CODEWITHSUNDEEP
arraysmodulefortran
muammar
muammar

Reputation: 967

Using an array coming from a module to be used in another subroutine or main program in Fortran

I'd be glad if somebody could help me with this. I'm studying modules in fortran, and I have a question. Let's say that my module creates a matrix [A(3,3)] that is read from user's input. Then, I'd like to use such a matrix in a new subroutine so that I can do an operation with it (for the sake of simplicity let's say a sum). My code looks like this:

     module matrixm                                                                                                                                                                                               
          contains                                                              
      subroutine matrixc                                                        
      integer i,j                                                               
      real, DIMENSION(3,3) :: a                                                 
      do 10 i=1,3                                                               
         do 20 j=1,3                                                            
        read(*,*) a(i,j)                                                        
20    continue                                                                  
10    continue                                                                                                          
      end subroutine matrixc                                                    
      end module matrixm                                                        

      program matrix                                                            
          use matrixm                                                           
      real, dimension(3,3) :: b,c                                               
      integer i,j                                                               
          call matrixc                                                          
          b=10.0                                                                
          c=a+b
      write statements here...                                                                 

      end

If the input of A is: 1 2 3 4 5 6 7 8 9 one would expect C[3,3] to be 11 12 13 14 15 16 17 18 19. However, the result shows only a matrix C whose elemets are all of them equal to 10.0. What is the error I have in my program?, and much more important, am I correct on what's the use of a module?. I have a similar issue on a big problem I'm working on right now. Thanks.

Upvotes: 0

Views: 75

Answers (1)

Stefan
Stefan

Reputation: 2518

The problem you have in your program is the visible memory:

You read the data in the matrix a which is local to your subroutine matrixc. This means, that this change is not visible to the program.

The next thing is, that the variable a in your program is implicitely defined as real and as a result, doesn't throw an error (keyword: IMPLICIT NONE).

There are two easy solutions:

1: Put the definition of matrix a in the definition part of your module:

module matrixm
  REAL, DIMENSION(3,3) :: a

  CONTAINS                                                         
    subroutine matrixc                                                        
      integer i,j                                                               

      do i=1,3                                                               
        do j=1,3                                                            
          read(*,*) a(i,j)        
        end do                                                
      end do                                                                 
    end subroutine matrixc                                                    
end module matrixm

2: Use a as a parameter to your subroutine and define it in the main program:

module matrixm
  CONTAINS                                                         
    subroutine matrixc(a)                                          
      integer i,j
      REAL, DIMENSION(3,3) :: a

      do i=1,3                                                               
        do j=1,3                                                            
          read(*,*) a(i,j)        
        end do                                                
      end do                                                                 
    end subroutine matrixc                                                    
end module matrixm

program matrix                                                        
  use matrixm 
  IMPLICIT NONE                                                          
  real, dimension(3,3) :: a,b,c                                              
  integer i,j

  call matrixc(a)                                                          
  b=10.0                                                                
  c=a+b

  write statements here...                                                                 

end program

Upvotes: 1

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