float safety clarification

[John W. Baxter]

In article <Pine.BSF.4.10.10006260856360.16661-100000 at chi.pair.com>, 
Michal Wallace <sabren at manifestation.com> wrote:

> Hey all,
> 
> I'm building a shopping cart, so I want to be able to store and work
> with prices. I've always been told that you shouldn't use floats for
> money. From past discussions here, I even understand why floats can be
> inaccurate... But in actually trying it out, I can't seem to produce
> any real-world situations relevant to my cart where using floats would
> screw me up... I guess I'm asking for a sanity check here.
> 
> Are floats safe to use for money values if you're only going to be
> adding and subtracting them, or multiplying them?
> 
> I tried this:
> 
> >>> for x in range(100):
> ...    print "%.2f" % (float(x)/100),
> 
> And it looks right. I haven't gotten any funny results with just
> adding or subtracting.. 
> 
> Here are the operations I need to perform:
> 
>   - assign a price
>   - multiply price by (whole number) quantity
>   - add a bunch of prices together
>   - calculate a sales tax
>   - possibly some day give a percent discount 
> 
> I thought sales tax might be a problem, but I haven't been able to
> come up with a problem where a percent sales tax represented as a
> float causes any float problems, either.
> 
> Basically, I can't find a reason not to use floats. 
> Am I asking for trouble?
> 
> Cheers,
> 
> - Michal

You'll probably be OK as long as you don't compare (with 0.00, or with 
any other specific value).  Are you doing something like offering free 
shipping on orders of $100 or more?  Comparison against 100 or 100.00 
could produce a wrong answer if the sum of the items is slightly less 
than 100.  Same for free shipping "over $100"...you might have a total 
"equal" to 100 for which sum > 100.00 is true (less bad:  you aren't 
failing to give something you should have, landing in FTC hell).

I wouldn't do it...I see no reason to work with decimal money with a 
method which expresses only 4% of the possible 100 different cents 
values exactly (.00, .25, .50, .75).

   --John

-- 
John W. Baxter   Port Ludlow, WA USA  jwbnews at scandaroon.com