Q:

Java Program for Prime numbers

Problem
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. 
Attempted Solution
class Prime {
    public static void main(String[] args) {
        int num1 = 0;
        int num2 = 0;
        int num3;
        for (num1 = 1; num1 < 101; num1++)
            System.out.println(num1);
        for (num2 = 1; num2 < 101; num1++)
            System.out.println(num2);
        num3 = num2 % num1;
        if (num3 == 0)
            System.out.println("The prime numbers are " + num1);
        else
            System.out.println("The prime numbers are " + (num1 += 1));
    }
}

A:

Ben, it looks like you are attempting something that is far beyond your current capability.  Start with some much simpler problems.  Talk to your teacher and consider taking a more rudimentary course.  You don't appear to understand either what the program is supposed to do, or how to write a program that might satisfy the requirements, and nothing we say here can overcome that - you have to develop more understanding of math and programming.  We're happy to help with that, but just writing your program here won't help you, and you are too far away from a solution for suggestions to help.  I'm sorry if this sounds harsh; honestly, I mean it constructively.  Please stay with it - but start simpler.

A:

Your example solution doesn't really follow the problem's specification at all. You should focus first on writing the static boolean isPrime(int m, int[] P) method. All that method needs to do is:

Iterate over the contents of P
If an element evenly divides m, m is composite -- return false
If an element's square is greater than m, m is prime -- return true. It sounds like from the problem description this won't ever happen, P will only have the primes from 2 to the one just before crossing the sqrt(m) boundary
If all the elements of P have been tested, m is prime -- return true

After that you can write main to make the primes array and build it up using the described loop, and finally do argument checking and implement the static void Usage() function to call if the arguments are invalid

