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javamathbiginteger
Stephen
Stephen

Reputation: 3992

Int to BigInteger, what's the difference?

I transfer the modExp function from int to BigInteger, but the result is different, what the different of these two functions?

Thanks!!!

The function with BigInteger, the result always 1:

public static BigInteger modExp(BigInteger a, BigInteger b, BigInteger n) {
    BigInteger two = new BigInteger("2");       
    if (b == BigInteger.ZERO)
        return BigInteger.ONE;
    BigInteger t = modExp (a, b.divide(two), n);
    BigInteger c = (t.pow(2)).mod(n);
    if (b.mod(two) == BigInteger.ONE)
        c = (c.multiply(a)).mod(n);
    return c;       
}

The function with int:

public static int modexp(int a, int b, int n) {
    if (b == 0) return 1;
    long t = modexp(a, b/2, n);  // use long for intermediate computations to eliminate overflow
    long c = (t * t) % n;
    if (b % 2 == 1)
       c = (c * a) % n;
    return (int) c;
}

The function is to calculate a^b mod p, For example:

a=4 b=6 p=11  result1 = 1  result2 = 4
a=9 b=2 p=11  result1 = 1  result2 = 4
a=5 b=6 p=23  result1 = 1  result2 = 8 ...

Upvotes: 4

Views: 23947

Answers (2)

Jib'z
Jib'z

Reputation: 1003

In java int can count from -2^31 up to 2^31-1 because int are coded over 4 bytes but long can count from -2^63 up to 2^63-1 because long are coded over 8 bytes.

In the second method with this : 

return (int) c;

you may loose data (the 4 first byte)

That could explain why your result are different because BigInteger are coded over much more byte than long

Upvotes: 0

Danail Alexiev
Danail Alexiev

Reputation: 7772

The obvious difference is the difference between int and BigInteger.

One difference is that int is a primitive type and BigInteger is a reference type. As such, it is better to use equals() when comparing BigIntegers. So b == BigInteger.ZERO should be BigInteger.ZERO.equals(b).

BigInteger is more suited to working with big numbers and will prevent you running into problems with overflowing the max int value, supported by Java.

Overflowing may be the cause you are getting a different result from the two functions as well. When this occurs, it doesn't cause any exception to be thrown, but the values of the ints get messed up.

Upvotes: 9

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