Introduction to Fortran Programming. -External subprograms-

Transcription

1 Introduction to Fortran Programming -External subprograms-

2 Subprograms Subprograms are used to split a program into separate smaller units. Internal subprograms are dependent parts of a program. Fortran 90 has two types of subprograms: functions and subroutines. Once each piece is working correctly, we can put the pieces together to build the program. 1

3 Program unit It is a physically complete sequence of program lines with end statement. Main program: The main program unit containing the first statement of the actual program. External subprogram: User-defined subprogram which is an independent program unit. Module (since Fortran90): A program unit which contains only declarations and external subprograms. 2

4 Program unit Main program (program-name) Internal functions Internal subroutines External Subprograms External functions External subroutines Today s topic!! But not recommended. Module 3

5 External Subprograms Internal subprograms are described in the main program. Internal subprograms are not an independent of a main program. Variables of the main program and internal subprograms are shared. External subprograms are defined outside the main program. External subprograms are independent of a main program. Variables of the main program and external subprograms are not shared. 4

6 External functions program program_name [Specification part] [Execution part] end program program_name End of main program function function_name (arg1,arg2, ) [Specification part] [Execution part] end function function_name External functions are defined outside the main program. Be sure to write implicit none in specification part. 5

7 External subroutines program program_name [Specification part] [Execution part] end program program_name End of main program Subroutine subroutine_name (arg1, arg2, ) [Specification part] [Execution part] end subroutine subroutine_name Same as external functions. 6

8 External Statement The general form of the statement is data_type, external ename1, ename2,, where ename is the name of some external function. Required when using external functions. Not recommended (interface statement is preferable). program examp real(8), external :: expfunc end program examp function expfunc( ) result(res) real(8) :: res end function It is not necessary for external subroutine. Declaration of external function expfunc. real(8) is the data type of return value of expfunc. Defined outside the main program. The data type of return value of expfunc. This matches real (8) in main program. 7

9 Example 1 Let r be a double-precision real type variable and assign a value appropriately. Create external functions and external subroutines to calculate the volume of the sphere with radius r. 8

10 Example 1 External function program examp1 real(8) :: r real(8), external :: volfunc r = 1d0 V = volfunc(r) write(*,*) V end program examp1 function volfunc(r) result(v) real(8), intent(in) :: r real(8) :: pi = acos(-1d0) V = 4d0/3d0*pi*r**3 end function volfunc Declaration of external function volfunc. real(8) is the data type of return value of volfunc. Use external function volfunc. External function is declared outside the main program. 9

11 Example 1 External subroutine program examp1 real(8) :: r For external subroutines, r = 1d0 call volsub(r,v) write(*,*) V end program examp1 subroutine volsub(r,v) real(8), intent(in) :: r real(8) :: pi = acos(-1d0) real(8), intent(out) :: V V = 4d0/3d0*pi*r**3 end subroutine volsub external statements are not required. External subroutine is declared outside the main program. 10

12 Comparison of internal and external functions External function program examp1 real(8) :: r real(8), external :: volfunc r = 1d0 V = volfunc(r) write(*,*) V end program examp1 function volfunc(r) result(v) real(8), intent(in) :: r real(8) :: pi = acos(-1d0) V = 4d0/3d0*pi*r**3 end function volfunc Internal function program examp1 real(8) :: r r = 1d0 V = volfunc(r) write(*,*) V contains function volfunc(r) result(v) real(8), intent(in) :: r real(8) :: pi = acos(-1d0) V = 4d0/3d0*pi*r**3 end function volfunc end program examp1 11

13 Comparison of internal and external functions External function Internal function program examp1 program examp1 What implicit happens none if you declare pi which was already implicit declared none in the main program? Deleting real(8) the pi :: declared r in the main program. real(8) :: r real(8) :: pi = acos(-1d0) real(8) :: pi = acos(-1d0) real(8), external :: volfunc r = 1d0 r = 1d0 V = volfunc(r) V = volfunc(r) write(*,*) V write(*,*) V contains end program examp1 function volfunc(r) result(v) function volfunc(r) result(v) real(8), intent(in) :: r real(8), intent(in) :: r V = 4d0/3d0*pi*r**3 end function volfunc V = 4d0/3d0*pi*r**3 end program examp1 end function volfunc 12

14 Comparison of internal and external functions External function Internal function program examp1 program examp1 What implicit happens none if you declare pi which was already implicit declared none in the main program? Deleting real(8) the pi :: rdeclared in the main program. real(8) :: r real(8) :: pi = acos(-1d0) real(8) :: pi = acos(-1d0) real(8), external :: volfunc r = 1d0 r = 1d0 V = volfunc(r) V = volfunc(r) write(*,*) V write(*,*) ERROR V OK! contains end program examp1 function volfunc(r) result(v) function volfunc(r) result(v) real(8), intent(in) :: r Variables of the main program real(8) Variables :: V of the main program and real(8), external intent(in) subprograms :: r V and = 4d0/3d0*pi*r**3 internal subprograms are real(8) not shared. :: V end are function shared. volfunc V = 4d0/3d0*pi*r**3 end program examp1 There end function are two volfunc program units. There is one program unit. 13

15 Common statement The general form of the statement is common /block_name/ variable_name, The Common statement defines a block of main memory storage so that different program units can share the same data without using arguments. (Not recommended.) program examp real(8) :: pi common /pidata/ pi pi = acos(-1) end program examp subroutine expsub( ) real(8) :: pi common /pidata/ pi end function Declare pi to be a shared variable. The block name is decided individually. By writing the block name and pi as a common statement, it is associated with pi of main program. 14

16 Example 2 Let r be a double-precision real type variable and assign a value appropriately. Create external functions and external subroutines to calculate the volume of the sphere with radius r. However, pi assigns the value with the main program and shares the value with common statement. 15

17 Example 2 program examp2 real(8) :: r real(8) :: pi real(8),external :: volfunc common /pidata/ pi pi = acos(-1d0) r = 1d0 V = volfunc (r) write(*,*) V end program examp2 function volfunc(r) result(v) real(8), intent(in) :: r real(8) :: pi common /pidata/ pi V = 4d0/3d0*pi*r**3 end function volfunc Declare pi to be a shared variable. Assign a value to pi in the main program. Use pi assigned with the value in the main program. 16

18 Example 2 program examp2 real(8) :: r real(8) :: pi common /pidata/ pi pi = acos(-1d0) r = 1d0 call volsub(r,v) write(*,*) V end program examp2 subroutine volsub(r,v) real(8), intent(in) :: r real(8) :: pi real(8), intent(inout) :: V common /pidata/ pi V = 4d0/3d0*pi*r**3 end subroutine volsub Declare pi to be a shared the variable. Assign the value to pi in the main program. Use pi assigned with the value in the main program. 17

19 Split compilation Program can be divided into smaller files for each unit. aaa.f90 program aaa end program aaa gfortran bbb.f90 aaa.f90 bbb.f90 function bbb( ) end function bbb subroutine ccc( ) end subroutine ccc Write all the necessary files. 18

20 Example 3 Let r be a double-precision real type variable and assign a value appropriately. Create an external subroutine that calculates the volume of the sphere with radius r. However, save the external subroutine in a separate file from the main program. 19

21 Example 3 examp3.f90 program examp2 real(8) :: r real(8) :: pi common /pidata/ pi pi = acos(-1d0) r = 1d0 call volsub(r,v) write(*,*) V end program examp2 ex3sub.f90 subroutine volsub(r,v) real(8), intent(in) :: r real(8) :: pi real(8), intent(inout) :: V common /pidata/ pi V = 4d0/3d0*pi*r**3 end subroutine volsub Compilation gfortran ex3sub.f90 examp3.f90 20