How to alias a function name in Fortran

Question

Not sure if the title is well put. Suggestions welcome.

Here's what I want to do. Check a condition, and then decide which function to use in a loop. For example:

if (a < 0) then
    loop_func = func1
else
    loop_func = func2
endif

I can then use loop_func as a pointer when writing my loop. Both functions take exactly the same inputs, and are different approaches on tackling the problem based on the value of a. This will allow me to only have one block of code, instead of two nearly identical blocks. This could apply to subroutines too.

Any ideas how this might be implemented?

Thank you.

Answer 1

Yes, Fortran has procedure pointers, so you can in effect alias a function name. Here is a code example which assigns to the function pointer "f_ptr" one function or the other. Thereafter the program can use "f_ptr" and the selected function will be invoked.

module ExampleFuncs

   implicit none

contains

function f1 (x)
  real :: f1
  real, intent (in) :: x

  f1 = 2.0 * x

  return
end function f1


function f2 (x)
   real :: f2
   real, intent (in) :: x

   f2 = 3.0 * x**2

   return
end function f2

end module ExampleFuncs


program test_func_ptrs

    use ExampleFuncs
    implicit none

   abstract interface
      function func (z)
         real :: func
         real, intent (in) :: z
      end function func
   end interface

   procedure (func), pointer :: f_ptr => null ()

   real :: input

   write (*, '( / "Input test value: ")', advance="no" )
   read (*, *) input

   if ( input < 0 ) then
      f_ptr => f1
   else
      f_ptr => f2
   end if

   write (*, '(/ "evaluate function: ", ES14.4 )' )  f_ptr (input)

   stop

end program test_func_ptrs

Answer 2

Most Fortran implementations do not have a standard way to manipulate function pointers or procedure pointers. However, Fortran 2003 and later have something. (See page 6 of this.)

For the given situation, this will work pretty well in its place:

 function func1 (p1, p2, etc)
 ...  as you have it already
 end

 function func2 (p1, p2, etc)
 ...  as you have it already
 end

 function funcselect (a, p1, p2, etc)
     if (a < 0) then
          x = func1 (p1, p2, etc)
     else
          x = func2 (p1, p2, etc)
     endif
 end

Then just call funcselect with the extra parameter instead of what you would have done with loop_func.