Re: fixed point vs floating point

[Stephen Leake <stephen_leake@stephe-leake.org>]

tmoran@acm.org writes:

>> The only place I have found fixed point to be useful is for time;
>> everything else ends up needing to be scaled, so it might as well be
>> floating point from the beginning.
>
>   Also for matching instrument or control values, formatting output,
> saving memory, interfacing to C stuff, or future proofing.
>
> In embedded devices measurements usually come in implicitly scaled
> integers, not float, as do output control values.

Well, yes. I do declare fixed point types that match hardware values.
But they immediately get turned into float (or time fixed point); they
are not used in computations.

> If Degrees is fixed point, Degrees'image is much more readable than
> if it's in floating point.

Put (item, fore, aft, exp) gives the same control.

> Usually real world physical values don't need 32 or more bits of float
> for either their range or precision.  If memory size (or IO time) is
> an issue, they can be stored in much smaller fixed point format.

Yes; these are reasonable criteria.

> Very often values passed to C et al are scaled, eg durations are
> milliseconds or seconds or hundreths of seconds, represented as integers,
> angles are tenths of a degree integers, and so forth.  Trying to do
> calculations remembering the proper scaling is error-prone, but the
> compiler will do it correctly if you use fixed point.

Yes. Luckily, I don't have to do that often at all :). 

-- 
-- Stephe