Function pointers in fortran - 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..