Recursion in Matlab vs Java

Question

I hacked up a recursive function in Java for a homework problem in my Stats class, that looked something like this:

public static int d (int k, int n) {
 if (n == 1) return 1;
 else if (n > k) return 0;
 else return n*d(k-1, n) + n*d(k-1,n-1);
}

I then plugged (20, 8) into this function, and got 998,925,952. My professor, however, said that this answer was wrong, and after rethinking my code over and over again, I decided to try the same thing in Matlab:

function t = d(k,n)
  t = 0;
  if n == 1
    t = 1;
  elseif n > k
    t = 0;
  else
    t = n*d(k-1, n) + n*d(k-1, n-1);
  end

This function, apparently, gave me the right answer with the above input, 6.1169 * 10^17.

This has been bugging me all day, and I have absolutely no idea why two seemingly identical programs in two different languages would give me completely different results. Can anyone help explain this?

Answer 1

Your Matlab routine is probably working on floating-point input, so it will compute in floating-point.

Your Java routine has integer types; 6.1169e17 is way outside the supported range, so it overflows. Try changing your types to float or double .

Answer 2

611692004959217300 is much larger than 2147483647 which is the integer MAX_VALUE in Java.

I got 611692004959217300 by running

function d (k, n) {
 if (n == 1) return 1;
 else if (n > k) return 0;
 else return n*d(k-1, n) + n*d(k-1,n-1);
}

console.log(d(20,8));

in Firebug.

Answer 3

Consider what the maximum value an int can have, which is what you've got in Java. Now consider what the maximum value a double can have, which is MATLAB's default type.

Answer 4

Java integers are 4 bytes in size, so the number looks too big (greater than 2^31). You should try again using "long" or "double" as datatype for your variables.