(j3.2006) Signaling NaN

[Bill Long]

The standard does specify some of this, and other parts are "processor 
dependent".

For this example program:

 > cat ieee_nan.f90
       program ieee_test_nan

       use, intrinsic :: ieee_arithmetic
       implicit none
       real(4) :: r, s
       real(8) :: x, y

       r = ieee_value(r,ieee_signaling_nan)
       s = ieee_value(s,ieee_quiet_nan)
       x = ieee_value(x,ieee_signaling_nan)
       y = ieee_value(y,ieee_quiet_nan)

       write (*,*) "snan, 32-bit prints as: ", r
       write (*,*) "qnan, 32-bit prints as: ", s
       write (*,*) "snan, 64-bit prints as: ", x
       write (*,*) "qnan, 64-bit prints as: ", y

       end

we print:

 > ftn ieee_nan.f90
 > aprun -n1 ./a.out
  snan, 32-bit prints as:  NAN
  qnan, 32-bit prints as:  NAN
  snan, 64-bit prints as:  NAN
  qnan, 64-bit prints as:  NAN
 >

Note that the standard (inconsistently) requires that on INPUT, the text 
is expected to be "NAN" and on output it is "NaN".  We print NAN on 
output so is consistent with input expectations.


 From the standard, for INPUT:

...an optional sign, followed by the string ?NAN?, optionally followed 
by zero or more alphanumeric characters enclosed in parentheses,...

The NaN value is a quiet NaN if the only nonblank characters in the 
field are ?NAN? or ?NAN()?; otherwise, the NaN value is processor 
dependent. The interpretation of a sign in a NaN input field is 
processor dependent.


For OUTPUT:

For an internal value that is an IEEE NaN, the output field consists of 
blanks, if necessary, followed by the letters ?NaN? and optionally 
followed by one to w?5 alphanumeric processor-dependent characters 
enclosed in parentheses, right justified within the field. If w is less 
than 3, the field is filled with asterisks.


Cheers,
Bill



On 3/11/11 10:26 AM, Fred J. Tydeman wrote:
> Are there any Fortran implementations that support IEEE
> Signaling NaNs?
>
> If yes, what is printed for them?  And how does that
> differ from what is printed for Quiet NaNs?
>
> The C committee is working on a Technical Report on
> adding support for IEEE-754-2008 to C.  One of the
> items we are considering is Signaling NaNs.  We would
> like to be consistent with any other work already done.
>
>
> ---
> Fred J. Tydeman        Tydeman Consulting
> tydeman at tybor.com      Testing, numerics, programming
> +1 (775) 358-9748      Vice-chair of PL22.11 (ANSI "C")
> Sample C99+FPCE tests: http://www.tybor.com
> Savers sleep well, investors eat well, spenders work forever.
>
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-- 
Bill Long                                           longb at cray.com
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