Introduction to Fortran Programming. -Internal subprograms (1)-

Transcription

1 Introduction to Fortran Programming -Internal subprograms (1)-

2 Subprograms Subprograms are used to split the program into separate smaller units. Internal subprogram is not an independent part 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 Internal subprogram Internal subprogram: Internal subprograms are routines that are defined within the main program. Internal subprograms are not an independent part of a main program. Let s reduce bugs using subprograms. 3

5 Program units Main program (program-name) Internal functions Internal subroutines External Subprograms Today s topic!! Fortran s program units are the main program, external subprograms, and modules. Module 4

6 Internal functions syntax program program_name [Specification part] [Execution part] stop contains function function_name (arg1,arg2, ) end function function_name end program program_name 5

7 Internal functions syntax function function_name (arg1, ) [Specification part] [Execution part] end function function_name Or function function_name (arg1, ) result (out) [Specification part] [Execution part] end function function_name 6

8 Internal functions syntax function function_name (arg1, ) [Specification part] [Execution part] end function function_name Specification part includes formal arguments. The name of the return variable is function_name. Specification part is used to define the type of the return value function_name. 7

9 Internal functions syntax function function_name (arg1, ) result (out) [Specification part] [Execution part] end function function_name Specification part includes formal arguments. The name of the return variable is out. Specification part is used to define the type of the return value out. 8

10 Example 1 1. Declaration - Declare the variable i and j as integers. - Substitute specific value for i. 2. Make an internal function addone - Return the value i Call the internal function addone and substitute the return value for j. 9

11 Example 1 program test1 implicit none integer :: i, j i = 1 write(*,*) i j = addone(i) write(*,*) j stop contains function addone(i) integer :: i integer :: addone addone = i + 1 end function addone end program test1 A dummy argument for initialization. However, variable needs to be defined. Specification part: declare the variable i. Specification part: declare the variable addone. Substitute i+1 for addone. 10

12 Example 1 program test1 implicit none integer :: i, j i = 1 write(*,*) i j = addone(i) write(*,*) j stop contains function addone(fff) integer :: fff integer :: addone addone = fff + 1 end function addone end program test1 For example, we set a dummy argument fff. Specification part: declare the variable fff. Substitute fff+1 for addone. 11

13 Example 1 program test1 implicit none integer :: i, j i = 1 write(*,*) i j = addone(i) write(*,*) j stop contains function addone(fff) result(ggg) integer :: fff integer :: ggg ggg = fff + 1 end function addone end program test1 For example, we set the variable of return value ggg. Specification part: declare the variable ggg. Substitute fff+1 for ggg. 12

14 Call by reference If this code is executed, how do we output the result? program test1 implicit none integer :: i, j i = 1 write(*,*) i=, i j = addone(i) write(*,*) i=, i, j=, j Display i and j. stop contains function addone(fff) result(ggg) integer :: fff integer :: ggg ggg = fff + 1 fff = fff + 10 Substitute fff+10 for fff. end function addone end program test1 We try to change the value of dummy argument. 13

15 Call by reference If this code is executed, how do we output the result? program test1 implicit none integer :: i, j i = 1 write(*,*) i=, i j = addone(i) write(*,*) i=, i, j=, j stop contains function addone(fff) result(ggg) integer :: fff integer :: ggg ggg = fff + 1 fff = fff + 10 end function addone end program test1 Output of i and j i = 1 i = 11 j = 2 The value i is changed from 1 to 11 Fortran passes arguments by reference. (No copy is made!!) 14

16 Intent attribute data_type, intent (attribute) :: variable_name When declaring variables that need to be passed in or out or inout, intent may be added to the declaration. Intent attribute is not necessary for the return value. Intent: in : the variable value can enter, but not be changed. out : the variable is set inside the procedure and sent back to the main program. inout : the variable comes in with a value and leaves with a value. To optimize your code, you need to use the intent attribute. 15

17 Example 2 1. Declaration - Declare the variable i and j as integers. - Substitute specific value for i. 2. Make an internal function addone - Return the value i+1. - Declare the argument with intent(in). 3. Call the internal function addone and substitute the return value for j. 16

18 Example 2 program test1 implicit none integer :: i, j i = 1 write(*,*) i j = addone(i) write(*,*) j stop contains function addone(i) integer, intent(in) :: i integer :: addone addone = i + 1 end function addone end program test1 A dummy argument with the intent(in) can enter, but not be changed. For example, if we set i = i+10 in function addone, then an error occurs. 17