[netCDF #DHW-277522]: ncdump doesn't display enough digits

Hello Ryo,

In addition to the response from Dennis, I think I can clarify the ncdump 
issue.  The default precision (which I don't have off the top of my head) is 
causing the rounding/issue you have reported.  You can correct this using the 
'-p' argument.  For example, when I run the following on the file you've 
provided, I see more precise output:

    $ ncdump -p 20,20 -v Z_ISO ncdump-bug-precision.nc

(...)

 Z_ISO =
  -9.9900000981736855962e+20, -9.9900000981736855962e+20, 

(...)

So, the precision displayed can be modified.  It's important to note that this 
is not affecting the underlying data in the file, it is only a cosmetic issue 
issue.

I notice you are doing a straight equality check on your values in the sample 
file you provided; this is typically not a good idea, for the reasons outlined 
here:

* http://floating-point-gui.de/errors/comparison/
* http://wiki.seas.harvard.edu/geos-chem/index.php/Floating_point_math_issues

When comparing floating point values, you must often take additional steps to 
ensure a very small difference in values, such as you've observed here, due to 
rounding errors.  Granted, I am coming at this from a C developer standpoint, 
as I am not a Fortran expert, but I believe the issue remains the same with 
programming in Fortran (as supported by the second link above).

I hope this information is helpful,

-Ward

> Dear netCDF maintainers,
> 
> I've found ncdump doesn't always display enough digits. If you display the
> attached netCDF file, you'll see
> 
> Z_ISO:_FillValue = -9.99000009817369e+20 ;
> 
> and
> 
> Z_ISO =
> -9.99000009817369e+20, [ . . . elided . . .]
> 
> How is that possible?  _Fillvalue doesn't work.  Actually the value of the
> variable is -9.9900000981736856e+20 .
> 
> This is confusing at least.  Moreover, because the output from ncdump can
> be used to generate another netCDF file, this problem can be the source of
> a subtle bug.  As the attached Fortran program demonstrates, gfortran is
> designed in such a way that ascii output retains enough digits to exactly
> recover the original value.
> 
> The Fortran program also illustrates how I was hit by this problem.  An
> application program converted my original single-precision data to double
> precision and as a result, it was difficult to set a correct fill value.
> But that's an aside.
> 
> Regards,
> 
> Ryo
> ------------------------------------------------------------
> ! The following program will print
> !
> ! -9.9900000981736856E+020  -9.9900000000000000E+020
> -9.9900000000000000E+020
> ! d1 == d2
> !
> ! using gfortran 4.8  .
> !
> program tmp
> implicit NONE
> real(8):: d, d1, d2 ! double precision variables
> character(16):: str = "-9.99e20"
> 
> d  = -9.99e20 ! single-precision constant
> d1 = -9.99d20 ! doulbe-precision constant
> read(str, *) d2 ! double precision read
> 
> write(*,*) d, d1, d2
> if (d1 == d ) write(*,*) "d1 == d"
> if (d2 == d ) write(*,*) "d2 == d"
> if (d1 == d2) write(*,*) "d1 == d2"
> end program tmp
> 
> 


Ticket Details
===================
Ticket ID: DHW-277522
Department: Support netCDF
Priority: Normal
Status: Closed
===================
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