Re: whole number constants

"Dick Hendrickson" <dick.hendrickson@[EMAIL PROTECTED]
> wrote in message
news:he7hk.129353$102.58099@[EMAIL PROTECTED]
> dancerchris@[EMAIL PROTECTED]
 wrote:
> > This may be a dumb question but what is "best practice" when coding
> > whole numbers in real (kind specified) formulas.  I am looking for
> > accuracy, consistancy and prortability reagardless of what value dp
> > has below.  Many of my calcs have numerical cancellations or accuracy
> > checks as per the error function below.  What are the pitfalls?
> >
> > for example:
> >
> > integer, parameter :: dp = selected_real_kind(15, 307)
> >
> > real(dp)  :: x,xnew
> >
> > real(dp), parameter :: one = 1.0_dp
> > real(dp), parameter :: onend = 1_dp
> > real(dp), parameter :: onenk = 1.0
> > real(dp), parameter :: onei = 1
> >
>
> For real constants that are exact integers and are small enough
> to fit in one word, those 4 are (almost) exactly equivalent.
> No processor in the world will possibly mess up the conversion
> of 1 or 1.0 or 1.0_dp into some other kind.  My advice would
> be to do what looks natural, as long as you declare the result
> kind, via the "real(dp)".
>
> The interesting exception here is the "1_dp" form.  This
> declares an integer of kind dp.  It's not guaranteed that
> this will be ****table.

It's worse than that.

It isn't correct, and may appear to execute OK
on some systems.
If you are lucky, a particular compiler may issue
an error message because that kind doesn't exist
among the kinds of integers.

>  Many processors only sup****t one kind
> of integer and the selected_real_kind refereence you used
> to get the value of "dp" isn't likely to return a valid
> integer kind.
>
> As you almost certainly know, it's different for values that
> aren't exactly representable.  3.14 and 3.14_dp are not the
> same thing (or at least aren't ****tably the same).
>
> > ! which should I use ?
> >
> > 10 error = one-xnew/x
> > 20 error = onend-xnew/x
> > 30 error = onenk-xnew/x
> > 40 error = onei-xnew/x
> >
> > ! or the more ledgable ?
> >
> > 50 error = 1-xnew/x
> This would be my choice, because it's clearer about
> what you are doing.
>
> The bad thing about getting in the habit of using just
> plain constants is with dummy arguments.  Here, you need
> to specify the kind value as there is no automatic conversion.
>
> Dick Hendrickson
> >
> > ! or the slightly less ledgable ?
> >
> > 60 error = 1_dp-xnew/x
> >
> > Thanks for any advice.