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Date: 22 Nov 91 21:10:01 GMT
From: dove!bruce.nist.gov@uunet.uu.net  (Jonathan Parker)
Subject: Re: Red-faced professor gets bitten in search for portability
Message-ID: <1042@dove.nist.gov>
List-Id: <comp.lang.ada>

In article <1991Nov22.090650.28@v7.vitro.com> vaxs09@v7.vitro.com writes:
> In article <11919@spim.mips.COM>, murphy@mips.com (Mike Murphy) writes:
> > Why not just
> > check whether you indeed rounded down after doing the integer conversion?
> > For example:
> > 	FUNCTION Trunc (X: Float) RETURN Integer IS
> > 		itrunc : integer = integer(x - 0.5);
> > 	BEGIN
> > 		if x - float(itrunc) >= 1.0 then
> > 			-- rounded down, so add back a 1
> > 			return itrunc+1;
> > 		else
> > 			return itrunc;
> > 		end if;
> > 	END Trunc;
> >
> I think this version focuses more clearly on the original problem.
> 
> 	FUNCTION Trunc ( X : Float ) RETURN Integer IS
> 		i : integer := integer ( x );
> 		if x < float(i) then
> 			return i - 1;
> 		else
> 			return i;
> 		endif;
> 	END Trunc;
  There's still more to it, if I understand the Pascal Trunc function
correctly.  Trunc doesn't round, it truncates.  For example, Trunc(-3.999)
is -3, not -4.  Below is an excerpt from W. A. Whitaker's implementation
of the elementary math functions in Cody and Waite's famous book on the
subject.  I took this unmodified from files in the Ada repository on
SIMTEL-20. Floating is a generic Float type.   -- Jonathan  

    function Truncate (X : Floating) return Floating is
    --  Optimum code depends on how the system rounds at exact halves
        Z : Floating := Floating (Integer (X));
    begin
        if X = Z then
            return Z;
        else
            if X > Zero then
        --  For instance, you can get into trouble when 1.0E-20 is overwhelmed
        --  by HALF and X - HALF is -HALF and that is "truncated" to -1
        --  so a simple floating (Integer (X - Half)) does not work in all  
cases
                if X < Z then
                    return Z - One;
                else
                    return Z;
                end if;
            elsif X = Zero then
                return Zero;
            else
                if X < Z then
                    return Z;
                else
                    return Z + One;
                end if;
            end if;
        end if;
    end Truncate;