bit manipulation macros

As part of a data storage project, I am storing floats in 3 byte ints as
follows:

* First 21 bits represent number
* Last 3 bits represent the number of decimal places

I need to write macros that will convert TO/FROM THREE_BYTE_VALUE and float

#define FLOAT_TO_TBV(fval,prec) //may need some input checks to make
sure no overflows
#define TBV_TO_FLOAT(bval, prec) // checks needed here to

Any ideas how to implement this (especially with the overflow checks)?

re: bit manipulation macros

(2b|!2b)==? wrote:What do you do if they don't fit?

Do they have a fixed range?
Why macros?

--
Ian Collins

re: bit manipulation macros

Ian Collins wrote:they will fit (see below)Yes, and the range is covered by the bit representation prescribedI generellay subscribe to the 'macros are evil' mantra - but in this
case, using macros would be a lot easier (read quicker to implement),
than say using template functions (with specializations) for the data
types to be stored.

re: bit manipulation macros

(2b|!2b)==? schrieb:Is that 21 highest bits and 3 lowest bits?
Is that 21 bits mantissa and 3 bits exponent, or what means the number
of decimal places?
Why not functions like

unsigned int float_to_tbv(float)?
Write some code. It helps us to understand what you want.
(And it shows that you at least tryed to make your homework ;-) )

--
Thomas

re: bit manipulation macros

On Oct 21, 12:43*pm, "(2b|!2b)==?" <void-s...@ursa-major.comwrote:Why would the functions have to be templates? What's wrong with
just a regular function?

Writing a safe macro to do this is relatively tricky, but a
function shouldn't be that hard, using standard functions like
ldexp and frexp. Something like:

unsigned int
toExternalRepresentation(
float value )
{
int exp ;
value = frexpf( value, &exp ) ;
return static_cast< int >( value * 2097152 )
| (static_cast< unsigned >( exp ) << 21) ;
}

This obviously requires some additional error checking, and may
contain an off by one error---I'm two lazy to verify whether the
results of frexpf are in the range [0.5,1.0) or [1.0,2.0). But
you get the idea.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
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re: bit manipulation macros

On Oct 21, 4:03*pm, "Thomas J. Gritzan" <phygon_antis...@gmx.de>
wrote:Because that would be too easy:-).
I doubt it's homework; this sort of problem occurs often enough
in real life, and isn't the sort of thing I'd expect to see in a
course. But you're right that the problem isn't well specified.
(On the other hand, my suggestion in another post isn't really
fully fleshed out either, and should be adaptable to any of the
meanings of his specification.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34