 5 * (4 + 3) - 1 is evaluated as follows:

Debugging
The process of finding and correcting errors is called debugging.
A common approach to debugging is to use a combination of methods to narrow down to the part of the program where the bug is located.
You can hand-trace the program (i.e., catch errors by reading the program), or you can insert print statements in order to show the values of the variables or the execution flow of the program. This approach might work for a short, simple program.
But for a large, complex program, the most effective approach for debugging is to use a debugger utility.

Debugger
Debugger is a program that facilitates debugging. You can use a debugger to
Execute a single statement at a time.
Trace into or stepping over a method.
Set breakpoints.
Display variables.
Display call stack.
Modify variables.