Mark Dickinson <dicki...@gmail.com> added the comment: About the footnote:
floor(log(n, 2)) is poor code. This is not supposed to be a dramatic statement, just a statement of fact. Its correctness is dependent on minute details of floating point. It is poor code in exactly the same way that "while x < 1.0: x += 0.1" is poor code---behaviour in boundary cases is almost entirely unpredictable. If 1 + floor(log(n, 2)) happens to give the correct result in the common corner case where x is a power of 2, then that's due to little more than sheer luck. Correct rounding by itself is nowhere near enough to guarantee correct results. In the case of IEEE 754 doubles, a large part of the luck is that the closest double to log(2) just happens to be *smaller* than log(2) itself, so that the implicit division by log(2) in log(x, 2) tends to give a larger result than the true one; if things were the other way around, the formula above would likely fail for many (even small) n. So I don't like seeing this poor code in the Python reference manual, for two reasons: (1) it might get propagated to real world code, and (2) its presence in the docs reflects poorly on the numerical competence of the Python developers. IMO, either: (1) the warning needs to be stronger, or (2) the formulation should be given purely mathematically, without any explicit code, or (3) the formula should be left out of the docs altogether. Mark _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue3439> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
