Re: Little Endian -> Big Endian (Ada95 / GNAT), Whats with floating point types?

["Warren W. Gay VE3WWG" <warren@ve3wwg.tk>]

pburnand0-news@yahoo.com wrote:
> Warren W. Gay VE3WWG wrote:
>>Marius Amado Alves wrote:
>>>>... Also, there is a small performance penalty paid for
>>>>shuffling bits around to achieve platform independence.
>>>But surely less than converting to an ASCII image and back. (I suspect
>>>that, contrary to what has been indicated, the significant performance
>>>loss in the ASCII solution is of time, not space.)
>>>
>>>But the ASCII solution is a nice, safe one if you don't have the time or
>>>the will or the possibility to learn to use or just to use the
>>>compiler's. Remember Ada has the wonderful 'Image and 'Value attributes
>>>which take care of the conversion for you.
>>
>>I'm no floating point numeric expert, but by converting to
>>a standard, say IEEE floating point format, you can still
>>represent a machine dependant number with repeating decimal
>>places in a IEEE floating point format (I assume).  Mind
>>you, this depends heavily upon the differences between the
>>two formats.
> 
> Even if there is a standard format, using binary format, there is always the 
> problem of the byteorder and such...

Of course, but that was not my point.

> Using ASCII is cool, because a number is always read from left to right...
> So you can write a floating point number in Ada on Linux and read it in a C 
> program on Windows to store it in a double or a float.  All combinations 
> are possible...

ASCII is also more expensive in time and space. Not much
of a consequence when you transmitting a few numbers,
but is much different if you are transmitting millions
of them. It makes to plan for the worst case in some
of these situations.

>>By converting to ASCII, you lose that, unless you encode
>>the fact that the last digit repeats, in some way (like
>>the number "1.33333*", where * might represent "repeat").
> 
> There are always losses.  

Agreed.

> If you want to keep the number as exact as 
> possible, you should use your Ada packages in a way to print the maximal 
> number of decimals.  You should always be able to control the format of the 
> output.
> 
> So there should be no numer so short as "1.33333", even with the lowest 
> precision format...

I think you missed my point (repeating decimals).

>>If they don't do that, then the number has been rounded,
>>and thus does not quite represent the original value any
>>more ;-)   (at a minimum, it is less faithful than it
>>could be).
> 
> The floating point numbers are always rounded even in an FPU.  No software, 
> no hardware can handle an infinite number of digits...

Of course. But my point was that some hardware does keep
a bit (IIRC), that indicates that this is a repeating
digit (binary/decimal).  The point of this is to reduce
error.

> Doing exactly the same calculation on different computer systems can even 
> give largely different results.

No kidding.

-- 
Warren W. Gay VE3WWG
http://ve3wwg.tk