CODEWITHSUNDEEP
javabiginteger
Sparrow
Sparrow

Reputation: 305

BigInteger modulus not positive exception

I have a function which decides whether a given BigInteger is a prime number or not. In the main class I made a call to that function with an argument passed. Now when I try to compile, I get the following error.

C:\Users\me\Downloads>java RSA_n_prime2_using_int
Exception in thread "main" java.lang.ArithmeticException: BigInteger: modulus not positive
        at java.math.BigInteger.mod(Unknown Source)
        at RSA_n_prime2_using_int.prime_check(RSA_n_prime2_using_int.java:92)
        at RSA_n_prime2_using_int.main(RSA_n_prime2_using_int.java:20)

My code looks something like this

public static boolean prime_check(BigInteger val)
{
    BigInteger prime_chk=new BigInteger("0");
    //System.out.println("in the function");
    boolean isprime=true;
    BigInteger prime_value=val.add(BigInteger.ZERO);

    if(val.equals(BigInteger.ZERO)||val.equals(BigInteger.ONE)) 
        return false;
    for(prime_chk.valueOf(2);prime_chk.compareTo(prime_value)<0;prime_chk.add(BigInteger.ONE))
    {
        if((prime_value.mod(prime_chk)).equals(BigInteger.ZERO))
            {
                isprime=false;
                break;
            }
    }
    return isprime;
}

In the main function, the call is made as follows

s1 = new BigInteger("1021");//s1=(int)Math.round(Math.random()*1000)%30;
if(prime_check(p1))
{
        System.out.println(p1+" is prime");
}

Please help me in finding, where I have went wrong.

Upvotes: 1

Views: 913

Answers (1)

paxdiablo
paxdiablo

Reputation: 881293

Having set prime_chk to zero at the start of the function, you then do:

for (prime_chk.valueOf(2); blah; blah ) {
    if((prime_value.mod(prime_chk)) {
        blah;
    }
}

That first part of the if statement (prime_chk.valueOf(2)) doesn't actually change prime_chk, it simply evaluates the 2 and creates a big integer of that value (which you appear to then throw away). Thus, when you execute prime_value.mod(prime_chk), prime_chk is still set to zero, hence your exception (some may argue the case but zero is neither negative nor positive - in any case, x mod 0 is a problematic operation regardless of the arguments raised).

Perhaps you might want to change the initial part of the if to something like:

for (prime_chk = BigInteger.valueOf(2); blah; blah ) {

This will actually change prime_chk to be 2 and hence avoid your exception.

One other point, you may be able to save a lot of cycles if you limit prime_chk to between 2 and the square root of the test number. If you haven't found a divisor below that, it's mathematically guaranteed you won't find one above. The normal method would be along the lines of (pseudo-code, obviously):

def isPrime (val):
    for (chk = 2; chk * chk <= val; chk++):
        if (val % chk) == 0:
            return false
    return true

Upvotes: 1

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