Outline. Numbers and Operations. What is binary digit? Dr. Fotis Liarokapis

Transcription

1 Numbers and Operations Dr. Fotis Liarokapis Outline What is a binary digit? Word and byte Java primitive types Arithmetic calculations Precedence of arithmetic operations Concept of casting Variables and assignment Memory concepts Variable names Application for adding integers Program for adding integers Adding integers program output Explanation Lines 4, 6 to 7 Explanation Lines 10 to 11 Explanation Lines 12 to 14 Explanation Lines 16 to 18 Explanation Lines 19 to 21 Explanation Lines 23 to 25 Explanation Lines 27 to 28 Explanation Lines 30 to 33 Final overview of adding integers program Taking input from keyboard Example Java program reading input from keyboard What is binary digit? Word and byte CPU can understand only two conditions 1 current on 0 current off All further computer processing is based on this simple scheme On machine level all data and programs ultimately represented by combination of binary digits (bits) consists of two symbols 0 and 1 Some binary patterns Binary pattern Number Binary pattern Letter A B C D E Data and programs stored in groups of binary digits (bits) A certain number of bits is called word (or machine word) Word length depends on particular hardware and can be: 8, 16, 32, 64 or 128 bits Calculation accuracy depends on the length of word Longer word higher accuracy Byte (= 8 bits) is the amount of memory required to hold a character (letter, digit or punctuation mark) Size of computer memory is described byte units 1 KiloByte (K) = 2 10 = 1024 bytes 1 MegaByte (M) = 2 20 = bytes 1 GigaByte (G) = 2 30 = bytes 1

2 Java primitive types Arithmetic calculations The two major categories of data types in Java are: Primitive Reference Following table illustrates the most commonly used data types Type Size Description byte 8-bit Byte-length integer short 16-bit Short integer int 32-bit Integer long 64-bit Long integer float 32-bit Single-precision floating point double 64-bit Single-precision floating point char 16-bit A single Unicode character boolean True, False A boolean value Integer numbers Real numbers Other types Operator ( + ) Operation addition Operator ( - ) Operation subtraction Operator ( * ) Operation multiplication Operator ( / ) Operation division Note that if both operands are integers, integer division will be performed automatically i.e. 7 / 3 evaluates to 2 Operator ( % ) Operation modulus (returns the reminder from integer division) i.e. 7 % 3 evaluates to 1 Precedence of arithmetic operations The rules of operator precedence (order of evaluation) enable Java to apply operators in the correct order In general operators evaluate from left to right Operator ( ) *, / or % + or - Operations Parenthesis Multiplication, Division, Modulus Addition, Subtraction Precedence Evaluated first. Nested expression innermost first. Same level left to right Evaluated second. If several left to right Evaluated last. If several left to right Concept of casting A simple casting example: Student's grade can be represented by integer numbers (ranging from 1 to 6). Need to calculate average grade for three students where two of them have grade 2 and one has grade 4 Using simple mathematics we can calculate total = 2*2 + 4 = 8 average = total / numberofstudents Note that: total and numberofstudents numberofstudents are integer numbers, and average is double (floating-point real number). But then the average grade is calculated as: Average = 8 / 3 = 2 Wrong answer, because integer division was performed Both operands are integers 2

3 Concept of casting (cont) Variables and assignment For the right answer use Java unitary cast operator average = (double) total / 3 Here total is still an integer but cast operator has created a temporary floating-point copy of its operand 8.0 instead of 8 The right answer for the average is calculated as: average = 8.0 / 3 = 2.67 This kind of casting is called explicit conversion Example: : Calculation of a discount price for some item. Old price was 10 and discount is 2 Variables used: double oldprice,, discount, newprice Calculation procedure: oldprice = 10.0; (assignment operator =) discount = 2.0; newprice = oldprice discount; Assignment operator assigns a particular value to a variable Value of variables may change any number of times during program execution, but only latest value matters. Variables and assignment (cont) Variables and assignment (cont) Consider memory as a large set of boxes that can hold the values of potential variables Boxes for different types of values are different int,, double, char, boolean etc. When we declare variable (describe its name and type), the box of correspondent type allocated and name of variable stick on it int boolean double char declaration oldprice Assignment discount newprice oldprice 10.0 discount 2.0 newprice 8.0 3

4 Memory concepts Variable names Variables Every value has a name, a type, a size and a value Name corresponds to a location in memory When new value is placed into a variable it replaces (and destroys) the previous value Reading values from memory does not change them Visual representation oldprice 10.0 A program refers to a variable s value by its name In Java the following must be true for a variable name It must be a legal Java identifier comprised of a series of Unicode characters Legal Java identifier is a sequence of letters, symbols and digits, started from letter of symbol It must not be a keyword Special reserved words in Java It must not have the same name as another variable declared in the t same scope Variable names (cont) Application for adding integers By Java convention variables names always begin with lower case i.e. average, total, etc. In addition, class names always begin with UPPER case i.e. Vehicle, String, etc. In cases where name consists of more that one word, the words are joined together and each word after first begins with an UPPER case For variable name - averagemark; For class name - StockMarket; The upcoming program Uses input dialogs to input two values from user Uses message dialog to display sum of the two values 4

5 Program for adding integers 1 //application to receive two integers as an input from the user 2 //and print in dialog window their sum 3 4 import java.swing.joptionpane; //import class JOptionPane 5 6 public class AddIntegers 7 { 8 public static void main (String args[]) 9 { 10 String firstnumber, //first String entered by user 11 secondnumber; //second String entered by user 12 int number1, //first number to add 13 number2, //second number to add 14 sum; //sum of number1 and number // read in first number from the user as a String 17 firstnumber = 18 JOptionPane.showInputDialog("Enter first integer"); Program for adding integers (cont) 19 // read in second number from the user as a String 20 secondnumber = 21 JOptionPane.showInputDialog("Enter first integer"); //convert numbers from type String to type int 24 number1 = Integer.parseInt(firstNumber); 25 number2 = Integer.parseInt(secondNumber); //add the numbers 28 sum = number1 + number2; //display the results 31 JOptioPane.showMessageDialog( 32 null, "The sum is " + sum, "Results", 33 JOptionPane.PLAIN_MESSAGE); System.exit(0); //terminate the program 36 } //end of main method 37 } //end of AddIntegers class Adding integers program output Explanation Lines 4, 6 to 7 4 import java.swing.joptionpane; //import class JOptionPane Location of JOptionPane class for the compiler 6 public class AddIntegers 7 { Begins public class AddIntegers Remember that the file name must be AddIntegers.java Line 8, main method declaration 5

6 Explanation Lines 10 to String firstnumber, //first String entered by user 11 secondnumber; //second String entered by user Variable declaration must be declared with names and data type before used in order to locate the required memory space Both variables ( firstnumber firstnumber and secondnumber secondnumber ) are of type String Declarations end with semicolons ( ; ) Can declare multiple variables of the same type at a time Use comma to separate the variables Can add comments to describe the purpose of variables Explanation Lines 12 to int number1, //first number to add 13 number2, //second number to add 14 sum; //sum of number1 and number2 Declare variables number1, number2,, and sum of type int Data type int holds integer values (whole numbers) i.e. 0, 1, 2, 3, -1, -2, -3, etc. Data types float and double can hold decimal numbers i.e. 0.23, 1.34, -5,05, etc. Data type char can hold a single character i.e. a, b, c, etc. Explanation Lines 16 to // read in first number from the user as a String 17 firstnumber = 18 JOptionPane.showInputDialog("Enter first integer"); Read input from the user as a String using JOptionPane dialog window Method JOptionPane.showMessageDialog displays the following: Message called a prompt Directs user to perform an action Argument appears as prompt text If wrong type of data entered (non-integer) then an error occurs! Explanation Lines 16 to 18 (cont) 16 // read in first number from the user as a String 17 firstnumber = 18 JOptionPane.showInputDialog("Enter first integer"); Result of call to showinputdialog given to the variable firstnumber using the assignment operator ( = ) Assignment is a binary operation, takes two operands i.e. calculatenumber = ; Expression on the right evaluated and assigned to a variable on the left Read as: firstnumber gets the value of JOptionPane.showInputDialog( Enter first integer ); 6

7 Explanation Lines 19 to // read in second number from the user as a String 20 secondnumber = 21 JOptionPane.showInputDialog("Enter second integer"); Similar operation with previous slide Only difference is that it is for the second integer value from the user Explanation Lines 23 to //convert numbers from type String to type int 24 number1 = Integer.parseInt(firstNumber); 25 number2 = Integer.parseInt(secondNumber); Method used here is Integer.parseInt Converts String argument into an integer (type int) Class Integer from package java.lang Integer returned by Integer.parseInt is assigned to variable number1 (see line 24) Remember that number1 was declared as type int Line 25 similar to the above Explanation Lines 27 to //add the numbers 28 sum = number1 + number2; Assignment statement Calculates sum of number1 and number2 Right hand side Uses assignment operator = to assign result to variable sum Read as sum gets the value of: number1 + number2 Explanation Lines 30 to //display the results 31 JOptioPane.showMessageDialog( 32 null, "The sum is " + sum, "Results", 33 JOptionPane.PLAIN_MESSAGE); Use showmessagedialog to display results "The sum is " + sum Uses the operator + to add the string literal "The sum is " + sum Performs concatenation of a String and another data type Results in a new String If sum contains the value 117, then "The sum is " + sum results in the following new String "The sum is 117" Note the space in "The sum is " 7

8 Explanation Lines 30 to 33 (cont) 30 //display the results 31 JOptioPane.showMessageDialog( 32 null, "The sum is " + sum, "Results", 33 JOptionPane.PLAIN_MESSAGE); Different version of showmessagedialog Requires four arguments (instead of two from previous example) First argument null for now Second argument String to display Third argument String in the title bar Fourth argument type of message dialog JOptionPane.PLAIN_MESSAGE No icon JOptionPane.ERROR_MESSAGE JOptionPane.INFORMATION_MESSAGE JOptionPane.WARNING_MESSAGE JOptionPane.QUESTION_MESSAGE Final overview of adding integers program 1 // clas AddIntegers.java 2 // An addition program 3 4 import javax.swing.joptionpane; // import class JOptionPane 5 6 public class AddIntegers { Declare variables: name and data 7 public static void main( String args[] ) type. 8 { 9 String firstnumber, // first string entered by user 10 secondnumber; // second string entered by user 11 int number1, // first number to add 12 number2, // second number to add 13 sum; // Input sum first of number1 integer as and a number2 String, read in first assign to firstnumber. number from user as a string // 16 firstnumber = 17 JOptionPane.showInputDialog( "Enter first integer" 18 ); 19 // read in second number from user as a string 20 secondnumber = Final overview of adding integers program (cont) 21 JOptionPane.showInputDialog( "Enter Convert second strings to integers. integer" 22 ); 23 // convert numbers from type String to type int 24 number1 = Integer.parseInt( firstnumber ); 25 number2 = Integer.parseInt( secondnumber Add, place ); result in sum // add the numbers 28 sum = number1 + number2; // display the results 31 JOptionPane.showMessageDialog( 32 null, "The sum is " + sum, "Results", 33 JOptionPane.PLAIN_MESSAGE ); System.exit( 0 ); // terminate the program 36 } 37 } Taking input from keyboard Import java.io.package Data from keyboard treats as a stream of data Special java classes desigened to deal with streams of data: InputStreamReader BufferedReader IOException Full list of related classes can be found at Java API specification ion Methods in these classes design to perform different operations with data stream readline() method from class BufferedReader return input data as a String Full list of methods can be found at Java API specification 8

9 Example Java program reading input from keyboard //This application reads in a String input from keyboard and //displays it in command window import java.io.*; public class KeyboardInputTest { public static void main(string args[]) throws IOException { InputStreamReader mystream = new InputStreamReader(System.in); BufferedReader mybuffer = new BufferedReader(myStream); System.out.print("Please, type your message:"); System.out.flush(); String myinput = mybuffer.readline(); System.out.println("You typed in the following message:"); System.out.println(myInput); } } Please, type your message: Hello, Java! You typed in the following message: Hello, Java! Program output 9