Prompting and Basic Branching

Prompting

• there is a way to ask for input while the program is running
• to do this, we use the Scanner class (much like the System.out class was used for printing)
• requires an import statement; pre-built code is available in packages
• the java.lang package is automatically imported; all others, we have to import explicitly
• the Scanner class pulls information from System.in which can be thought of as representing the keyboard (System.out can be thought of as the screen)
• we specify what we expect from the keyboard using various next methods (e.g. nextInt, nextDouble, etc)
• normally stuff we type from the keyboard is broken into tokens separated by white space
• this is called parsing the input
• so the "next" token is everything typed in up to the next white space
• Consider the following example:

    import java.util.Scanner;  // necessary to use Scanner
  
    public class AskForStuff {
      public static void main (String [ ] input) {
        Scanner keyboard = new Scanner(System.in);
        String person;
        int numArms;
        double gpa;
  
        // get user to give name
        System.out.print("What is your name? ");
        person = keyboard.nextLine();
  
        // get the user's number of arms
        System.out.print("How many arms do you have? ");
        numArms = keyboard.nextInt();
  
        // get the user's GPA
        System.out.print("Give your GPA: ");
        gpa = keyboard.nextDouble();
  
        // now print back what we just received
        System.out.println(person + " has " + numArms
            + " arms.");
        System.out.println("The GPA of " + person +
            " is " + gpa + ".");
      }
    }
    

Practice for Prompting

• P3: write a java program that asks the user for first name, last name (separately from first name), three test grades (prompted separately), and a final exam grade. Then the program calculates and prints the final numeric average in which the average of the test grades is 60% and the final exam is 40%. The full name should printed as well somewhere along the way. [solution]
• P4: try running your P3 program and give it bogus answers to the test grades and see what happens. In particular, try to give letters instead of numbers for test scores. Consider how your prompts should change to make this situation less likely.
• P5: change P3 so that it asks for three test grades to be entered on the same line. That is, just prompt once for the user to enter three items all at once. Again, consider how the user might misinterpret your prompt and try and make it better. [solution]
• P6: Write a java program that reads two floating point numbers and prints their sum, difference, and product. [solution]
• P7: Write a java program that reads in a time duration in seconds and prints the equivalent amount of time in hours, minutes, and seconds. For example, 9999 seconds is equivalent to 2 hours, 46 minutes, and 39 seconds. [solution]
• P8: Write a java program that determines the value of the coins in a jar and prints the total in dollars and cents. Read integer values that represent the number of quarters, dimes, nickels, and pennies. [solution]

Basic Branching with if

• the if statement is used to determine whether or not a set of statements will be run
• Consider this example:

    import java.util.Scanner;
    public class Branch {
      public static void main(String [ ] args) {
        int num;
        Scanner kbd = new Scanner(System.in);
        System.out.print("Gimme a number---> ");
        num = kbd.nextInt();
        if (num > 0)
          System.out.println("Your number is greater than zero!");
      }
    }

• there are various conditional operators we can use to ask questions
• note that we must use the ".equals" method when comparing variables of type String
• multiple conditions can be combined into a single condition using logical operators
• expression can be evaluated inside conditions as well
• an if statement can have statements to run when the condition is true and when it is false
• the true and false branches can have more than one statement if you use braces
• the recommended vertical and horizontal spacing of an if-then-else statement is a matter of debate; the compiler doesn't care for the most part
• if statements can show up inside if statements; these are called nested if statements

Practice for Branching

• What is wrong with the following code fragment?

  if (total == max)
    if (total < sum)
      System.out.println("total == max and is < sum.");
  else System.out.println("total is not equal to max");

• What is wrong with the following code fragment?

  if (length = min_length)
    System.out.println("The length is minimal.");

• What is printed by the following code fragment?

  int num = 87, max = 25;
  if (num >= max*2)
    System.out.println("apple");
    System.out.println("orange");
  System.out.println("pear");

• What is printed by the following code fragment?

  int limit = 100, num1 = 15, num2 = 40;
  if (limit <= limit)
  {
    if (num1 == num2)
      System.out.println("lemon");
    System.out.println("lime");
  }
  System.out.println("grape");

• P9: write a java program that prompts for two integers and prints the larger [solution]
• P10: write a java program that prompts for three real numbers and prints how many of them are greater than zero [solution]