[Developers] Fortran 77/90 formatted output "gotcha"

[Jonathan Thornburg]

Hi,

I've just run across a Fortran 77/90 "gotcha" which I'd like to warn
other Fortran programmers about.  It's best expressed by an example:

 	program test_gformat
 	implicit none
 	double precision :: x

 	x = 1.234d-300
 	write (6, '(g25.10)') x
 	write (6, '(g25.10e3)') x

 	stop
 	end

When compiled with either g77 3.3.3 or ifort 8.0, this produces the
output
          0.1234000000-299
         0.1234000000E-299

Notice the first line -- that number will be read by most other
software as 0.1234 followed by some trailing junk, i.e. the exponent
has essentially been lost!

Since IEEE double precision has an exponent range of around 1e-308 to
1e308, the moral of the story is to always use the "e3" suffix on output
formats.  I didn't know this before this past weekend...

ciao,

-- 
-- Jonathan Thornburg <jthorn at aei.mpg.de>
    Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
    Golm, Germany, "Old Europe"     http://www.aei.mpg.de/~jthorn/home.html
    "Washing one's hands of the conflict between the powerful and the
     powerless means to side with the powerful, not to be neutral."
                                       -- quote by Freire / poster by Oxfam