Question

I am learning java myself these days. This function is to calculate the Combination. However, I found that there is a very small limit of the number n and k in this function. Each time if I type a large n or k, for instance, 100, it gives me

Exception in thread "main" java.lang.ArithmeticException: / by zero
at Combination.combination(Combination.java:29)
at Combination.main(Combination.java:47)

Or gives me a negative number…

Is there a way to make it work for large number like 10000?

Thanks!

import java.util.HashMap; import java.util.Map;

public class Combination {

private Map<Long,Long> factorialMap = new HashMap<Long,Long>();

public Long getFactorial(int number) {
    Long val = factorialMap.get(number);
    if(val != null) {
        return val;
    } else {
        val = getFactorialRecursive(number);
        factorialMap.put((long) number, val);
        return val;
    }
}

public Long getFactorialRecursive(int number) {
    if(number == 1 || number == 0) {
        return 1L;
    } else {
        return number * getFactorialRecursive(number-1);
    }
}

public Long combination(int fromVal, int chooseVal) {
    return getFactorial(fromVal)/(getFactorial(chooseVal)*getFactorial(fromVal-chooseVal));
}


public static void main(String[] args) {
    int   n, k;
    Combination comb = new Combination();
    java.util.Scanner console = new java.util.Scanner(System.in);

    while (true)  // will break with k > n or illegal k or n
    {  System.out.print ("Value for n:  ");
       n = console.nextInt();
       if ( n < 0 ) break;
       System.out.print ("Value for k:  ");
       k = console.nextInt();;
       if ( k > n || k < 0 )
          break;
       System.out.print(n +" choose " + k + " = ");
       System.out.println(comb.combination(n,k));
    }
    console.nextLine();    // Set up for "Press ENTER...


} }

Answer 1

You should use the BigInteger object: http://docs.oracle.com/javase/7/docs/api/java/math/BigInteger.html

In particular, your problem is that 21! is too large for even a long and therefore overflows. Another option would be to use a double, but that will lose precision, so if you need integer accuracy BigInteger is the way to go.

Using BigInteger you will need to convert your integer to BigInteger:

BigInteger bi = new BigInteger(intVal+"");

Then use the add, multiply, divide and subtract (amongst others) to manipulate your values (like):

bi = bi.add(bi2);

Then you can use the method longValue() to get the value back (assuming it fits in a long):

return bi.longValue();