Function pointers in fortran

Question

is it possible to have function pointers in Fortran? Right now I have a code with some lines like this:

subroutine flag(iflag,a,b)

integer n, a, b, ii, iflag

do ii = 1, n
     if (iflag.eq.0) a+b
     else a-b  
end do    

return
end

The variable "n" has a huge value and so to run this code, I feel like I am wasting a lot of time with the "if" command. Is it possible to write something like a function pointer (I am not sure what I mean by that), such that at the code does something like this:

subroutine flag(iflag,a,b)

*depending on the iflag the subroutine flag is automatically 
precompiled to call either flag_plus or flag_minus*     

return
end

subroutine flag_plus(a,b)

integer n, a, b, ii

do ii = 1, n
     a+b
end do    

return
end

subroutine flag_minus(a,b)

integer n, a, b, ii

do ii = 1, n
     a-b  
end do    

return
end

If it is possible I can save a lot of time by avoiding the "if" loop. Is anything like this remotely possible?

Answer 1

In Fortran 77 you can have a limited variant of function pointers, namely that you can pass the name of a procedure as an argument to another procedure (a function pointer, essentially). You cannot have a variable that contains the address of a procedure and then "call" that variable, though.

As of Fortran 2003, procedure pointer variables are part of the language.

That being said, I think your example problem could be solved even easier by something like

if (iflag == 0) then  ! Why is iflag not of type logical?
  do ii = 1, n
    a + b
  end do
else
  do ii = 1, n
    a - b
  end do
end if

Make sure to profile it to see whether it has any effect, as well. Modern CPU's have pretty good branch predictors, and a branch test which doesn't change during the entire loop is pretty much the best case scenario (rule of thumb: predicted branches are close to free). Heck, your compiler might even be able to do the above kind of optimization..