1. PHY-102 SAPVariables and OperatorsSlide 1 Variables and Operators In this section we will learn how about variables in Java and basic operations one can perform on them:  Variables  Constants  Assignment Statements  Arithmetic Operations  Displaying Variables  The pitfalls of Integer Division  Type Conversion  Incrementing and Decrementing  Operator Precedence  Assignment Operators  Other Operators

2. PHY-102 SAPVariables and OperatorsSlide 2 Variables Variables are names that refer to memory locations where information can be stored. The value of variables can be changed. All variables must be declared before use. int a = 5; This creates a variable called a, sets aside memory for it and writes 5 in that memory location. Variable names have to start with a letter, underscore _ or dollar $. Can have numerals in the name but not at the front i.e. 1a is not allowed but a1 is OK. Variable names are Case Sensitive - myVar and myvar are different variables. Java keywords such as int, class public etc are not allowed as variable names. Memory This creates a variable called myVar, sets aside memory for it but does not put a value in it. int myVar; myVar a 10 10 000 0 0 00 0 00 0 00 0 0 0 0 00 0 0 0 0 0 00 0 0

3. PHY-102 SAPVariables and OperatorsSlide 3 Integer Variables byte - holds values from -128 to +127, takes up 1 byte (8 bits). short - holds values from -32768 to +32767, takes up 2 bytes (16 bits). int - holds values from -2147483648 to +2147483647, takes up 4 bytes (32 bits). long - holds values from -9223372036854775808 to +9223372036854775807, takes up 8 bytes (64 bits). byte smallerValue; short pageCount; int wordCount; long bigValue; Note: Conventional to start with lower case but to Capitalise any words within the name.

4. PHY-102 SAPVariables and OperatorsSlide 4 Floating Point Variables float - holds values from -3.4E38 (-3.4x10 38 ) to +3.4E38 (+3.4x10 38 ), takes up 4 bytes (32 bits). The smallest non zero number is  1.4E-45 (  1.4x10 -45 ). Values have approximately 7 digit accuracy. double - holds values from -1.7E308 (-1.7x10 308 ) to +1.7E3-8 (+1.7x10 308 ), takes up 8 bytes (64 bits). The smallest non zero number is  4.9E-324 (  4.9x10 -324 ). Values have approximately 17 digit accuracy. float speedofLight = 2.998E8; double electronMass = 9.109E-31; For scientific applications suggest you stick to int and double.

5. PHY-102 SAPVariables and OperatorsSlide 5 Other Variables boolean - holds values true or false, takes up 1 bit. char - holds a single character a to z, A to Z, number 0 to 9 or other symbol *, $ / etc, takes up 1 byte (8 bits). String- holds a group of characters, takes as much memory as necessary. boolean gameOver = false; char key = 'H'; String courseName = "Scientific Application Programming"; Note: Capital S for string (it's an Object not a simple variable)

6. PHY-102 SAPVariables and OperatorsSlide 6 Constants Constants are declared like normal variables but have the keyword final in front to indicate that the value cannot change. final double PI = 3.141592654; Note: Conventional to capitalise constant names final double PI = 3.141592654;... PI = 62.3; This would give an error

7. PHY-102 SAPVariables and OperatorsSlide 7 Assignment Statements The simplest assignment just gives a variable some value. sum = 0; numberofTries = 10; Other assignments consist of a variable name followed by an assignment operator followed by a mathematical expression. sum = sum + 1; total = a + b; Takes the value of sum, adds one to it and stores the result back in sum. Takes the values of a and b, adds them together and stores the result in total. Operator

8. PHY-102 SAPVariables and OperatorsSlide 8 Arithmetic Operators + Addition total = total + number; - Subtraction * Multiplication / Division % Remainder numberLeft = total - used; cost = items * price; mean = total / number; remainder = 245 % 3; Takes the value of total, adds number to it and stores the result back in total. Takes the value of total, subtracts used from it and stores the result in numberLeft. Takes the value of items, muliplies it by price and stores the result in cost. Takes the value of total, divides by number and stores the result in mean. Be CAREFUL with INTEGERS! Takes 245, divides by 3 and stores the remainder (2) in remainder. Spaces not necessary but makes it easier to read.

9. PHY-102 SAPVariables and OperatorsSlide 9 Displaying Variables In applets, we use the method drawString from the graphics class to display text on the screen: g.drawString("Hello World!", 50, 50); We can also use the + operator to join two strings together and display them: g.drawString("Hello" + " World!", 50, 50); These give the same result Hello World! If the + operator acts on a string and a number, the number is converted from its binary representation to its character representation and then joined (concatenated) to the string. You can then display the whole thing with drawString: int result = 5; g.drawString("Result = " + result, 50, 50); This can be any string "Result" is not related to your variable result result is converted from 0101 to "5" and joined to "Result = " to give Result = 5 You must have at least one real string otherwise the number will be passed directly as a number to drawString and the compiler will complain. This does not work: g.drawString(result, 50, 50); 

10. PHY-102 SAPVariables and OperatorsSlide 10 Example Calculation import java.awt.*; import java.applet.Applet; public class Calculation extends Applet { public void paint(Graphics g) { int length;// declarations int breadth; int area; length = 20;// assignments breadth = 10; area = length * breadth; // * means multiply g.drawString("Area is " + area, 100, 100); } This example calculates the area of a rectangle:

11. PHY-102 SAPVariables and OperatorsSlide 11 Using Expressions Any expression that gives a result can be used where a variable of the same type as the result can be used: int x = 50; int y = 100; g.drawString("Hello World!", 50, 100); g.drawString("Hello World!", x, y); g.drawString("Hello World!", 5*10, 10+90); g.drawString("Hello World!", x, 2*x); These all write Hello World! at the same point on the display

12. PHY-102 SAPVariables and OperatorsSlide 12 Integer Division If you divide two integers, the answer will be truncated to an integer value - this is usually NOT what you want. import java.awt.*; import java.applet.Applet; public class IntDiv extends Applet { public void paint(Graphics g) { int i = 2/3; double d1 = 2/3; double d2 = 2.0/3.0; g.drawString("i = " + i, 50, 50); g.drawString("d1 = " + d1, 50, 75); g.drawString("d2 = " + d2, 50, 100); } Try this:

13. PHY-102 SAPVariables and OperatorsSlide 13 import java.awt.*; import java.applet.Applet; public class IntDiv extends Applet { public void paint(Graphics g) { int i = 2/3; double d1 = 2/3; double d2 = 2.0/3.0; g.drawString("i = " + i, 50, 50); g.drawString("d1 = " + d1, 50, 75); g.drawString("d2 = " + d2, 50, 100); } Integer Division Divides 2 by 3 then truncates it to store it as an integer. Since the RHS are both integers does an integer division as before. Then converts the result to a double.

14. PHY-102 SAPVariables and OperatorsSlide 14 Type Conversion Sometimes you need to convert from one type to another. This is called casting and is done by putting the required type in brackets before the variable. double d1 = (double)2/3; int i = 2; double x; x = (double)i; This can be used to solve the integer division problem: Converts (casts) i to a double. Converts integer 2 to a double 2.0 before the division. The result is then a double. If you do the reverse and cast a double to an integer you truncate it and lose the decimal places. double x = 2.4; int i; i = (int)x; i becomes 2 and the 0.4 is lost.

15. PHY-102 SAPVariables and OperatorsSlide 15 Incrementing and Decrementing A common task is to increase a number by 1 (incrementing) or decrease it by 1 (decrementing). There are two operators for this: ++ Increment -- Decrement total = total + 1;total++; is equivalent to total = total - 1;total--; is equivalent to

16. PHY-102 SAPVariables and OperatorsSlide 16 Prefix and Postfix int x = 3; int y = 3; int pre = ++x * 10; int post = y++ * 10; g.drawString("pre = " + pre + " x = " + x, 50, 50); g.drawString("post = " + post + " y = " + y, 50, 75); pre = 40 x = 4 post = 30 y = 4 Increments x before multiplication. Increments y after multiplication. To avoid confusion suggest you stick to postfix and don't use it in expressions. int post = y * 10; y++; If the ++ or -- comes after the variable (postfixed) the number is updated after any calculation. If they come before the variable (prefixed) the number is updated before the calculation.

17. PHY-102 SAPVariables and OperatorsSlide 17 Operator Precedence int y = 10; int x = y * 3 + 5; 10 x 3 = 30 plus 5 = 35 or 3 + 5 = 8 x 10 = 80? Answer 35. The following order (precedence) is used:  Incrementing and Decrementing first then  Multiplication, Division and Remainder then  Addition and Subtraction then  The equals sign to set a value. Operators with equal precedence are evaluated left to right int x = 4; int number = ++x * 6 + 4 * 10 /2; 5 x 6 = 30 30 + 20 = 50 4 x 5 = 20 If you want to change the precedence use brackets ( ). int y = 10; int x = y * (3 + 5); Now gives 80 What value is x?

18. PHY-102 SAPVariables and OperatorsSlide 18 Assignment Operators In addition there are a set of 'assignment and operator' operators of the form = These are equivalent to = y = x; The simple assignment operator = sets the variable on the left of the = sign to the value of the variable or expression on the right of the = sign. x += 2; x -= 2; x *= 2; x /= 2; x %= 2; are equivalent to x = x + 2; x = x - 2; x = x * 2; x = x / 2; x = x % 2; myRatherLongName += 2;myRatherLongName = myRatherLongName + 2;

19. PHY-102 SAPVariables and OperatorsSlide 19 Other Operators In addition there are: Comparison Operators - used to compare variables or objects < less than <= less than or equal to > greater than >= greater or equal to == equal to != not equal to Logical Operators - used to perform logical operations && AND || OR Bitwise Operators - used to perform binary operations. We shall use some of these when we make decisions later on.