strings (dollar.cents) into floats

MRAB google at mrabarnett.plus.com
Fri Aug 31 19:03:47 CEST 2007


On Aug 31, 5:28 pm, "Chris Mellon" <arka... at gmail.com> wrote:
> On 8/31/07, Steve Holden <st... at holdenweb.com> wrote:
>
> > sturlamolden wrote:
> > > On 31 Aug, 02:12, Wildemar Wildenburger
> > > <lasses_w... at klapptsowieso.net> 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...




More information about the Python-list mailing list