CS 117
Lab 7 - Random Numbers

Time estimate: not long, an hour or so should suffice.

Due: By classtime Thur, 10/5. (If you've more pressing work and desire an extension, send me a request via e-mail.)

Collusion: You may work together on the lab; but for the ensuing homework you each should maintain and submit your own program. If you wish to share the results of your lab before starting on the homework, simply email the file to the partner whose account you are not in. S/he can retrieve it from the mail.

What's emphasized in this lab:

• Math.random()
• The while, do-while statement and for statement.
• The switch statement.
• Defining our own functions (methods).
• Making your own non-instantiable class

• 1. Log in, make a Lab7 subdirectory, get to an editor.

• 2. Modify this code so it uses a while rather than the do-while loop:

  int i = 0;
  System.out.println(); //What's this statement do?
  do
  {   
      double x = Math.random();
      System.out.println(i+"  "+x);
  	i++;
  } while(i<10);
  

• 3. Compile and run a program testing your code. Notice where the output occurs. Run it again; check the output. Are the numbers the same? If all is going well, you're generating random numbers (doubles) between 0 and 1. Note that the do-while is similar to the while except the test is at the end.

• 4. Modify the code so it uses a for rather than a do-while:

  for(int i=0; i<10; i++)
  {   
      double x = Math.random();
      System.out.println(i+"  "+x);
  }
  

  Notice carefully the syntax of the for-statement; note how compact it is. The int i can be declared right in the loop rather than before it.

  Try to output the value of i after the loop. What happens?

  Note when the autoincrementation occurs. Is it at the beginning or the end of the loop?

  When is the condition, i<10, tested - at the beginning or end?

• 5.
  • Multiply Math.random() by 6. What results do you get?
  • Add 1 to that. Now you should be getting numbers greater than 1 but less than 7. Right?
  • If you use an explicit cast to make the result an int (or take the floor first if you want) you should get ints between 1 and 6, inclusive. These numbers are called pips in dice games. (By adding two such random numbers together we could play craps.)
  • Before continuing, define a method which has two int parameters and returns a random int between a and b, inclusive.

    public static int ranInt( int a, int b)
        {
            return (int) (a+(what goes here?)*Math.random());
        } 
    

    You would call this in main by using the actual arguments, e.g.

       int pips = ranInt(1,6)
    

• 6. Let's test the die by rolling it 60 times. Do that by changing the number of iterations from 10 to 60. (If the first value of n is 0, the last will be 59.) You should be getting about the same numbers of 1's, 2's,...,6's. You could count them by hand, but that's what computers are for. Let's declare several counters and initialze them all to 0.

     int counter1 = 0, counter2 = 0,...,counter6 = 0;   
  

  Now in the loop where pips is generated, let's increment the appropriate counter, which is counter1 for 1 pip, etc, each time pips is generated.

          for(int i=0; i<60; i++)
          {   
              int pips = ranInt(1,6);
              if (pips == 1) counter1++;
              if (pips == 2) counter2++;
                ...
              if (pips == 6) counter6++; 
          }
      
  

  Note that there's no output in the loop. Rather, after the loop let's output the counters...

    
         System.out.println("There were:");
         System.out.println(counter1 + "1's");
         System.out.println(counter2 + "2's");
          ...
         System.out.println(counter6 + "3's");
  

  Run the program. Check that the counters add up to 60.

• 7. Next let's use "else"'s in between the "if"'s. This is better, because once the correct choice is found, subsequent possibilities aren't unnecessarily checked.

• 7. Still Better, in my opinion, is to use a switch. Then the whole main loop looks like:

          for(int i=0;i<60;i++)
          {   
              int pips = ranInt(1,6);
  			
              switch(pips)
              {  case 1: counter1++; //break;
                 case 2: counter2++; //break;
                 case 3: counter3++; //break;
                 case 4: counter4++; //break;
                 case 5: counter5++; //break;
                 case 6: counter6++;
              }
          }
  

  Test it. Notice this has almost the same effect. However, now were getting the cumulative counts. Uncomment the breaks and it should come out right. Once you're use to it, I think switch is easier to read then if-else's and logically better than if's. Switch just picks out the right case and increments that counter.

• 9. Now separate your program into two files Ran.java and RanTest.java. Ran.java contains the Ran class which has all the methods just dealing with random numbers. The class should be non-instatiable and so all its members should be static. In the future you should be able to use this class with other programs.

  The purpose of the RanTest or TestRan class is simply to test the Ran class.

• 10. Add at least one more method to your Ran class and test it. Some possibilities are:
  • a random double between a and b,
  • a random capital letter,
  • a random lower case letter,
  • a random integer digit,
  • a random character digit.

    Ran r = Ran();
    int pips = r.ranInt(1,6);

    int pips = Ran.ranInt(1,6);