% operator for BigInteger in java

Question

How to use a%b with big integers? like

...
BigInteger val = new BigInteger("1254789363254125");
...

boolean odd(val){
   if(val%2!=0)
      return true;
   return false;
...

Eclipse says that operator % is undefined for BigInteger.

Any ideas?

Answer 1

Like this:

BigInteger val = new BigInteger("1254789363254125");
public boolean odd(BigInteger val) {
    if(!val.mod(new BigInteger("2")).equals(BigInteger.ZERO))
        return true;
    return false;
}

Or as user Duncan suggested in a comment, we can take out the if statement altogether like so:

BigInteger val = new BigInteger("1254789363254125");
public boolean odd(BigInteger val) {
    return !val.mod(new BigInteger("2")).equals(BigInteger.ZERO));
}

Answer 2

A much more efficient way is to check the last bit. If it is 0 (aka false) the number is even, otherwise it is odd.

public boolean odd(BigInteger i){
    return i.testBit(0);
}

odd(BigInteger.valueOf(1));//true
odd(BigInteger.valueOf(2));//false
odd(BigInteger.valueOf(101));//true
odd(BigInteger.valueOf(100));//false

Also its less lines of code.

Answer 3

I'd use the method remainder of the class BigInteger in this way:

BigInteger result = a.remainder(b);

The assignment is due to the fact that BigInteger is immutable, so a won't be changed by the method.

Answer 4

Use val.mod(2).

BigInteger is an object. You can't use arithmetic operators on objects, that works only with primitives.

% only works with java.lang.Integer because that is implicitly cast (actually, it is called unboxed) to int. But BigInteger can not be unboxed. unboxing / baxing (that means object to primitive / primitive to object conversion) only works with int, float, double, short and byte.

Answer 5

As BigInteger is a class and not a primitive*1, you do not use operators with it. Check the methods for BigInteger: http://docs.oracle.com/javase/1.5.0/docs/api/java/math/BigInteger.html#mod(java.math.BigInteger)

*1: In the case of Integer, Float, you can use operators because the JVM automatically converts the object to its primitive value (autoboxing)