25 January 2013Birkbeck College, U. London1 Introduction to Programming Lecturer: Steve Maybank...

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25 January 2013 Birkbeck College, U. London 1

Introduction to Programming

Lecturer: Steve Maybank

Department of Computer Science and Information Systems

sjmaybank@dcs.bbk.ac.ukSpring 2013

Week 3: Variables and Number Types

Overview

Review of week 2: declaring variables

Number types Keyboard input Mathematical functions See Java for Everyone, Chapter 2

Recall Variable Declaration

int cansPerPack = 6;

/*int: integer number type. All variables have a

type.=: assignmentcansPerPack: descriptive name in camel case.6: initial value*/

Alternative Declaration

int cansPerPack; // variable declarationcansPerPack = 6; // initialisation

Always initialise a variable before using it,

even if there is a default initialisation.

Example Code

int cansPerPack = 6;System.out.println(cansPerPack);int cansPerCrate = 4 * cansPerPack;

/* A variable can be initialised using an expression.

Number Types

Type Description

int integer, range –231 to 231 -1

byte integer, range –128 to 127

short integer, range –215 to 215 -1

long integer, range –263 to 263 -1

double double precision floating point (15 decimal digits, range ±10308)

float single precision floating point (7 decimal digits, range ±1038). Now rarely used.

char character, encoded in Unicode

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

Examples of Variable Names

counter

counter67

largeNumber

large_Number

large Number

6counter

counter?

days/month

Counter

cou67ter

_counter

largenumber

$largeNumber

Conventionaland accepted bythe compiler

Not conventional butaccepted by the compiler

Rejected bythe compiler

Comments

double canVolume=0.355; //Litres in a can

/* This is a long comment */

/** This is a comment which explains thepurpose of a class.*/

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 canSystem.out.print(“A six-pack of 12-ounce cans contains ”);System.out.print(cansPerPack * canVolume);System.out.println(“ litres.”);

Overflowint 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. 10400.

Assignment of a Value to a Variable

int counter = 0;int increment = 1;counter = counter+1;counter++;counter- -;counter += 1;counter += increment;counter *= 2;

Constantsfinal 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;

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. */

Recall Key Board Inputimport 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 */

Types of Key Board InputSystem.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” */

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”);}

Arithmetic Operators

+ 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.

Examples

Expression

2+6/2 5

(2+6)/2 4

6.2/2 3.1

(2+6.0)/16

(2+6)*2+3

Powers and Roots

Math function Method

x Math.sqrt(x)

x2 Math.pow(x,2)

xy, 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)

log10(x), x>0. Math.log10(x)

|x| Math.abs(x)

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);

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=435System.out.println(cents); // Prints 434!

}/* 4.35 cannot be represented exactly as a number of type double

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/apiSee also JFE Appendix D.

25 January 2013Birkbeck College, U. London1 Introduction to Programming Lecturer: Steve Maybank...

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