On Aug 31, 5:28 pm, "Chris Mellon" <[EMAIL PROTECTED]> 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. > FYI, 1 pound 4 shillings was 1.20 pounds; 1.333... pounds was 1 pound 6 shillings 8 pence. I think...
-- http://mail.python.org/mailman/listinfo/python-list
