I realize that if you write
Real (Kind(0.d0))::x,y
x = sqrt(-1.d0)
y = sqrt(-1.d0)
if (x == y) then
write(*,*)'yep, they are equals', x
endif
It compiles ok using ifort. But nothing is written, the conditional is always false, did you notice that? why is this so?
NaN signifies not a number, and since there are many, different, reasons a calculation could give that result, they generally do not compare as equal against themselves. If you want to do nan-testing, fortran compilers that support the f2003 standard (which is recent versions of most compilers) have ieee_is_nan
in the ieee_arithmetic
module:
program testnan
use ieee_arithmetic
real (kind=kind(0.d0)) :: x,y,z
x = sqrt(-1.d0)
y = sqrt(-1.d0)
z = 1.d0
if ( ieee_is_nan(x) ) then
write(*,*) 'X is NaN'
endif
if ( ieee_is_nan(y) ) then
write(*,*) 'Y is NaN'
endif
if ( ieee_is_nan(x) .and. ieee_is_nan(y) ) then
write(*,*) 'X and Y are NaN'
endif
if ( ieee_is_nan(z) ) then
write(*,*) 'Z is NaN, too'
else
write(*,*) 'Z is a number'
endif
end program testnan
Compiling and running this program gives
ifort -o nan nan.f90
X is NaN
Y is NaN
X and Y are NaN
Z is a number
Unfortunately, gfortran still doesn't implement ieee_arithmetic
as time of writing, so with gfortran you have to use the non-standard isnan
.