123.3 + 0.1 is 123.3999999999 ?

[Simon Brunning]

> From:	Isaac To [SMTP:kkto at csis.hku.hk]
> >>>>> "Simon" == Simon Brunning <SBrunning at trisystems.co.uk> writes:
> 
>     >> From: Isaac To [SMTP:kkto at csis.hku.hk] Somehow people keep yelling
>     >> that they can't stand the inaccuracy of floating points, without
>     >> actually looking at how inaccurate (or actually, accurate) they
> are.
>  
>     Simon> Ah, but try this:
> 
>     >>>> sum = 0.0 for i in range(10): sum += 0.1
>     Simon> ...
>     >>>> sum == 1
>     Simon> 0
> 
>     Simon> So, if you do any equality tests, even tiny inaccuracies can
> trip
>     Simon> you up.  Using FixedPoint:
> 
>     >>>> import FixedPoint as fp sum = fp.FixedPoint(0.0) for i in
>     >>>> range(10): sum += sum += 0.1
>     Simon> ...
>     >>>> sum == 1
>     Simon> 1
> 
>     Simon> This works as expected.
> 
> But what if you divide the number by 3, multiply it by 3 again, and
> compare
> with 1?
 
In the context of monetary values, (which is what we are talking about, I
believe) then you wouldn't expect this to sum to 1.

£1 / 3 = £0.33. Multiply by 3, and you get £0.99. Accountants expect
rounding errors like this. So, in a *financial* context, fixed point does
the wrong thing, and FixedPoint works as expected.

In *other* contexts, of course, FixedPoint does the wrong thing and fixed
point is right.

Cheers,
Simon Brunning
TriSystems Ltd.
sbrunning at trisystems.co.uk




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