1 Interactive Applications (CLI) and Math Interactive Applications Command Line Interfaces The Math Class Flow of Control / Conditional Statements The if Statement Reading for this class: L&L, 3.5, 5.1, 5.2

2 Interactive Applications (CLI) An interactive program with a command line interface contains a sequence of steps to: –Prompt the user to enter input data –Read and save the user’s responses –Process the data after all input(s) are received We can prompt the user: System.out.println(“prompt text”); We can read and format user responses: type variable = scan.nextType();

3 Interactive Applications (CLI) Similar to Quadratic.java (Page 129) int a, b, c;// integer coefficients Scanner scan = new Scanner(System.in); System.out.println(“Enter coefficient A”); a = scan.nextInt(); System.out.println(“Enter coefficient B”); b = scan.nextInt(); System.out.println(“Enter coefficient C”); c = scan.nextInt(); // we have the data to solve the equation // ax-squared + bx + c = 0 for it’s roots

4 We have the input values, now what? To solve the quadratic equation, we need to program in Java the formulas learned in high school algebra: discriminant = b squared – 4ac root1 = (-b + squareroot of discriminant)/2a root2 = (-b - squareroot of discriminant)/2a How do we program those equations? We need to use the Math Class Library, Expression Evaluation, and Assignment

5 The Math Class The Math class is part of the java.lang package The Math class contains methods that perform various mathematical functions These include: –absolute value –square root –exponentiation –trigonometric functions

6 The Math Class The methods of the Math class are static methods (also called class methods) Static methods can be invoked through the class name – no object of the Math class is needed value = Math.cos(90) + Math.sqrt(delta); Similar to Quadratic.java (page 129)Quadratic.java discriminant = Math.pow(b, 2) – 4.0 * a * c; root1 = (-1.0 * b + Math.sqrt(discriminant))/(2.0 * a); root2 = (-1.0 * b – Math.sqrt(discriminant))/(2.0 * a); Note: We can’t program the + in the formula on page 130 in Java. We need to calculate each root separately

7 Flow of Control Unless specified otherwise, the order of statement execution through a method is linear: –one statement after another in sequence Some programming statements allow us to: –decide whether or not to execute a particular statement –execute a statement over and over, repetitively These decisions are based on boolean expressions (or conditions) that evaluate to true or false The order of statement execution is called the flow of control

8 Conditional Statements A conditional statement lets us choose which statement will be executed next Therefore they are sometimes called selection statements Conditional statements give us the power to make basic decisions The Java conditional statements are the: –if statement –if-else statement –switch statement

9 The if Statement The if statement has the following syntax: if ( condition ) statement; if is a Java reserved word The condition must be a boolean expression. It must evaluate to either true or false. If the condition is true, the statement is executed. If it is false, the statement is skipped.

10 Conditions/Boolean Expressions A condition is often obtained using an equality operator and/or relational operator which create boolean expressions that return boolean results: == equal to != not equal to < less than > greater than <= less than or equal to >= greater than or equal to Note the difference between the equality operator ( == ) and the assignment operator ( = )

11 The if Statement An example of an if statement: First the condition is evaluated -- the value of sum is either greater than the value of MAX, or it is not If the condition is true, the assignment statement is executed -- if it isn’t, it is skipped. Either way, the call to println is executed next See Age.java (page 208)Age.java if (sum > MAX) delta = sum - MAX; System.out.println ("The sum is " + sum);

12 Indentation The statement controlled by the if statement is indented to indicate that relationship The use of a consistent indentation style makes a program easier to read and understand Although it makes no difference to the compiler, proper indentation is crucial to human readers "Always code as if the person who ends up maintaining your code will be a violent psychopath who knows where you live." -- Martin Golding