CODEWITHSUNDEEP
javabiginteger
microslayer
microslayer

Reputation: 155

Java - Converting to BigInteger

I build a program to find out the largest prime factor of 2^1000. It worked, and gave the right answers to my smaller test numbers, but then I realized that for my number I would have to use BigInteger. I've never used it before so I'm pretty sure I did something wrong. The BigInteger program doesn't give me back anything and doesn't finish running.

Here's the program, using primitive data types, that works: 

public class PE3 { public static void main(String[] args) {
  int num = 13195; 

  //find factors of 'num'
  for (int i=(num); i>2; i--) {
    if ((num%i)==0)
      testPrime(num, i); 
  }
 }

  // find if factor is prime
  public static void testPrime(int num, int i){
    for (int j=2; j<i; j++) {
      if ((i%j)==0) 
        break; 
      if (j==(i-1))
        System.out.println(i); 
}
  }}

Here's (what I think is) the same program using BigInteger: 

import java.math.BigInteger; 

public class PE3BI { public static void main(String[] args) {
  BigInteger num = new BigInteger("600851475143"); 
  BigInteger zero = new BigInteger("0"); 
  BigInteger one = new BigInteger("1"); 
  BigInteger two = new BigInteger("2"); 

  //find factors of 'num'
  for (BigInteger i = new BigInteger("600851475143"); i.compareTo(two)==1; i.subtract(one)) {
    if ((num.mod(i))==zero)
      testPrime(num, i, one, zero); 
  }
  }

  // find if factor is prime
  public static void testPrime (BigInteger num, BigInteger i, BigInteger one, BigInteger zero){
    for (BigInteger j = new BigInteger("2"); j.compareTo(i)==-1; j.subtract(one)) {
      if ((i.mod(j))==zero) 
        break; 
      if (j.compareTo(i.subtract(one))==0)
      {System.out.println(i); 
        System.exit(0); }
    }}}

Upvotes: 1

Views: 117

Answers (3)

Forseth11
Forseth11

Reputation: 13

It might be better to use a long instead of the BigInteger class?

I just made this using your code to check if a number is prime or not prime:

long num = 600851475143L; 

boolean prime = true;//Check this after for loop for prime.
for (long i=2; i<num; i++) {//Loops through all numbers between 2 and the number given to test.
    if ((num%i)==0){//This is true if number is not prime.
        prime = false;
        System.out.println("Not prime: " + i);
        break; 
    }
}

Upvotes: 0

k_g
k_g

Reputation: 4483

While, piotrek's answer will solve one of your problems, your program still won't work.

Need to do's:

  1. Don't use == for comparisons. It doesn't work for non-primitives, such as BigInteger(piotrek)
  2. i.subtract(one) as the last item in a loop does nothing. Use i = i.subtract(one) and the equivalent for j.

Would be good:

Although technically correct, a.compare(b) == -1 and a.compare(b) == +1 are typically not the best practice for code readability and it might not be the case that compare returns one of these three values in all situations, so don't get in the habit. Use a.compare(b) < 0 and a.compare(b) > 0 instead.

Upvotes: 1

piotrek
piotrek

Reputation: 14550

you can't do comparisons like object == zero. it works only for primitives. now, your numbers are objects so it checks if reference is the same. change it to compare or equals

Upvotes: 0

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