Accuracy of the log function

From: O.K.Man <okman_at_netvigator.com>
Date: Wed, 29 Aug 2001 23:06:21 +0800
Message-ID: <9mj0ee$keh1@imsp212.netvigator.com>

Dear all,

I've tried to write a function to determine the number of bits of an integer, the implementation is like this:

FUNCTION bit_cnt

        (pi_number NUMBER
        ) RETURN NUMBER IS

BEGIN
  IF pi_number > 0 THEN
    RETURN TRUNC(LOG(2,pi_number)) + 1;
  ELSE
    RETURN 1;
  END IF;
END bit_cnt;

However, I found that the function return wrong result if the number is the power of 2 (i.e. 2, 4, 8, 16, 32, etc) and then discovered that the values returned by the LOG function on these values are actually 0.99999..., 1.99999..., 2.99999..., 3.9999..., 4.9999... etc. After the TRUNC function, the values returned by my function are 1, 2, 3, 4, 5, etc and obviously are wrong.

I believe that replacing the TRUNC by ROUND, FLOOR or CEIL will not solve the problem.

And I finally modified the function like this

FUNCTION bit_cnt

       (pi_number NUMBER
       ) RETURN NUMBER IS

  vl_bit NUMBER;
BEGIN
  IF pi_number > 0 THEN
    vl_bit := TRUNC(LOG(2,pi_number)) + 1;

• counter check the result IF POWER(2,vl_bit) <= pi_number THEN vl_bit := vl_bit + 1; END IF; RETURN vl_bit; ELSE RETURN 1; END IF; END bit_cnt;

My question is that can we consider the behaviour of the LOG function as a bug ? Is there a better way to implement my function ?

Best regards,

O.K.Man Received on Wed Aug 29 2001 - 10:06:21 CDT