import java.util.Scanner;

/**
 * Provides a simple command-line calculator that supports single addition, subtraction,
 * multiplication, and divison.
 * 
 * <p>Valid expressions are of the following form: NUM OP NUM, where NUM represents a decimal
 * number and OP is either a +, -, *, or /. Spaces are required to separate the numbers
 * from the operator.
 * 
 * <p>This program demonstrates good use of if-else-if constructs, as well as defensive programming.
 * I think that this program will respond in a well-behaved manner (i.e. no crashes) to any
 * user input. If you find it to be otherwise, please let me know!
 * 
 * @author pollen
 */
public class Calculator {
    public static void main(String[] args) {
        
        Scanner in = new Scanner(System.in);
        
        // Give the user a prompt.
        System.out.print("Enter expression: ");
        
        // Guard against bad numeric input.
        if (in.hasNextDouble()) {
            double firstNum = in.nextDouble();
            
            // Read the operator in as a string, since we can't parse a plus or minus
            // sign as a double or an int.
            String operator = in.next();
            
            // Guard against bad numeric input.
            if (in.hasNextDouble()) {
                double secondNum = in.nextDouble();
                
                // Check the operator and perform the actual operation.
                if (operator.equals("+")) {
                    System.out.println(firstNum + secondNum);
                    
                } else if (operator.equals("-")) {
                    System.out.println(firstNum - secondNum);
                    
                    // I know we didn't do multiplication and division in lecture, but I told you
                    // they were pretty easy to add. Here they are:
                    
                } else if (operator.equals("*")) {
                    System.out.println(firstNum * secondNum);
                    
                } else if (operator.equals("/")) {
                    // And you probably thought you could crash the program by having it divide
                    // by zero. Try it out (1/0) and you will learn something about how Java handles
                    // doubles. You could also try 0/0 -- the result is different.
                    
                    System.out.println(firstNum / secondNum);
                    
                } else {
                    // The operator was not a plus or minus sign. Signal to the user that this is an error.
                    System.out.println("Invalid operator: " + operator);
                }
                
            } else { // The second number was badly formed.
                System.out.println("Invalid number");
            }
            
        } else { // The first number was badly formed.
            System.out.println("Invalid number");
        }
        
    }
}