Transcript Boolean expressions, part 2: Logical operators

Boolean expressions, part 2:
Logical operators
Previously discussed
• Recall that there are 2 types of operators that return a
boolean result (true or false):
• Compare operators:
• A compare operator compares 2 numerical
expressions and return a Boolean result.
• Example:
• the compare operation 3 > 1 returns the
Boolean value true
Previously discussed (cont.)
• Logical operators:
• A logical operator compares 2 Boolean (logical)
expressions and return a Boolean result.
•Example:
• the logical operation true AND false
returns the Boolean value false
Logical operators
• Logical operators in Java:
Operator symbol
Meaning
Comment
&&
The logical AND
operator
The logical OR
operator
The logical NOT
operator
Binary operator
||
!
Binary operator
Unary operator
The logical AND operator &&
• The && operator only operate on the Boolean values true
and false
• The results are as follows:
Operation
Result
true && true
true
true && false
false
false && true
false
false && false
false
The logical AND operator && (cont.)
• Example 1:
(3 > 1) && (3 < 5)
= true && true
= true
The logical AND operator && (cont.)
• Example 2:
(3 > 1) && (3 > 5)
= true && false
= false
Example program: test if a number is
between 10 and 20
• Problem description:
• Write a Java program that reads in a number a
• The program prints "yes" when 10 ≤ a ≤ 20 and
"no" otherwise.
Example program: test if a number is
between 10 and 20 (cont.)
• Wrong solution:
if ( 10 <= a <= 20 )
System.out.println("yes");
else
System.out.println(“no");
Example program: test if a number is
between 10 and 20 (cont.)
Because 10 <= a <= 20 is evaluated as follows:
Expression:
Operators:
Evaluated as:
10 <= a <= 20
<= <=
10 <= a <= 20 (10 <= a is
either true or false)
true <= 20
or false <= 20
It is illegal to use the compare <= operator between a Boolean
value and a number
Example program: test if a number is
between 10 and 20 (cont.)
• Correct solution:
if ( 10 <= a && a <= 20 )
System.out.println("yes");
else
System.out.println(“no");
Because only numbers that are between 10 and 20 will
satisfy the condition 10 <= a && a <= 20
Example program: test if a number is
between 10 and 20 (cont.)
• Java program:
import java.util.Scanner;
public class Between01
{
public static void main(String[] args)
{
int a;
Scanner in = new Scanner(System.in); // Construct Scanner object
a = in.nextInt(); // Read in number into a
if ( 10 <= a && a <= 20 )
{
System.out.println("Number is between 10 and 20");
}
else
{
System.out.println("Number is NOT between 10 and 20");
}
}
}
Example program: test if a number is
between 10 and 20 (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/
Between01.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac Between01.java
• To run:
java Between01
Priority of the logical operators
• Priority ranking of the logical operators against the
previously discussed operators:
Priority level
Operator(s)
Description
1
()
Brackets
2
(int) −
!
Casting, negation,
logical NOT
Associativity
Right to left
3
++, --
Increment, decrement
4
* / %
Multiple, divide,
remainder
Left to right
5
+ -
Add, subtract
Left to right
6
< <= > >=
== !=
Compare operators
7
&&
logical AND
Left to right
8
||
logical OR
Left to right
9
= += -= ...
Assignment
operators
Right to left
Priority of the logical operators (cont.)
• Reference:
http://introcs.cs.princeton.edu/java/11precedence/
Priority of the logical operators (cont.)
• Example 1:
boolean a;
Statement:
a = 3 > 1 && 4 < 5 ;
Operators in statement: = > && <
Executed as follows:
a = 3 > 1 && 4 > 5 ; // > and < have highest priority
a = true && true ;
a = true;
Priority of the logical operators (cont.)
• Example 2:
boolean a;
Statement:
a = 3 > 1 && ! 4 < 5
Operators in statement: = > && ! <
Executed as follows:
a = 3 > 1 && ! 4 < 5 // ! has the highest priority
Result: error
Logical NOT operator (!) cannot be applied to a number
Priority of the logical operators (cont.)
• Example 3:
boolean a;
Statement:
a = 3 > 1 && ! (4 < 5)
Operators in statement: = > && ! ( < )
Executed as follows:
a = 3 > 1 && ! (4 < 5); // ( ... ) has the highest priority
a = 3 > 1 && ! true;
// ! has the highest priority now
a = 3 > 1 && false;
// > has the highest priority now
a = true && false;
// && has the highest priority now
a = false;
Programming example: Leap year using a
complicated Boolean expression
• Leap year description (Wikipedia):
• In the Gregorian calendar, the current standard
calendar in most of the world, most years that are
evenly divisible by 4 are leap years.
• Years that are evenly divisible by 100 are not leap
years, unless they are also evenly divisible by 400, in
which case they are leap years
Programming example: Leap year using a
complicated Boolean expression (cont.)
• Constructing the Boolean expression for "leap year":
• most years that are evenly divisible by 4 are leap
years:
Year is leap year if:
year % 4 == 0
(divisible by 4)
Programming example: Leap year using a
complicated Boolean expression (cont.)
• Years that are evenly divisible by 100 are not leap years:
Year is leap year if:
(year % 4 == 0) && !(year % 100 == 0)
divisible by 4 AND not divisible by 100
Programming example: Leap year using a
complicated Boolean expression (cont.)
• unless they are also evenly divisible by 400, in which case
they are leap years
Year is leap year if:
(year % 4 == 0)
&& !(year % 100 == 0)
|| (year % 400 == 0)
Programming example: Leap year using a
complicated Boolean expression (cont.)
• Java program:
import java.util.Scanner;
public class LeapYear02
{
public static void main(String[] args)
{
int year;
boolean leap;
Scanner in = new Scanner(System.in); // Construct Scanner object
Programming example: Leap year using a
complicated Boolean expression (cont.)
System.out.print("Enter year:");
year = in.nextInt();
// Read in year
if ( (year % 4 == 0) && !(year % 100 == 0) || (year % 400 == 0) )
{
System.out.println("It is a leap year");
}
else
{
System.out.println("It is NOT a leap year");
}
}
}
Programming example: Leap year using a
complicated Boolean expression (cont.)
• Example Program: (Demo above code)
– Prog file:
http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/06/Progs/
LeapYear02.java
• How to run the program:
• Right click on link and save in a scratch directory
• To compile: javac LeapYear02.java
• To run:
java LeapYear02