Re: Floating-Point Numbers and Internal Representation

[Martin Krischik <krischik@users.sourceforge.net>]

Matthias Kretschmer wrote:

> Hello,
> 
> I had a problem in one of my programs, that was caused by the internal
> representation of floating-point numbers in the fpu. Concrete:
> calculating the value of an optimum for some large number of objects,
> then in a second doing something with all optimal objects. The problem
> was, that when doing the calculation the second time, the compiler left
> the floating-point number in the fpu which had a higher precision than
> the representation I choosed, so comparing for equality returns always
> "False". The problem would be solved by some operation truncating the
> floating-point number to the precision I orginally wanted or used. I
> could of course put all the values in an array or list and then finding
> optimum and optimal objects, but I don't want to go this way. In C iirc
> I could use a volatile variable to ensure the compiler will put the
> value in and read from the variable before comparing, but to achieve
> something similiar (truncating the precision to that of the type used)
> in Ada?

Yes it is called "pragma Volatile".

> My current solution is to enhance precision to compiler maximum which
> seems to be the machine maximum. But I would like to know if there are
> any better solutions?

Comparting Floatingpoing values in equality is an allways tricky. And I
could bet I answered this question before.

Independent of CPU/Language used I would allways suggest to use

abs (X - Y) < epsilon

with a sufficient but not to small epsilon instead.

Martin

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Ada programming at: http://ada.krischik.com