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Date: Sun, 3 May 2015 17:51:57 -0700 (PDT)
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Subject: Re: getting same output as gfortran, long_float
From: robin.vowels@gmail.com
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Xref: news.eternal-september.org comp.lang.ada:25717
Date: 2015-05-03T17:51:57-07:00
List-Id: <comp.lang.ada>

On Saturday, May 2, 2015 at 3:27:29 AM UTC+10, Nasser M. Abbasi wrote:
> On 5/1/2015 10:39 AM, Waldek Hebisch wrote:
> > Nasser M. Abbasi <nnospam@12000.org> wrote:
> >>
> >> Thanks, this is very useful. I noticed small difference in output:
> >> side-by-side:
> >>
> >> Ada:          0.00182898948331047096479195244627343392775491540..
> >> Mathematica*: 0.00182898948331047096479195244627343392775491540..
> >> gfortran:     0.00182898948331047112025871115292829927
> >>
> >> at digit 18, gfortran result is different. But your Ada rational
> >> package gives same result as Mathematica. This tells me your
> >> result is the accurate one !
> >
> > Your previous gfortran result was:
> >
> > 1.82898948331047096479195244627343369E-3
> >
> > which agrees with others...
> >
> 
> Thanks. My mistake. So the command line flag was needed after all!!
> 
> >gfortran -Wall -fdefault-real-8  foo2.f90
> >./a.out
>     1.82898948331047096479195244627343369E-0003
> 
> Without the flag, the digit is different
> 
> >gfortran -Wall foo2.f90
> >./a.out
>     1.82898948331047112025871115292829927E-0003
> 
> as you say, this is due to D0 in code using double, while
> the result "x" is quad.  With the flag in, gfortran will
> treate all doubles as "quad", hence the correct result.
> 
> I thought with -Wall it will catch all these things, but I think
> I need more flags to detect this user error
> 
> >> for reference:
> >>
> >> -------------------
> >> PROGRAM foo
> >> IMPLICIT NONE
> >> REAL(KIND = 16) :: x  !-- kind=16 tells it is double quad
> >> x = 12.0D0 * 0.0001D0/(1.0D0 * (1.0D0 - 0.1D0)**4 )
> >> PRINT *, x
> >> END PROGRAM
> >
> > AFAICS you have 34 digit x, but 17 digit constants.  So
> > it seems that you assign 17 digit number to x, no
> > wonder there is no gain in accuracy.
> >
> 
> Yes, needed the command line flag.

Not really. he correct way is to specify quad precision constants,
e.g.,
0.0001_quad