The misteries of Fortran Functions

Fortran has a 2 ways to define sub-procedures: functions and subroutines. Functions allow many input variables, but only one output variable, which has to be a number. Subroutines, can take a few variables and output a few variables. At first glance, this separation looks unnecessary, but it helps you keep your code easy to understand, provided you use functions. In the following a I give a few examples of how I found I can use functions in Fortran:

PROGRAM function_demo1
! *** Program to demonstrate a function
IMPLICIT NONE
REAL :: a,b,c ! *** Declare a,b,c
PRINT*,"Enter number one"
READ*,a
PRINT*,"Enter number two"
READ*,b
c=calc(a,b)
PRINT '("Answer = ",f10.4)',c
CONTAINS
!
FUNCTION calc(d,e)
REAL :: d,e
REAL :: calc
! *** Dummy variables
! *** Local Variables
calc=SQRT(d**2+e**2)
END FUNCTION calc
END PROGRAM function_demo1

Notice the word "CONTAINS" above.

PROGRAM function_demo2
! *** Program to demonstrate a function
IMPLICIT NONE
!External Function declaration
REAL, EXTERNAL :: calc
REAL :: a,b,c ! *** Declare a,b,c
PRINT*,"Enter number one"
READ*,a
PRINT*,"Enter number two"
READ*,b
c=calc(a,b)
PRINT '("Answer = ",f10.4)',c
END PROGRAM function_demo2
!
REAL FUNCTION calc(d,e)
IMPLICIT NONE
REAL :: d,e  ! *** Dummy variables
!REAL :: calc !  *** Local Variables
calc=SQRT(d**2+e**2)
END FUNCTION calc

The third way will be:

PROGRAM function_demo3
! *** Program to demonstrate a function
IMPLICIT NONE
!External Function declaration
REAL, EXTERNAL :: calc
REAL :: a,b,c ! *** Declare a,b,c
PRINT*,"Enter number one"
READ*,a
PRINT*,"Enter number two"
READ*,b
c=calc(a,b)
PRINT '("Answer = ",f10.4)',c
END PROGRAM fuction_deom3
!
REAL FUNCTION calc(d,e) RESULT (CR)
IMPLICIT NONE
REAL :: d,e  ! *** Dummy variables
!REAL :: calc !  *** Local Variables
CR=SQRT(d**2+e**2)
END FUNCTION calc

And there is even a forth way !

PROGRAM function2
! *** Program to demonstrate a function
        IMPLICIT NONE
        !External Function declaration
        REAL, EXTERNAL :: calc
        REAL :: a,b,c ! *** Declare a,b,c
        PRINT*,"Enter number one"
        READ*,a
        PRINT*,"Enter number two"
        READ*,b
        c=calc(a,b)
        PRINT '("Answer = ",f10.4)',c
END PROGRAM function2
        !
        !REAL FUNCTION calc(d,e)
FUNCTION calc(d,e)
       IMPLICIT NONE
        REAL :: d,e  ! *** Dummy variables
        REAL :: calc !  *** Local Variables
       calc=SQRT(d**2+e**2)
END FUNCTION calc

Notice that the return value of the function has a different name from the function name. All these programs compile equally under gfortran 4.3 in Debian Squeeze.