Transcript Chapter 1 Introduction to Java

Lecture #4
By
Dr. Basheer M. Nasef
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Dr. Basheer M. Nasef
Control Statements I
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Home work
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Home work
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Home work
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Motivations
If you assigned a negative value for radius in
ComputeArea.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?
Compute Area
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 To
declare boolean type and use Boolean values true and
false .
 To
apply relational operators (<, <=, ==, !=, >, >=) and logic
operators (!, &&, ||, ^) to write Boolean expressions.
 To
 To
use Boolean expressions to control selection statements.
implement selection control using if and nested if
statements.
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The boolean Type and Operators
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);
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Comparison Operators
Operator Name
<
less than
<=
less than or equal to
>
greater than
>=
greater than or equal to
==
equal to
!=
not equal to
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Boolean Operators
Operator Name
!
not
&&
and
||
or
^
exclusive or
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Truth Table for Operator !
p
!p
true
false
!(1 > 2) is true, because (1 > 2) is false.
false
true
!(1 > 0) is false, because (1 > 0) is true.
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Example
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Truth Table for Operator &&
p1
p2
p1 && p2
false
false
false
false
true
false
true
false
false
true
true
true
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Example
(3 > 2) && (5 >= 5) is true, because (3 >
2) and (5 >= 5) are both true.
(3 > 2) && (5 > 5) is false, because (5 >
5) is false.
Dr. Basheer M. Nasef
Truth Table for Operator ||
p1
p2
p1 || p2
false
false
false
false
true
true
true
false
true
true
true
true
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Example
(2 > 3) || (5 > 5) is false, because (2 > 3)
and (5 > 5) are both false.
(3 > 2) || (5 > 5) is true, because (3 > 2)
is true.
Dr. Basheer M. Nasef
Truth Table for Operator ^
p1
p2
p1 ^ p2
false
false
false
false
true
true
true
false
true
true
true
false
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Example
(2 > 3) ^ (5 > 1) is true, because (2 > 3)
is false and (5 > 1) is true.
(3 > 2) ^ (5 > 1) is false, because both (3
> 2) and (5 > 1) are true.
Dr. Basheer M. Nasef
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)));
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Problem:
Determining Leap Year?
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)
LeapYear
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Run
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Problem:
A Simple Math Learning Tool
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.
AdditionQuiz
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Control Statements

if Statements

switch Statements

Conditional Operators
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Simple if Statements
if (booleanExpression) {
statement(s);
}
Boolean
Expression
if (radius >= 0) {
area = radius * radius * PI;
System.out.println("The area"
+ " for the circle of radius "
+ radius + " is " + area);
}
false
false
(radius >= 0)
true
true
area = radius * radius * PI;
System.out.println("The area for the circle of " +
"radius " + radius + " is " + area);
Statement(s)
(A)
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(B)
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Note
Outer parentheses required
Braces can be omitted if the block contains a single
statement
if ((i > 0) && (i < 10)) {
System.out.println("i is an " +
+ "integer between 0 and 10");
}
(a)
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Equivalent
if ((i > 0) && (i < 10))
System.out.println("i is an " +
+ "integer between 0 and 10");
(b)
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Caution
Adding a semicolon at the end of an if clause is a common
mistake.
if (radius >= 0);
Wrong
{
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.
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The if...else Statement
if (booleanExpression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}
true
Statement(s) for the true case
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Boolean
Expression
false
Statement(s) for the false case
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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");
}
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Multiple Alternative if Statements
if (score >= 90.0)
grade = 'A';
else
if (score >= 80.0)
grade = 'B';
else
if (score >= 70.0)
grade = 'C';
else
if (score >= 60.0)
grade = 'D';
else
grade = 'F';
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Equivalent
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
Dr. Basheer M. Nasef
Trace if-else statement
Suppose score is 70.0
The condition is false
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
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Trace if-else statement
Suppose score is 70.0
The condition is false
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
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Trace if-else statement
Suppose score is 70.0
The condition is true
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
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Trace if-else statement
Suppose score is 70.0
grade is C
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
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Trace if-else statement
Suppose score is 70.0
Exit the if statement
if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
else if (score >= 70.0)
grade = 'C';
else if (score >= 60.0)
grade = 'D';
else
grade = 'F';
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Note
The else clause matches the most recent if clause in
the same block.
int i = 1;
int j = 2;
int k = 3;
int i = 1;
int j = 2;
int k = 3;
Equivalent
if (i > j)
if (i > k)
System.out.println("A");
else
System.out.println("B");
(a)
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if (i > j)
if (i > k)
System.out.println("A");
else
System.out.println("B");
(b)
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Note
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.
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TIP
if (number % 2 == 0)
even = true;
else
even = false;
boolean even
= number % 2 == 0;
(b)
(a)
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Equivalent
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CAUTION
if (even == true)
System.out.println(
"It is even.");
if (even)
System.out.println(
"It is even.");
(b)
(a)
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Equivalent
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Problem:
An Improved Math Learning Tool
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
in the input dialog box, the program
displays a message dialog box to indicate
whether the answer is correct.
SubtractionQuiz
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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 2002 are shown in Table 3.1.
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Problem: Computing Taxes, cont.
if (status == 0) {
// Compute tax for single filers
}
else if (status == 1) {
// Compute tax for married file jointly
}
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
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Problem: Guessing Birth Date
The program can guess your birth date.
Run to see how it works.
= 19
+
1 3 5 7
9 11 13 15
17 19 21 23
25 27 29 31
2
10
18
26
Set1
3
11
19
27
6
14
22
30
Set2
7
15
23
31
4 5 6 7
12 13 14 15
20 21 22 23
28 29 30 31
8 9 10 11
12 13 14 15
24 25 26 27
28 29 30 31
Set3
Set4
GuessBirthDate
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16
20
24
28
17
21
25
29
18
22
26
30
Set5
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19
23
27
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Problem
Suppose, when you run the program, you enter input 2 3 6 from the console. What
is the output?
public class Test {
public static void main(String[] args) {
java.util.Scanner input = new java.util.Scanner(System.in);
double x = input.nextDouble();
double y = input.nextDouble();
double z = input.nextDouble();
System.out.println("(x < y && y < z) is " + (x < y && y < z));
System.out.println("(x < y || y < z) is " + (x < y || y < z));
System.out.println("!(x < y) is " + !(x < y));
System.out.println("(x + y < z) is " + (x + y < z));
System.out.println("(x + y < z) is " + (x + y < z));
}
}
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HW
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