Assignment #4 - Writing Methods

Objective

Task

• Dice.java
• Primes.java

Exercise 1

1. Write a method called rollDice that returns the result from rolling two standard six-sided dice)
   • The method will take no parameters, but it will return the resulting total of the dice roll as an integer
   • Remember that the rolling of a die involves randomness, so you'll need to use the random number generation library discussed in class (see online notes).
   • Note that rolling two 6-sided dice is NOT equivalent to picking a random number from 2 through 12. This is because when you roll 2 dice, there is a much larger change of hitting some totals than others. You need to roll two dice and add them together
   
   
2. To test this method, write a main() routine that does the following:
   • Ask the user to enter how many times they want to roll the dice, and let them enter a value (for this writeup, I'll call it N)
   • Call the rollDice method this many times (i.e. N times), and count how many times a total of 2 (known as "Snake Eyes") appears, as well as how many times a total of 7 appears.
   • For each of these totals (2 and 7), print out how many times that total appeared, as well as what percentage this is of the total number of rolls. Percentages should be printed to 2 decimal places.
   
   
3. Note: When you declare the Random object, instead of putting it inside of main(), declare this variable inside the class block, but NOT inside of either of the methods. Declare it to be static. This way, it will be accessible to both your rollDice method, as well as to main

Sample Run 1

How many times would you like to roll the two dice? 10000

Snake eyes (double 1s) appeared
   268 times
   2.68 % of the time

A roll of 7 appeared
   1675 times
   16.75 % of the time

Sample Run 2

How many times would you like to roll the two dice? 500000

Snake eyes (double 1s) appeared
   13706 times
   2.74 % of the time

A roll of 7 appeared
   83300 times
   16.66 % of the time

Sample Run 3

How many times would you like to roll the two dice? 500000

Snake eyes (double 1s) appeared
   13921 times
   2.78 % of the time

A roll of 7 appeared
   83789 times
   16.76 % of the time

Exercise 2

1. Recall that an integer is a prime number if it is divisible only by 1 and itself. Write a method called isPrime that takes in one integer parameter X and determines whether or not X is prime. The method should return a boolean value:
   • return true if X is a prime number
   • return false if X is not a prime number.
   (Hint: The % operator is good for checking for divisibility of one integer by another)
    
2. To test this method, write a main() routine that asks the user to input a positive integer N. Using your isPrime() method, find and print all the prime numbers less than or equal to N, where the output has 8 numbers per line (you can use tab characters to separate numbers on a line).

Sample Run

	 Please input a positive number: 500
 The prime numbers less than or equal to 500 are:
 2	 3	 5	 7	 11	 13	 17	 19
 23	 29	 31	 37	 41	 43	 47	 53
 59	 61	 67	 71	 73	 79	 83	 89
 97	 101     103     107     109     113     127     131
 137     139     149     151     157     163     167     173
 179     181     191     193     197     199     211     223
 227     229     233     239     241     251     257     263
 269     271     277     281     283     293     307     311
 313     317     331     337     347     349     353     359
 367     373     379     383     389     397     401     409
 419     421     431     433     439     443     449     457
 461     463     467     479     487     491     499

Requirements for each program

• The required tasks must be performed with the methods specified (not just with a single main() routine)
  • Note that each exercise requires the writing of a method, and a main routine to test that method.
  • Each method should do exactly the task specified in item 1 of each exercise
  • Item 2 of each exercise specifies what to do in main() -- this will always involve CALLING the method, sometimes more than once
  • Note that there is NO keyboard-input/screen-output specified in the methods themselves (i.e. "return" does not mean "print") -- this means you should NOT have any print statements or Scanner usage inside these named methods. Any printing and/or keyboard input is done by main() in each exercise
• When you write source code, it should be readable and well-documented.

Submitting:

 Dice.java
 Primes.java