Help with Java Program for Prime numbers
- by Ben
Hello everyone, I was wondering if you can help me with this program. I have been struggling with it for hours and have just trashed my code because the TA doesn't like how I executed it. I am completely hopeless and if anyone can help me out step by step, I would greatly appreciate it.
In this project you will write a Java program that reads a positive integer n from standard input, then
prints out the first n prime numbers. We say that an integer m is divisible by a non-zero integer d if there
exists an integer k such that m = k d , i.e. if d divides evenly into m. Equivalently, m is divisible by d if
the remainder of m upon (integer) division by d is zero. We would also express this by saying that d is a
divisor of m. A positive integer p is called prime if its only positive divisors are 1 and p. The one
exception to this rule is the number 1 itself, which is considered to be non-prime. A positive integer that
is not prime is called composite. Euclid showed that there are infinitely many prime numbers. The prime
and composite sequences begin as follows:
Primes: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, …
Composites: 1, 4, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 25, 26, 27, 28, …
There are many ways to test a number for primality, but perhaps the simplest is to simply do trial
divisions. Begin by dividing m by 2, and if it divides evenly, then m is not prime. Otherwise, divide by 3,
then 4, then 5, etc. If at any point m is found to be divisible by a number d in the range 2 d m-1, then
halt, and conclude that m is composite. Otherwise, conclude that m is prime. A moment’s thought shows
that one need not do any trial divisions by numbers d which are themselves composite. For instance, if a
trial division by 2 fails (i.e. has non-zero remainder, so m is odd), then a trial division by 4, 6, or 8, or any
even number, must also fail. Thus to test a number m for primality, one need only do trial divisions by
prime numbers less than m. Furthermore, it is not necessary to go all the way up to m-1. One need only
do trial divisions of m by primes p in the range 2 p m . To see this, suppose m 1 is composite.
Then there exist positive integers a and b such that 1 < a < m, 1 < b < m, and m = ab . But if both
a m and b m , then ab m, contradicting that m = ab . Hence one of a or b must be less than
or equal to m .
To implement this process in java you will write a function called isPrime() with the following
signature:
static boolean isPrime(int m, int[] P)
This function will return true or false according to whether m is prime or composite. The array
argument P will contain a sufficient number of primes to do the testing. Specifically, at the time
isPrime() is called, array P must contain (at least) all primes p in the range 2 p m . For instance,
to test m = 53 for primality, one must do successive trial divisions by 2, 3, 5, and 7. We go no further
since 11 53 . Thus a precondition for the function call isPrime(53, P) is that P[0] = 2 , P[1] = 3 ,
P[2] = 5, and P[3] = 7 . The return value in this case would be true since all these divisions fail.
Similarly to test m =143 , one must do trial divisions by 2, 3, 5, 7, and 11 (since 13 143 ). The
precondition for the function call isPrime(143, P) is therefore P[0] = 2 , P[1] = 3 , P[2] = 5, P[3] = 7 ,
and P[4] =11. The return value in this case would be false since 11 divides 143. Function isPrime()
should contain a loop that steps through array P, doing trial divisions. This loop should terminate when
2
either a trial division succeeds, in which case false is returned, or until the next prime in P is greater
than m , in which case true is returned.
Function main() in this project will read the command line argument n, allocate an int array of length n,
fill the array with primes, then print the contents of the array to stdout according to the format described
below. In the context of function main(), we will refer to this array as Primes[]. Thus array Primes[]
plays a dual role in this project. On the one hand, it is used to collect, store, and print the output data. On
the other hand, it is passed to function isPrime() to test new integers for primality. Whenever
isPrime() returns true, the newly discovered prime will be placed at the appropriate position in array
Primes[]. This process works since, as explained above, the primes needed to test an integer m range
only up to m , and all of these primes (and more) will already be stored in array Primes[] when m is
tested. Of course it will be necessary to initialize Primes[0] = 2 manually, then proceed to test 3, 4, …
for primality using function isPrime().
The following is an outline of the steps to be performed in function main().
• Check that the user supplied exactly one command line argument which can be interpreted as a
positive integer n. If the command line argument is not a single positive integer, your program
will print a usage message as specified in the examples below, then exit.
• Allocate array Primes[] of length n and initialize Primes[0] = 2 .
• Enter a loop which will discover subsequent primes and store them as Primes[1] , Primes[2],
Primes[3] , ……, Primes[n -1] . This loop should contain an inner loop which walks through
successive integers and tests them for primality by calling function isPrime() with appropriate
arguments.
• Print the contents of array Primes[] to stdout, 10 to a line separated by single spaces. In other
words Primes[0] through Primes[9] will go on line 1, Primes[10] though Primes[19] will go
on line 2, and so on. Note that if n is not a multiple of 10, then the last line of output will contain
fewer than 10 primes.
Your program, which will be called Prime.java, will produce output identical to that of the sample runs
below. (As usual % signifies the unix prompt.)
% java Prime
Usage: java Prime [PositiveInteger]
% java Prime xyz
Usage: java Prime [PositiveInteger]
% java Prime 10 20
Usage: java Prime [PositiveInteger]
% java Prime 75
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
%
3
As you can see, inappropriate command line argument(s) generate a usage message which is similar to
that of many unix commands. (Try doing the more command with no arguments to see such a message.)
Your program will include a function called Usage() having signature
static void Usage()
that prints this message to stderr, then exits. Thus your program will contain three functions in all:
main(), isPrime(), and Usage(). Each should be preceded by a comment block giving it’s name, a
short description of it’s operation, and any necessary preconditions (such as those for isPrime().) See
examples on the webpage.
