I was reading about Fortran’s intrinsic function REAL recently and discovered that it behaved inconsistently, so I fixed it by overloading the function in a module that I generally use everywhere.
I use REAL to extract the real part of a complex number. The KIND of the return value is equal to that of it’s argument, but only if the argument is COMPLEX. If the argument itself is of type REAL, then it returns a value using the compiler’s default REAL KIND. For me, since I program in double precision, this can cause me to accidentally lose precision, if I’m not careful.
I see three ways to fix this:
- Always include the
KINDwhen callingREAL, for exampleREAL(foo,8). I’m likely to forget, and it clutters my code without adding information to the reader. - Change the compiler default
KIND. In gfortran, I just need a command-line argument:gfortran -fdefault-real-8. However, this option can change from compiler to compiler. - Overload the intrinsic function to return a
REALvalue with the sameKINDas the argument.
So, I went with the last option. To get this to work, I put the function in my module prec_def, which is where I set the precision for the entire code. I used an interface for the overloading, so that I only overrode the intrinsic for the argument names and types that I specify.
Here’s the result:
use iso_fortran_env, only : REAL64 !to get the KIND parameter for 64-bit precision
implicit none
integer, parameter :: long=REAL64
interface real
module procedure real_real
end interface
contains
real(Long) function real_real(num)
real(Long), intent(in) :: num
!call the intrinsic function, specifying KIND
real_real = real(num,Long)
end function real_real
end module prec_def
Let me know what you think!
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