Karatsuba multiplication not working recursively with BigInteger

Question

I am trying to multiply n digit number using karatsuba multiplication.

I am getting output for single digit number (Example 4 and 5 = 20).

But, when I multiply 1234 and 5678 I am getting error and no output result.

I have updated the code and error message. Please check.

import java.math.BigInteger;

class karatsubamultiplication{

public static BigInteger karat(BigInteger number1, BigInteger number2){

    if(number1.bitLength() < 4 && number2.bitLength() < 4)
        return number1.multiply(number2);

    //Finding maximum length of two 
    //int m = Math.max(number1, number2);
    String first = number1.toString();
    String second  = number2.toString();

    BigInteger a = new BigInteger(first.substring(0,  first.length()/2));
    BigInteger b = new BigInteger(first.substring(first.length() - first.length()/2, first.length()));

    BigInteger c = new BigInteger(second.substring(0,  second.length()/2));
    BigInteger d = new BigInteger(second.substring(second.length() - second.length()/2, second.length()));

    BigInteger ac = karat(a, c);
    BigInteger bd = karat(b, d);

    BigInteger abcd = karat(a.add(b), c.add(d));

    BigInteger z = abcd.subtract(ac.add(bd));

    int m = String.valueOf(number1).length();

    BigInteger len = new BigInteger(String.valueOf(Math.pow(10, m)));

    return (ac.multiply(len)).add(z.multiply(len.divide(BigInteger.valueOf(2)))).add(bd);
}


public static void main(String[] args){

    BigInteger result = new BigInteger("0");

    BigInteger x = new BigInteger("1234");

    BigInteger y = new BigInteger("5678");

    result = karat(x, y);

    System.out.println(result);
       }
   }

Please correct the code. I have not checked whether two numbers are equal or not and length is odd or even.

Thank you.

That's not the whole error, just the end of the stack trace. Please post the entire exception message. — meowgoesthedog, Aug 31 '17 at 12:41
Looks to me like an endless recursion, hence the repeats of exactly the same message. The first error message is probably the important one, not the repetitive follow ons. — rossum, Aug 31 '17 at 12:43
Ah sorry. But why didn't you first try to put in some **print** statements to understand what your code is doing? The time it took you to write this question ... might have uncovered the problem already?! — GhostCat, Aug 31 '17 at 12:45
Questions seeking debugging help ("why isn't this code working?") must include the desired behavior, a specific problem or error and the shortest code necessary to reproduce it in the question itself. Questions without a clear problem statement are not useful to other readers. See: How to create a [mcve]. Use the "edit" link to improve your *question* - do not add more information via comments. Thanks! — GhostCat, Aug 31 '17 at 12:46
Then: an exception stack trace is **text**. So please add stack traces as well formatted *text* to your question - instead of dumping screen shots on us for no good reason. — GhostCat, Aug 31 '17 at 12:46
Sorry for my unclear question. When I run the program from command line I am able to see this message only. Please feel free to delete this question because I am trying from 2 weeks to multiply numbers using karatsuba multiplication but still not successful. If you can please provide me with proper code. I don't want to use code that is already on net. — Sid, Aug 31 '17 at 12:51
@Sid you need to show us the *top* of this error message, i.e. what *kind* of exception has been raised (`NullReference`? `Overflow`? `InvalidOperation`? `OutOfBounds`? `OutOfMemory`?) — meowgoesthedog, Aug 31 '17 at 12:58
`BigInteger a = new BigInteger(first.substring(0, first.length() / 2));` — Joop Eggen, Aug 31 '17 at 13:15
Also 123456 * 7890 should maintain the same power of 10 when splitting: 1234*100+56, 78*100+90. — Joop Eggen, Aug 31 '17 at 13:22
You simply divide your string too far: a single character string cannot be divided anymore, so at least one of the parts is an empty string. That is what the error message is telling you: It can't make a BigInteger out of an empty string. — Rudy Velthuis, Sep 04 '17 at 09:03

Karatsuba multiplication not working recursively with BigInteger

0 Answers0