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25 January 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 3: Variables and Number Types

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Overview Review of week 2: declaring variables Number types Keyboard input Mathematical functions See Java for Everyone, Chapter 2 25 January 2013Birkbeck College, U. London2

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Recall Variable Declaration int cansPerPack = 6; /* int: integer number type. All variables have a type. =: assignment cansPerPack: descriptive name in camel case. 6: initial value */ 25 January 2013Birkbeck College, U. London3

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Alternative Declaration int cansPerPack; // variable declaration cansPerPack = 6; // initialisation Always initialise a variable before using it, even if there is a default initialisation. 25 January 2013Birkbeck College, U. London4

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Example Code int cansPerPack = 6; System.out.println(cansPerPack); int cansPerCrate = 4 * cansPerPack; /* A variable can be initialised using an expression. */ 25 January 2013Birkbeck College, U. London5

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Number Types 25 January 2013Birkbeck College, U. London6 TypeDescription intinteger, range –2 31 to 2 31 -1 byteinteger, range –128 to 127 shortinteger, range –2 15 to 2 15 -1 longinteger, range –2 63 to 2 63 -1 doubledouble precision floating point (15 decimal digits, range ±10 308 ) floatsingle precision floating point (7 decimal digits, range ±10 38 ). Now rarely used. charcharacter, encoded in Unicode

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Variable Names Consist of numbers, letters and underscore _ $ is legal, but reserved for names generated by software tools Must begin with a letter or underscore Case sensitive Avoid reserved words By convention: class names begin with a capital letter, variable names begin with a lower case letter 25 January 2013Birkbeck College, U. London7

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Examples of Variable Names 25 January 2013Birkbeck College, U. London8 counter counter67 largeNumber large_Number large Number byte 6counter counter? days/month Counter cou67ter IF _counter largenumber $largeNumber Conventional and accepted by the compiler Not conventional but accepted by the compiler Rejected by the compiler

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Comments double canVolume=0.355; //Litres in a can /* This is a long comment */ /** This is a comment which explains the purpose of a class. */ 25 January 2013Birkbeck College, U. London9

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Example Program /** This program computes the volume in litres of a six-pack of soda cans. */ public class Volume1 { public static void main(Strings[] args) { int cansPerPack = 6; double canVolume = 0.355; // Litres in a 12-ounce can System.out.print(A six-pack of 12-ounce cans contains ); System.out.print(cansPerPack * canVolume); System.out.println( litres.); } 25 January 2013Birkbeck College, U. London10

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Overflow int oneBillion = 1000000000; System.out.println(3 * oneBillion); /* Output: -1294967296. No compile time error is flagged. No error message appears at run time. Solution: use type double but note round off errors and overflow in double, e.g. 10 400. */ 25 January 2013Birkbeck College, U. London11

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Assignment of a Value to a Variable int counter = 0; int increment = 1; counter = counter+1; counter++; counter- -; counter += 1; counter += increment; counter *= 2; 25 January 2013Birkbeck College, U. London12

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Constants final double BOTTLE_VOLUME=2; /* The reserved word final ensures that the value of BOTTLE_VOLUME will never change. Use names for constants and avoid magic numbers, eg. compare the following.*/ double volume1=bottles * 2; double volume2=bottles * BOTTLE_VOLUME; 25 January 2013Birkbeck College, U. London13

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Strings and Characters H`: character of type char. H: string containing a single character, namely H`. String greeting = Harry; char start = greeting.charAt(0); // Value of start is `H`; int count = Integer.parseInt(34); /* parseInt is a static method in the class java.lang.Integer. It converts a string to the corresponding integer. */ 25 January 2013Birkbeck College, U. London14

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Recall Key Board Input import java.util.Scanner; // first line of program Scanner in = new Scanner(System.in); /* create a Scanner object to read keyboard input */ System.out.print(Enter the number of bottles: );// prompt int bottles = in.nextInt(); // read integer input /* Type digits at key board, then press the enter or return key */ 25 January 2013Birkbeck College, U. London15

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Types of Key Board Input System.out.print(Enter the price in the form a.b where a is the number of pounds and b is the number of pence: ); double unitPrice=in.nextDouble(); System.out.print(Enter your first name: ); String name = in.next(); // input a single word, e.g. John System.out.print(Enter your full name: ); String fullName = in.nextLine(); /* input a whole line, e.g. John Smith */ 25 January 2013Birkbeck College, U. London16

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A Test of the Key Board Input System.out.println(Type an integer: ); boolean test = in.hasNextInt(); if(test) { int i = in.nextInt(); } else { System.out.println(error); } 25 January 2013Birkbeck College, U. London17

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Arithmetic Operators 25 January 2013Birkbeck College, U. London18 +addition -subtraction *multiplication /division %modulus Multiplication and division take precedence over addition and subtraction. Use brackets to control the evaluation of expressions. If in any doubt, use brackets.

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Examples 25 January 2013Birkbeck College, U. London19 ExpressionValue 2+6/25 (2+6)/24 5/22 5%21 6.2/23.1 (2+6.0)/160.5 (2+6)*2+319

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Powers and Roots 25 January 2013Birkbeck College, U. London20 Math functionMethod xMath.sqrt(x) x2x2 Math.pow(x,2) x y, x>0 or x=0, y>0 or x<0, y integer Math.pow(x,y) Round x to nearest integer Math.round(x) (long integer returned) log 10 (x), x>0.Math.log10(x) |x|Math.abs(x)

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Conversion of Floating Point to Integer double balance = total+tax; int dollars1 = (int) balance; /* The value of balance is converted to an integer by discarding the fractional part. The operator (int) is a cast operator. */ int dollars2=(int)(total + tax); 25 January 2013Birkbeck College, U. London21

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Roundoff Errors /** This program demonstrates the effect of a round off error. */ public class RoundoffDemo { public static void main(String[] args) { double price = 4.35; int cents = (int)(100 * price); // Should be 100*4.35=435 System.out.println(cents); // Prints 434! } /* 4.35 cannot be represented exactly as a number of type double */ } 25 January 2013Birkbeck College, U. London22

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API Application Programming Interface: contains the classes and methods in the Java library. The API documentation is at http://java.sun.com/javase/7/docs/api See also JFE Appendix D. 25 January 2013Birkbeck College, U. London23

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