Failure of implicit real*8

[N.M. Maclaren]

On Feb 20 2011, Steve Kargl wrote:
>> 
>> >Note of caution when not using -fdefault-real-8:  1.0 is a default-real 
>> >variable, thus for, e.g.,
>> >  real*8 sqrt
>> >  sq2 = sqrt(2.0)
>> >you loose precision, unless you do
>> >  sq2 = sqrt(2.0d0)
>> 
>> Is it safe for all constants and the problematic intrinsics (like cmplx)?
>> If so, I may mention it a bit more definitely in my course, more as a
>> debugging tool than as something to rely on - i.e. if using this works,
>> look for unmodified constants and intrinsics without kind.
>
>The above is just one problem.  Consider the dusty deck
>(ie nonconforming, nonportable) method for manipulating
>the bits of a REAL:
>
>      EQUIVALENCE(X,I)
>      I = 2139095040
>      print *, I, x
>      end

Oh, very true (and the snipped points), but my course is modified from
Steve Morgan's and teaches only modern Fortran.  I give several dire
warnings about old Fortran features (like COMMON, implicit procedure
declaration and calling LAPACK).  The only advice I give about EQUIVALENCE
in any course is "Don't - REALLY don't", and I stress the use of IMPLICIT
NONE (everywhere, but EVERYWHERE) at least half a dozen times.

By far the most common gotchas in clean, modern Fortran are constants
and a few intrinsics.  I don't think that I know of another that is of
any consequence.

Regards,
Nick Maclaren.