8 february 2013birkbeck college, u. london1 introduction to programming lecturer: steve maybank...

8 February 2013 Birkbeck College, U. London 1

Introduction to Programming

Lecturer: Steve Maybank

Department of Computer Science and Information Systems

[email protected] 2013

Week 5: Boolean Operations

Overview Java Lab 3, Exercises 2 and 4 Relational Operators Equality of strings Boolean operators De Morgan’s laws See Java for Everyone, Ch. 3


Java Lab 3, Exercise 2 Integer calculations: write a

program that prompts the user for two integers and then prints

The sumThe difference…


Keyboard Input

import java.util.Scanner;…Scanner in = new Scanner(System.in);System.out.print(“Please input an integer: ”);int i = in.nextInt();System.out.print(“Please input a second

integer: ”);int j = in.nextInt();


Overflow Integers of type int must be in the range –231 to 231-

1. The following two integers are in this range:

int i=1500000000;int j = 1500000001;System.out.println(i+j);/* result: -1294967295 */System.out.println(i-j);/* result: -1 */

There are no error messages when i+j is evaluated.


Java Lab 3, Exercise 4 Separate digits: write a program

that reads in a five digit positive integer and prints out the individual digits, separated by spaces. For example 16348 is printed out as 1 6 3 4 8


Possible Solutions Input a five digit integer i Extract the digits, eg. d1=i%10; Print the digits and spaces. Alternative: use in.next() to input the five

digits of i in the form of a string. Divide the string into five substrings, one

for each digit. Reassemble the five substrings, with

spaces.


Solution

String digits=in.next();String d1=digits.substring(0,1);String d2=digits.substring(1,2);String d3=digits.substring(2,3);String d4=digits.substring(3,4);String d5=digits.substring(4,5);String gap=“ “;String result =

d1+gap+d2+gap+d3+gap+d4+gap+d5;


Relational Operators


Java

Math

Description

> > greater than

>= greater than or equal

< < less than

<= less than or equal

== = equal

!= not equal

Relational Operator Examples


Expression Value Comment

3 <= 4 true <= is less than or equal

3 =< 4 error Use <=

3 > 4 false > is the opposite of <=

4 < 4 false < is strict inequality

3 != 5-1 true != tests for inequality

1.0/3.0 ==0.33333333

false The numbers are similar but not equal

“10” > 5 error A string cannot be compared with a number

Precedence

Relational operators have a lower precedence than arithmetical operators, eg.

int floor = 10;boolean v = floor-1 < 13;

The expression floor-1 is evaluated first and the value is then compared with 13.


Boolean Data Type A variable of type boolean has either

the value true or the value false, eg.boolean temp = true;

boolean is a reserved word, true and false are values.

It is not the case that true is 1 and false is 0.


Boolean Operators


Java Name Description

&& And Binary: a&&b is true if and only if a and b are both true.

|| Or Binary: a||b is true if and only if at least one of a, b is true.

! Not Unary: !a is true if and only if a is false.

Boolean Operator Examples


Expression Value Comment

0<20 && 20<10 false Only the first condition is true

0<20 || 20<10 true The first condition is true

0<x<100 error 0<x has a boolean value, 100 is an integer

!(0<200) false 0<200 is true

frozen==true frozen No need to compare a boolean value with true

Combining Conditions Think carefully about the

difference between && and || Buying a shirt:

white, cotton, size 15 Buying apples:

from the UK, from France,from South Africa


Lazy Evaluation of Boolean Expressions

Logical expressions are evaluated left to right. The evaluation stops as soon as the truth value is determined, eg.

int quantity = 0;boolean test1 = quantity > 0 && price/quantity < 10;/* test1 is false */boolean test2 = quantity == 0 || price/quantity < 10;/* test2 is true */


Compile Time Errors

int temp = 40;boolean test1 = (0 <= temp <=100);/* 0<=temp is true, true<=100 is an error

*/

boolean test2 = (temp == 40||50);/* 40||50 is an error because || cannot be

applied to integers */


Equality of Strings Use

boolean test = string1.equals(string2);

ExampleString str1 = “red”;String str2 = “blue”;System.out.println(str1.equals(str2));/* Prints: false */


More on Equality of Strings str1==str2 returns true if and only if str1 and

str2 are initialised with the same string literal, eg.

String str1=“Rob”, str2=“Robert”;boolean test1=(str1==“Rob”); //trueboolean test2 = (str1==str2.substring(0,3)); //false


Lexicographic Ordering

int sc=string1.compareTo(string2);

/* sc<0: string1 precedes string2 sc==0: string1 equals string2 sc>0: string1 follows string2

*/


De Morgan’s Laws

Let a, b be Boolean variables. Then!(a && b) and (!a) || (!b)

have the same truth table.

Similarly!(a || b) and (!a) && (!b)

have the same truth table.


Check the First Law Suppose a is false. Then

!(a && b) = !(false && b) = !false = true

(!a) || (!b)= (!false) || b = true||b = true

Suppose a is true. Then!(a && b) = !(true && b) = !b (!a)||(!b) = (!true)||(!b) = false||(!b)=!b


Example of a De Morgan’s Law

Buying apples:

boolean reject1 = (!fromUK) && (!fromFR);

boolean reject2 = !(fromUK || fromFR);

/* reject1 has the same value as reject2 */


Test Suppose that x and y are integers.

Test whether both of them are zero. Test whether at least one of x, y is

zero. Test whether exactly one of x, y is

zero. What is the value of !!(x>y)?


8 february 2013birkbeck college, u. london1 introduction to programming lecturer: steve maybank...

Documents

birkbeck college

string gap

solution string digits

string d1

string result

string d2

string d3

string d4