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8 February 2013Birkbeck College, U. London1 Introduction to Programming Lecturer: Steve Maybank Department of Computer Science and Information Systems sjmaybank@dcs.bbk.ac.uk Spring 2013 Week 5: Boolean Operations

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Overview Java Lab 3, Exercises 2 and 4 Relational Operators Equality of strings Boolean operators De Morgans laws See Java for Everyone, Ch. 3 8 February 2013Birkbeck College, U. London2

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Java Lab 3, Exercise 2 Integer calculations: write a program that prompts the user for two integers and then prints The sum The difference … 8 February 2013Birkbeck College, U. London3

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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(); 8 February 2013Birkbeck College, U. London4

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Overflow Integers of type int must be in the range –2 31 to 2 31 -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. 8 February 2013Birkbeck College, U. London5

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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 8 February 2013Birkbeck College, U. London6

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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. 8 February 2013Birkbeck College, U. London7

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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; 8 February 2013Birkbeck College, U. London8

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Relational Operators 8 February 2013Birkbeck College, U. London9 JavaMathDescription >>greater than >=greater than or equal <<less than <=less than or equal ===equal !=not equal

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Relational Operator Examples 8 February 2013Birkbeck College, U. London10 ExpressionValueComment 3 <= 4true<= is less than or equal 3 =< 4errorUse <= 3 > 4false> is the opposite of <= 4 < 4false< is strict inequality 3 != 5-1true!= tests for inequality 1.0/3.0 == 0.33333333 falseThe numbers are similar but not equal 10 > 5errorA string cannot be compared with a number

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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. 8 February 2013Birkbeck College, U. London11

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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. 8 February 2013Birkbeck College, U. London12

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Boolean Operators 8 February 2013Birkbeck College, U. London13 JavaNameDescription &&AndBinary: a&&b is true if and only if a and b are both true. ||OrBinary: a||b is true if and only if at least one of a, b is true. !NotUnary: !a is true if and only if a is false.

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Boolean Operator Examples 8 February 2013Birkbeck College, U. London14 ExpressionValueComment 0<20 && 20<10falseOnly the first condition is true 0<20 || 20<10trueThe first condition is true 0<x<100error0<x has a boolean value, 100 is an integer !(0<200)false0<200 is true frozen==truefrozenNo need to compare a boolean value with true

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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 8 February 2013Birkbeck College, U. London15

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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 */ 8 February 2013Birkbeck College, U. London16

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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 */ 8 February 2013Birkbeck College, U. London17

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Equality of Strings Use boolean test = string1.equals(string2); Example String str1 = red; String str2 = blue; System.out.println(str1.equals(str2)); /* Prints: false */ 8 February 2013Birkbeck College, U. London18

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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); //true boolean test2 = (str1==str2.substring(0,3)); //false 8 February 2013Birkbeck College, U. London19

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Lexicographic Ordering int sc=string1.compareTo(string2); /* sc<0: string1 precedes string2 sc==0: string1 equals string2 sc>0: string1 follows string2 */ 8 February 2013Birkbeck College, U. London20

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De Morgans 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. 8 February 2013Birkbeck College, U. London21

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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 8 February 2013Birkbeck College, U. London22

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Example of a De Morgans Law Buying apples: boolean reject1 = (!fromUK) && (!fromFR); boolean reject2 = !(fromUK || fromFR); /* reject1 has the same value as reject2 */ 8 February 2013Birkbeck College, U. London23

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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. London24

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