Download ppt - Boolean expressions, part 2: Logical operators. Previously discussed Recall that there are 2 types of operators that return a boolean result (true or

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

Binary operator

|| The logical OR operator

Binary operator

! The logical NOT 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");


Because 10 <= a <= 20 is evaluated as follows:

It is illegal to use the compare <= operator between a Boolean

value and a number

Expression: 10 <= a <= 20 Operators: <= <=

Evaluated as: 10 <= a <= 20 (10 <= a is either true or false)

true <= 20 or false <= 20


• Correct solution:

Because only numbers that are between 10 and 20 will satisfy the condition 10 <= a && a <= 20

if ( 10 <= a && a <= 20 ) System.out.println("yes"); else System.out.println(“no");


• 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: (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 Associativity

1 ( ) Brackets

2 (int) − ! Casting, negation, logical NOT

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/


• 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;


• Example 2:

boolean a;

Statement: a = 3 > 1 && ! 4 < 5

Operators in statement: = > && ! <


a = 3 > 1 && ! 4 < 5 // ! has the highest priority

Result: error Logical NOT operator (!) cannot be applied to a number


• Example 3:

boolean a;

Statement: a = 3 > 1 && ! (4 < 5)

Operators in statement: = > && ! ( < )


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)


• Years that are evenly divisible by 100 are not leap years:


(year % 4 == 0) && !(year % 100 == 0)

divisible by 4 AND not divisible by 100


• unless they are also evenly divisible by 400, in which case they are leap years


(year % 4 == 0) && !(year % 100 == 0) || (year % 400 == 0)


• 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


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"); } } }


• 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