Download Relational Operators - NYU Computer Science

Introduction to Computers and
Programming in JAVA: V22.0002
Relational Operators
Control structures
Decisions using “if” statements
 2000 Prentice Hall, Inc. All rights reserved. Modified for use with this course.
2
Displaying Text in a Dialog Box
• Display
– Most Java applications use windows or a dialog box
• We have used command window
– Class JOptionPane allows us to use dialog boxes
• Packages
– Set of predefined classes for us to use
– Groups of related classes called packages
• Group of all packages known as Java class library or Java
applications programming interface (Java API)
– JOptionPane is in the javax.swing package
• Package has classes for using Graphical User Interfaces (GUIs)
 2000 Prentice Hall, Inc. All rights reserved.
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Review Packages in Java
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// Java packages
– Two groups of packages in Java API
– Core packages
• Begin with java
• Included with Java 2 Software Development Kit
– Extension packages
• Begin with javax
5
import javax.swing.JOptionPane;
// program uses OptionPane
– import declarations
• Used by compiler to identify and locate classes used in Java
programs
• Tells compiler to load class JOptionPane from javax.swing
package
 2000 Prentice Hall, Inc. All rights reserved.
// InputDataDemo.java: Entering input from input dialog boxes. Finding the square of a number
import javax.swing.JOptionPane;
public class Class5_Input_DataDemo {
public static void main(String args[]) {
int result; // declare the result
// Prompt the user to enter a number:
String numString = JOptionPane.showInputDialog(null,
"Enter a number from 1 to 10:", "Input Window Demo", JOptionPane.QUESTION_MESSAGE);
// Convert the string into an int value
int num = Integer.parseInt(numString);
result = num * num ;
// Display the result in a message dialog box
JOptionPane.showMessageDialog(null,
num + " squared is " + result, "Input Window Demo", JOptionPane.INFORMATION_MESSAGE);
System.exit(0);
}
}
 2000 Prentice Hall, Inc. All rights reserved.
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Three Basic Control Structures
• All programs can be written with just these types
of structures
– Sequence structure
• Statements run one after the other
– Selection structure
• Depending on a condition, do one thing (single selection
using if);
• otherwise, do something else (Double selection using if else)
• if, if-else, and switch (multiple selections).
– Repetition structure
• Repeat some actions over and over
• for loops, while loops, and do/while loops.
 2000 Prentice Hall, Inc. All rights reserved.
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Flow Chart Basics 1
Diamonds
(decision symbol)
contain
conditions
Rectangles represent
statements of work.
For example:
flow
line
print()
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
Flow Chart Basics
Sequence structure
Connector symbol
Triangle boxes represent statements
such as
x=30;
int a;
System.out.print(“x”);
x=20;
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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Decision Making: Equality and Relational
Operators
• if control statement
– If a condition is true, then the body of the if statement gets
executed
– Control always resumes after the if structure
if ( condition )
statement executed if condition true
• No semicolon needed after condition
– Else: otherwise the conditional task is not performed
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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The if structure
• If some condition is true
–
do this
• Example:
if ( x == y )
{
System.out.println(“ x is equal to y!\n” ) ;
}
• Every programming language has some form of an if
statement.
• Note the operator: = vs ==
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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Flow Chart Basics Selection structure Single
selection (if)
Connector symbol
grade >=60
true
print “passed”
false
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
Equality and Relational Operators
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The if structure
• If some condition is true
– do this
– else, do something else
• Example:
if ( x == y )
{
System.out.println(“ x is equal to y!\n“ ) ;
}
else
{
System.out.println(“ x is NOT equal to y!\n“);
}
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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if/else Flow Chart
False
Print “You
failed”
 2000 Prentice Hall, Inc. All rights reserved.
True
grade
>=60
Print “You
passed”
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Even Odd program
• Lets write a program in class that
will:
– input a number between 1-10
– test whether a number is even or odd
 2000 Prentice Hall, Inc. All rights reserved.
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Even or Odd numbers
// even & odd numbers: using "mod"
import javax.swing.JOptionPane;
public class even_odd {
public static void main(String args[]) {
// Prompt the user to enter a number:
String numString = JOptionPane.showInputDialog(null,
"Enter a number from 1 to 10:", "Input Window Demo",
JOptionPane.QUESTION_MESSAGE);
// Convert the string into an int value
int num = Integer.parseInt(numString);
// Display the result in a message dialog box
if ( ( num % 2 ) == 0 )
System.out.println(" The number " + num + " is even.");
else
System.out.println(" The number " + num + " is odd.");
System.exit(0);
}
}
 2000 Prentice Hall, Inc. All rights reserved.
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// Comparison.java
// Compare integers using if statements, relational operators
// and equality operators.
// Java packages
import javax.swing.JOptionPane;
public class Comparison {
1. import
// main method begins execution of Java application
public static void main( String args[] )
{
String firstNumber;
// first string entered by user
String secondNumber; // second string entered by user
String result;
// a string containing the output
int number1;
int number2;
Comparison.java
// first number to compare
// second number to compare
2. Class
Comparison
2.1 main
2.2 Declarations
// read first number from user as a string
firstNumber = JOptionPane.showInputDialog( "Enter first integer:" );
2.3 Input data
(showInputDialo
g)
// read second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:" );
2.4 parseInt
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
// initialize result to empty String
result = "";
2.5 Initialize result
 2003 Prentice Hall, Inc.
All rights reserved.
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if ( number1 == number2 )
result = result + number1 + " == " + number2;
if ( number1 != number2 )
result = result + number1 + " != " + number2;
Test for equality, createComparison.java
new string,
assign to result.
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2;
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2;
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2;
3. if statements
4.
showMessageDialo
g
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2;
// Display results
JOptionPane.showMessageDialog( null, result, "Comparison Results",
JOptionPane.INFORMATION_MESSAGE );
System.exit( 0 );
// terminate application
} // end method main
} // end class Comparison
Notice use of
JOptionPane.INFORMATION_MESSAGE
 2003 Prentice Hall, Inc.
All rights reserved.
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Program Output
 2003 Prentice Hall, Inc.
All rights reserved.
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2.8
Decision Making: Equality and
Relational Operators
– Lines 1-12: Comments, import JOptionPane, begin
class Comparison and main
– Lines 13-18: declare variables
• Can use comma-separated lists instead:
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String firstNumber,
secondNumber,
result;
– Lines 21-30: obtain user-input numbers and parses input
string into integer variables
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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2.8
32
Decision Making: Equality and
Relational Operators
result = "";
– Initialize result with empty string
34
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if ( number1 == number2 )
result = result + number1 + " == " + number2;
– if statement to test for equality using (==)
• If variables equal (condition true)
– result concatenated using + operator
– result = result + other strings
– Right side evaluated first, new string assigned to result
• If variables not equal, statement skipped
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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2.8
Decision Making: Equality and
Relational Operators
– Lines 37-50: other if statements testing for less than, more
than, etc.
• If number1 = 123 and number2 = 123
– Line 34 evaluates true (if number1 = = number 2)
• Because number1 equals number2
– Line 40 evaluates false (if number1 < number 2)
• Because number1 is not less than number2
– Line 49 evaluates true (if number1 >= number2)
• Because number1 is greater than or equal to number2
– Lines 53-54: result displayed in a dialog box using
showMessageDialog
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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2.8
Decision Making: Equality and
Relational Operators
• Precedence of operators
– All operators except for = (assignment) associates from left
to right
• For example: x = y = z is evaluated x = (y = z)
Op era tors
Assoc ia tivity
Typ e
* / %
left to right
multiplicative
+ left to right
additive
< <= > >=
left to right
relational
== !=
left to right
equality
=
right to left
assignment
Fig. 2.21 Prec ed enc e a nd a ssoc ia tivity of the op era tors d isc ussed so fa r.
 2003 Prentice Hall, Inc. All rights reserved (Modified) .
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• Lets write a program in class:
– Write a program that will input a number
from 1-10 (using input dialog box)
– Determine if the number entered is equal to
5
– OR less than 5
– Or larger than 5
 2000 Prentice Hall, Inc. All rights reserved.
// less than five!!
import javax.swing.JOptionPane;
public class less_than_5{
public static void main(String args[]) {
// Prompt the user to enter a number:
String numString = JOptionPane.showInputDialog(null,
"Enter a number from 1 to 10:", "Input Window Demo",
JOptionPane.QUESTION_MESSAGE);
// Convert the string into an int value
int num = Integer.parseInt(numString);
// Display the result in a message dialog box
if ( num < 5 )
System.out.println(" The number " + num + " is less than five.");
else
if (num == 5)
System.out.println(" The number " + num + " is equal to five.");
else
System.out.println(" The number " + num + " is greater than five.");
System.exit(0);
}  2000 Prentice Hall, Inc.
All rights reserved.
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