1. A simple program: Area of a circle Problem statement: Given the radius of a circle, display its area Algorithm: 4. Display area To store values, we need to learn about variables 1. Store the radius value 2. Compute area using formula: Area = radius x radius x Pi 3. Store the Area value

2. Variables Variables store data…input, output or inBetweeny stuff Variables are like shaped boxes…. Variables must be set up ("declared") before use Each variable is of a certain datatype ( int, float, double, String ) and each variable has a name (an identifier). Integers go in integer-shaped variables (labelled int ) Real numbers go in real-number shaped ones ( float, double) Strings (“hello”) can go in string-shaped variables (String)

3. Declaring variables……. ; int x; // a box called x that will hold an integer double radius; /* a box called radius that will hold a floating point no. */ char ch; // ch can hold a single character Int, double, float, char all begin with a lower case letter. String begins with a capital letter. String myName; // myName holds a string of characters

4. Identifiers Identifiers name things: variables, methods, classes….. Must start with a letter (or _ or $) Cannot contain an operator, e.g. +, - Cannot be a reserved word, e.g. public, static, double, int Can be as long as you like……… Should be descriptive Rules for identifiers (variable names, and names for methods, classes)

5. Identifiers…… Good or bad? area radius403 3rdRadius d+4 thisIsAVeryLongIdentifierIndeed i double ISTHISOK

6. Assigning to variables = ; x = 1; radius = 1.0; ch = 'a'; x = 1 + (2 * 3) + (3 * 4); = means put something into a variable.

7. Program: ComputeArea Public class ComputeArea { public static void main(String[] args) { double radius; double area; // step 1: store radius // step 2: compute and store area // step 3: display the area } (same as the Algorithm we saw earlier)

8. Program: ComputeArea Public class ComputeArea { public static void main(String[] args) { double radius; double area; // step 1: store radius radius = 1.23; // step 2: compute, store area area = radius * radius * 3.1416; // step 3: display the area System.out.print(“A circle of radius ”); System.out.print(radius); System.out.print(“ has an area of ”); System.out.println(area); } ‘Declare’ variables radius and area (boxes to hold radius and area values) Store radius value in the variable Compute area and store in area variable

9. Things to note about ComputeArea 3.1416 is the value of pi. We could have a variable called pi if we wanted, and put that value in it. What type? There is a sequence of print statements, ending with a println. This means everything will be printed on a single line. We can print strings (in quotes) and variables ( radius and area ). When we print a variable, its value comes out. We have to type the value of radius into the program. It’d be better to read it in: we’ll see how to do that later.

10. More assignment Datatype on LHS (Left Hand Side) must be compatible with datatype on RHS (Right Hand Side): int x; x = 3; // ok x = 3.1; // not ok, as we said x was of type int LHS: must be a variable 1 = x; // this is meaningless RHS: expression can include variables… x = x + 1; // ok if x already has some legal value

11. Shortcut: declare and initialize Initialization: giving a variable a value for the first time We can declare and initialize in the same line (good practice!) int x = 20; double d = 3.14; Variables must be declared before usage! myName = “Fintan”; String myName; // NOPE! String myName = “Fintan”;

12. Homework Read Liang Ch.2 pp. 25 – 32.