Chris Mellon wrote: > On 8/31/07, Steve Holden <[EMAIL PROTECTED]> wrote: >> sturlamolden wrote: >>> On 31 Aug, 02:12, Wildemar Wildenburger >>> <[EMAIL PROTECTED]> wrote: >>> >>>> I've heard (ok, read) that several times now and I understand the >>>> argument. But what use is there for floats, then? When is it OK to use >>>> them? >>> There are fractions that can be exactly represented by floats that >>> cannot be exactly represented by decimals. >> Would you care to give an example? >> >>> There are fractions that >>> can be exactly represented by decimals that cannot be exactly >>> represented by floats. >>> >>> Which one is better? Which do we prefer? >>> >>> What a float cannot do is to represent a decimal fractional number >>> (e.g. 1.1) exactly. If we need that, we cannot use floats. A notable >>> example is monetary computations, it covers 99% of the use for decimal >>> numbers in computers. For this reason, we should never use floats to >>> add 10 cents to a dollar. The use of decimals for monetary >>> calculations is mandatory. >>> >> That last sentence is patent nonsense, and completely untrue. Many >> satisfactory financial applications have been written using only >> floating-point arithmetic. Indeed I believe the accountant's Swiss army >> knife, the Excel spreadsheet, uses floating-point numbers exclusively. >> > This is true, although Excel munges it's FP to provide "expected" results. > > It depends on what you consider a "financial" application though. > Excel, while in extremely broad use, is not used to implement any of > the systems which actually "define" money, like the back end financial > systems at banks and credit unions. > > In my experience, by far the most common method of calculating > financial numbers is actually using integer amounts, and then applying > well-defined rounding rules which I can't be bothered to look up the > name for. > > For what it's worth, the work that I do with money (which is > middleware doing data transport, not calculations or billing) uses > either string representations or fixed point. > >> What you say about floating-point have speed advantages is true, but you >> go too far in claiming that decimal arithmetic is mandatory for monetary >> calculations. That's about as sensible as saying that base 12 and base >> 20 arithmetic units were required to calculate in pounds, shillings and >> pence. >> > > I believe that to the degree that "real" accounting was done in those > currencies it did in fact use non-decimal bases. Just as people don't > use decimal time values (except us crazy computer folk), you're write > 1 pound 4 shillings, not 1.333... pounds.
There is much in what you say (which, I am happy to note, doesn't really contradict much of what I wrote). While I agree that Excel isn't a fundamental component of back-end financial systems you might be surprised at just how many people rely on its "currency" cells for financial applications, albeit of a more lightweight kind. You let yourself down just a little right at the end by not knowing the intricacies of the old sterling currency - since there were twenty shillings to the pound and twelve pennies to a shilling that would have been £1.2. However, you are correct in assuming that nobody tried to represent £1 3s. 4d. as 23.333... shillings. regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC/Ltd http://www.holdenweb.com Skype: holdenweb http://del.icio.us/steve.holden --------------- Asciimercial ------------------ Get on the web: Blog, lens and tag the Internet Many services currently offer free registration ----------- Thank You for Reading ------------- -- http://mail.python.org/mailman/listinfo/python-list
