// Arithmetic demo for El Camino High School AP Computer Science

import java.util.Scanner;

public class Arithmetic {
  public static char ADD = '+';
  public static char SUBTRACT = '-';
  public static char MULTIPLY = '*';
  public static char DIVIDE = '/';
  public static char MODULUS = '%';

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);

    // Get input.
    System.out.print("Enter first number: ");
    double first = input.nextDouble();
    System.out.print("Enter operation: ");
    char operation = getOperation(input);
    System.out.print("Enter second number: ");
    double second = input.nextDouble();

    // Perform operation.
    System.out.print("Result: ");
    if (operation == ADD) {
      System.out.println(first + second);
    } else if (operation == SUBTRACT) {
      System.out.println(first - second);
    } else if (operation == MULTIPLY) {
      System.out.println(first * second);
    } else if (operation == DIVIDE) {
      System.out.println(first / second);
    } else if (operation == MODULUS) {
      System.out.println(first % second);
    } else {
      // This should not happen. Why?
    }
  }

  public static char getOperation(Scanner input) {
    // Get operation. from user.
    char operation = input.next().charAt(0);
    if (operation == ADD
        || operation == SUBTRACT
        || operation == MULTIPLY
        || operation == DIVIDE
        || operation == MODULUS) {
      // If operation is valid, return.
      return operation;
    } else {
      // Operation is invalid, so try again.
      System.out.println("Invalid operation. Try again.");
      return getOperation(input);
    }
  }
}