Subroutines

An external subroutine a program unit that does all the communication with the rest of the world via arguments (and global variables); it does not return a value. The syntax of a subroutine declaration is

For example, suppose we want to declare a subroutine that receives the three coordinates of a cartesion vector x,y,z and computes the 2-norm of the vector, ${\tt r = \sqrt{x^2 + y^2 + z^2}}$ (this is also called the 2-norm). The arguments x,y,z are REAL, and so is the computed value r.

The declaration could then be

 
subroutine norm2s(x,y,z,r)
 implicit none
 real :: x,y,z,r
 r = sqrt(x**2+y**2+z**2)
end subroutine norm2s

The calling program invokes the subroutine using

 
call norm2s(a,b,c,d)

The CALL statement builds the argument stack and passes control to the subroutine; when END SUBROUTINE is encountered, the control is returned to the calling program, at the first statement after CALL. Unlike functions, subroutines communicate with the calling program via arguments (or global variables) only - they do NOT return a value associated with the subroutine's name.

For example, the main program might read in the coordinates and print the 2-norm of the vector:

 
program print_norm
implicit none
external :: norm2s
real :: a, b, c, d
print*, "input~3~coordinates:"
read*,a,b,c
call norm2s(a,b,c,d)
print*,"the norm is",d
end program print_norm

The declaration EXTERNAL :: norm2s tells the main program that norm2 is an external subroutine (we do NOT have any type declaration associated with the name!). Again, in this particular context the declaration is superfluos, but, since it will be mandatory in other contexts, and since it improves readability, I strongly recommend its use.

The actual arguments a,b,c,d replace the formal (dummy) arguments x,y,z,r (in this order) at the time of call. The type and order of actual arguments should match exactly the type and order of their omologue formal (dummy) arguments.

An IMPLICIT NONE statement should be given in each subroutine, just as it is given in the main program.