Function in fortran passing array receiving array

Question

I have this feature pictured below. It runs in two vectors with three values ​​each and must skip one vector with three values. I call the function as follows:

Fr = Flux(W(:,i),W(:,i+1)) 

What I realized while fiddling with the code, trying clean functions and modules and examining the error operator (which I will include below) is that fortran reads my Flux function and thinks that input vectors are an attempt to call a record from an array . This is my best guess about what is happening. I asked around the laboratory, and most people suggested using routines, but that seemed awkward, and I thought there probably should be a more elegant way, but I haven't found it yet. I tried to determine the result by saying:

  DOUBLE PRECISION FUNCTION Flux(W1,W2) Result(FluxArray(3)) 

and then returns fluxarray, but this does not work as fortran cannot understand the syntax

The actual function is as follows:

 DOUBLE PRECISION FUNCTION Flux(W1,W2) USE parameters IMPLICIT NONE DOUBLE PRECISION, DIMENSION(3), INTENT(IN)::W1, W2 DOUBLE PRECISION, DIMENSION(3), INTENT(OUT):: Flux DOUBLE PRECISION, DIMENSION(3):: F1, F2 DOUBLE PRECISION::U1,U2,Rh1,Rh2,P1,P2,E1,E2,Rh,P,u,c,Lambda INTEGER:: k U1=W1(2)/W1(1) U2=W2(2)/W2(1) Rh1=W1(1) Rh2=W2(1) P1=(gamma_constant-1.d0)*(W1(3)-.5d0*Rh1*U1**2) P2=(gamma_constant-1.d0)*(W2(3)-.5d0*Rh2*U2**2) E1=W1(3) E2=W2(3) F1=[Rh1*U1,Rh1*U1**2+P1,(E1+P1)*U1] F2=[Rh2*U2,Rh2*U2**2+P2,(E2+P2)*U2] Rh=.5d0*(Rh1+Rh2) P=.5d0*(P1+P2) u=.5d0*(U1+U2) c=sqrt(gamma_constant*P/Rh) Lambda=max(u, u+c, uc) do k=1,3,1 Flux(k)=.5d0*(F1(k)+F2(k))-.5d0*eps*Lambda*(W2(k)-W1(k)) end do RETURN END FUNCTION Flux 

Here is the error instruction:

 Quasi1DEuler.f90:191.51: DOUBLE PRECISION, DIMENSION(3), INTENT(OUT):: Flux 1 Error: Symbol 'flux' at (1) already has basic type of REAL Quasi1DEuler.f90:217.58: Flux(k)=.5d0*(F1(k)+F2(k))-.5d0*eps*Lambda*(W2(k)-W1(k)) 1 Error: Unexpected STATEMENT FUNCTION statement at (1) Quasi1DEuler.f90:76.18: Fr = Flux(W(:,i),W(:,i+1)) 

The last error occurs for both Fr and Fl. Thanks for your time and any help or attention you can give!

EDIT / Follow-up :: Thanks for the help, I don't know a better way to introduce this, so I'm going to change the original question.

I did what you suggested and solved this problem, now it says:

 Fr = Flux(W(:,i),W(:,i+1)) 1 Error: The reference to function 'flux' at (1) either needs an explicit INTERFACE or the rank is incorrect 

I saw a similar problem in SO at this link:

Computing the cross product of two vectors in Fortran 90

where they suggested that he include all his functions in modules. Is there a better / easier way to fix this error?

Answer 1

With RESULT(FluxArray) , fluxArray is the name of the result variable. Thus, your attempt to declare characteristics in a result clause is incorrect.

Instead, the result variable should be specified inside the function body:

 function Flux(W1,W2) result(fluxArray) double precision, dimension(3), intent(in)::W1, W2 double precision, dimension(3) :: fluxArray ! Note, no intent for result. end function Flux 

Yes, you can declare the type of the result variable in the function statement, but the array cannot be declared there. I would not recommend having a separate dimension statement in the function body for the result variable.

Note that when accessing a function that returns an array, an explicit interface is required for the caller. One way is to place the function in the use d module. For more details, see Other sections of SO, or Language Tutorials.

Returning to the errors from your question without result .

 DOUBLE PRECISION FUNCTION Flux(W1,W2) DOUBLE PRECISION, DIMENSION(3), INTENT(OUT):: Flux 

Here, the Flux type has been declared twice. In addition, this is not a dummy function argument, since above it should not have an intent attribute.

Can write

 FUNCTION Flux(W1,W2) DOUBLE PRECISION, DIMENSION(3) :: Flux ! Deleting intent 

or (shudder)

 DOUBLE PRECISION FUNCTION Flux(W1,W2) DIMENSION :: Flux(3) 

The complaint about the function of the operator is unimportant here, following from a bad declaration.