Tips & Tricks for Fortran

This page contains a number of clarifications and explanations that will hopefully help you get a better understanding of Fortran

"Implicit none" 

The line 'implicit none' is needed in Fortran to avoid obscure errors when defining variables. In earlier Fortran versions all variables were implicitly given as floats (real) unless the first letter in the name between i and n. 'implicit none' makes sure all variables have to be given-type explicit. Remember to add this line to every program, function and sub-routine. It is a very common form of error.

Subroutine vs. function

The difference between a subroutine and a function can be confusing, as they often can be implemented in similar ways to do the same job.  The biggest difference is that subroutines work as pass-by-reference, while functions are pass-by-value.  Se the example below for clarity. Many prefer subroutines because of its simpler syntax. Functions has a few syntax requirements. As you see inn the example under the function has to be defined with correct return type (this case float number). 

program main
  implicit none
  real :: a,b, fn !Notice how the function has to be defined as real
  a = 1.0
  b = 1.0
  print *, 'a befor subroutine: ', a
  call sub(a)
  print *, 'a after subroutine: ', a
  
  print *, 'b befor function: ', b
  print *, 'Result from function', fn(b), ', b after function: ', b
 
end program

subroutine sub(a)
	implicit none
	real :: a 
	a = 2.0*a
end subroutine sub

function fn(b) result (c) !Defining c as a return variable
	implicit none
	real, intent(in) :: b !Many compilers require input variables to have intent(in)
	real :: c
	c = 2.0*b
end function fn

 a befor subroutine:    1.0000000
 a after subroutine:    2.0000000
 b befor function:    1.0000000
 Result from function   2.0000000 , b after function:    1.0000000
 Press ENTER to continue--

Recursive functions

In Fortran subroutines and functions that does a recursive call on itself needs to be labeled "recursive function" in its declaration. It's easy to forget when handling programs that intuitively doesn't look like a recursive function.  For an example see the calculator application.  Below is a simple program for writing the Fibonacci numbers.  

program main
  implicit none
  integer :: i, n, fib
  n = 12
  
  do 10 i=1,n !10 is the label for the do loop
  	print *, fib(i)
  10 continue !Terminate loop with label 10
end program

recursive function fib(n) result (x)
	implicit none
	integer, intent(in) :: n
	integer :: x
	
	if (n<=1) then
		x = n
	else
		x = fib(n-2) + fib(n-1)
	end if
end function fib