Transfer array from Fortran subprogram to main program - fortran

Question

I have a subroutine that opens and reads a file. The final result is an array that contains the data from the input file in a re-organized fashion. I want to call the subroutine in the main program to use the aforementioned array.
The subroutine has all the variables necessary for it to run as a separate program declared in its file. I'm new using Fortran so I'm not sure how to correctly employ subroutines. Do I need to assign any formal variables to subroutine's first line, or should I have an empty set of parenthesis?
The subroutine is in a file (subroutine.f03) that's separate from the main program's file (main.f03).
Main program code:
PROGRAM main
IMPLICIT NONE
CALL readBasis
WRITE(*,*) basis(1,1)
END PROGRAM
Subroutine code:
SUBROUTINE readBasis()
IMPLICIT NONE
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! io_open = IOSTATUS FOR OPENING THE BASIS FILE !!
!! io_red = IOSTATUS FOR READING THE BASIS FILE !!
!! atom_num = NUMBER ASSIGNED TO A PARTICULAR ATOM IN THE BASIS FILE !!
!! end_of_line = 0, DEFAULT BASIS SET INPUT FORMAT !!
!! end_of_line_1 = 0.00 DEFAULT BASIS SET INPUT FORMAT !!
!! atom_end = **** INDICATES THE END OF THE BASIS SET INFO FOR A GIVEN ATOM !!
!! primitives = NUMBER OF PRIMITIVES IN A CONTRACTION !!
!! basis_type = ANGULAR MOMENTUM ASSOCIATED WITH A CONTRACTION !!
!! expo = GAUSSIAN PRIMITIVE EXPONENT !!
!! coeff = CONTRACTION COEFFICIENT FOR AN S, P, D PRIMITIVE RESPECTIVELY IN A S, P, D SHELL !!
!! s_coeff & p_coeff = CONTRACTION COEFFICIENTS FOR S AND P PRIMITIVES IN AN SP SHELL !!
!! basis = ARRAY CONTAINING ALL OF THE BASIS SET INFORMATION. THE FORMAT IS GIVEN BELLOW: !!
!! BASIS NUMBER | PRIMITIVE TYPE | EXPONENT | S COEFF | P COEFF | D COEFF | X COORDS | Y COORDS | Z COORDS !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
INTEGER :: i, io_open, io_read, atom_num, end_of_line, primitives, gauss_i, gauss_f
INTEGER :: total_basis_functions, total_primitives, primitive_counter, primitive_num
INTEGER :: func_start, func_end, func_counter
CHARACTER (LEN=4) :: basis_type, atom_end
REAL :: scaling, end_of_line_1
REAL :: expo, coeff, s_coeff, p_coeff
REAL, ALLOCATABLE :: basis(:,:)
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!! atom_loop WILL LET YOU READ THE BASIS FUNCTIONS FOR EVERY ATOM !!
!! contraction_loop WILL LET YOU READ EACH BASIS FUNCTION PER ATOM !!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
OPEN(UNIT=10, FILE="BASIS", STATUS="OLD", ACTION="READ", IOSTAT=io_open)
READ(10,*) total_basis_functions
READ(10,*) total_primitives
ALLOCATE(basis(total_primitives,6))
READ(10,*,IOSTAT=io_read) atom_num, end_of_line
READ(10,*) basis_type, primitives, scaling, end_of_line_1, func_start, func_end
atom_end = basis_type
primitive_num = 1
atom_loop: DO WHILE (io_read .EQ. 0)
contraction_loop: DO WHILE (atom_end .NE. "****")
orbital_type_loop: IF (basis_type == "S ") THEN
DO func_counter = func_start, func_end
DO primitive_counter = 1, primitives
READ(10,*) expo, coeff
basis(primitive_num,1) = REAL(func_counter)
basis(primitive_num,2) = REAL(0)
basis(primitive_num,3) = expo
basis(primitive_num,4) = coeff
basis(primitive_num,5) = REAL(0)
basis(primitive_num,6) = REAL(0)
primitive_num = primitive_num + 1
END DO
IF (func_counter .LT. func_end) THEN
DO primitive_counter = 1, primitives
BACKSPACE(10)
END DO
ELSE
CONTINUE
END IF
END DO
ELSE IF (basis_type .EQ. "P ") THEN
DO func_counter = func_start, func_end
DO primitive_counter = 1, primitives
READ(10,*) expo, coeff
basis(primitive_num,1) = REAL(func_counter)
basis(primitive_num,2) = REAL(1)
basis(primitive_num,3) = expo
basis(primitive_num,4) = REAL(0)
basis(primitive_num,5) = coeff
basis(primitive_num,6) = REAL(0)
primitive_num = primitive_num + 1
END DO
IF (func_counter .LT. func_end) THEN
DO primitive_counter = 1, primitives
BACKSPACE(10)
END DO
ELSE
CONTINUE
END IF
END DO
ELSE IF (basis_type == "D ") THEN
DO func_counter = func_start, func_end
DO primitive_counter = 1, primitives
READ(10,*) expo, coeff
basis(primitive_num, 1) = REAL(func_counter)
basis(primitive_num,2) = REAL(2)
basis(primitive_num,3) = expo
basis(primitive_num,4) = REAL(0)
basis(primitive_num,5) = REAL(0)
basis(primitive_num,6) = coeff
primitive_num = primitive_num + 1
END DO
IF (func_counter .LT. func_end) THEN
DO primitive_counter = 1, primitives
BACKSPACE(10)
END DO
ELSE
CONTINUE
END IF
END DO
ELSE IF (basis_type .EQ. "SP ") THEN
DO func_counter = func_start, func_end
DO primitive_counter = 1, primitives
READ(10,*) expo, s_coeff, p_coeff
basis(primitive_num,1) = REAL(func_counter)
basis(primitive_num,2) = REAL(10)
basis(primitive_num,3) = expo
basis(primitive_num,4) = s_coeff
basis(primitive_num,5) = p_coeff
basis(primitive_num,6) = REAL(0)
primitive_num = primitive_num + 1
END DO
IF (func_counter .LT. func_end) THEN
DO primitive_counter = 1, primitives
BACKSPACE(10)
END DO
ELSE
CONTINUE
END IF
END DO
END IF orbital_type_loop
READ(10,*) atom_end
IF (atom_end .EQ. "****") THEN
READ(10,*,IOSTAT=io_read) atom_num, end_of_line
IF (io_read < 0) THEN
EXIT atom_loop
ELSE IF (io_read > 0) THEN
WRITE(*,*) "FILE COULD NOT BE READ."
EXIT atom_loop
ELSE
READ(10,*) basis_type, primitives, scaling, end_of_line_1, func_start, func_end
atom_end = basis_type
EXIT contraction_loop
END IF
ELSE
BACKSPACE(10)
READ(10,*) basis_type, primitives, scaling, end_of_line_1, func_start, func_end
END IF
END DO contraction_loop
END DO atom_loop
CLOSE(10)
RETURN
END SUBROUTINE

Answer 1

A subroutine has "dummy variables" identified in the parenthesis at its inception. These can be input or output arguements of mixed data types, i.e. a mixture of integers, integer arrays, reals ,etc.. Each dummy variable must have a data type assigned to in the variable declarations section of the subroutine, before any procedural statements. It is good practice, IMO, to use the intent modifier to ensure clarity between input and output varaibles. Varaibles that exist locally in the subroutine and are not explicitly input or ouput do not need to be in the parens but do do need to be declared, unless they have an implicit data type. Here is an example:
subroutine MEGA_SUBROUTINE(X,Y,Z,OUTPUT_ARRAY)
implicit none
real, intent(in):: X,Y,Z
real:: local_var
real, intent(out):: OUTPUT_ARRAY
! begin procedural section
! do stuff with your variables here, assign a value to output array
end subroutine MEGA_SUBROUTINE