The trouble with rounding floating point numbers. So, Prudence, computers do make mistakes… By Dan Clarke.
We all know of floating point numbers, so much so that we reach for them each time we write code that does math. But do we ever stop to think what goes on inside that floating point unit and whether we can really trust it?
I hate to cast aspersions on its good name but when I hear stories of space craft crashing, inconsistent information on bank statements and pensioners being short changed (all of which have happened: see, for example, Risks Digest entries here and here), I start to realise that there is a real danger of misusing floating point numbers. Indeed, anyone with a few years of experience under their belt will probably have either had the pleasure of dealing with a floating-point related bug; or have watched a colleague slowly go crazy over one.
Often, the underlying cause of such problems falls into common categories: a division by zero or a narrowing conversion that loses information. Other times however, it's not so evident – sometimes the cause is the futile attempt of a software developer to round a floating-point number.
That's right, one of the most basic operations in math, a thing that we learn to do before we can ride a bike, eludes the combined efforts of the finest engineers over the last 30 years. Of course, this is something that is intuitively nonsensical - why should it be impossible to round a floating-point number reliably?
more... http://www.regdeveloper.co.uk/2006/08/12/floating_point_approximation/ _______________________________________________ NetBehaviour mailing list [email protected] http://www.netbehaviour.org/mailman/listinfo/netbehaviour
