title: Function in fortran, passing array in, receiving array out
body: <p>I have this function, depicted below. It passes in two vectors with three values each, and should pass out one vector with three values as well. I call the function like this:</p>
  
  <pre><code>Fr       = Flux(W(:,i),W(:,i+1))
  </code></pre>
  
  <p>What I have realized through messing around with the code,  trying pure functions, and modules, and researching the error statement (that I will include at the bottom), is that fortran is reading my function Flux, and thinks that the input vectors are an attempt to call an entry from the array. That is my best guess as to what is going on. I asked around the lab and most people suggested using subroutines, but that seemed clunky, and I figured there should probably be a more elegant way, but I have not yet found it.  I tried to define a result by saying:</p>
  
  <pre><code> DOUBLE PRECISION FUNCTION Flux(W1,W2) Result(FluxArray(3))
  </code></pre>
  
  <p>and then returning fluxarray but that does not work, as fortran cannot understand the syntax</p>
  
  <p>The actual Function is this:</p>
  
  <pre><code>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, u-c)
  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
  </code></pre>
  
  <p>Here is the error statement:</p>
  
  <pre><code>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))
  </code></pre>
  
  <p>The last error occurs for both Fr and Fl. Thank you for your time and any help or consideration you can give!</p>
  
  <p>EDIT/Follow-up::
  Thanks for the help, I don't know a better way to present this so I'm going to edit the initial question.</p>
  
  <p>I did as you suggested and It solved that issue, now it says: </p>
  
  <pre><code>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  
  </code></pre>
  
  <p>I saw a similar issue on SO at this link: </p>
  
  <p><a href="https://stackoverflow.com/questions/6511711/computing-the-cross-product-of-two-vectors-in-fortran-90">Computing the cross product of two vectors in Fortran 90</a> </p>
  
  <p>where they suggested that he put all his functions into modules. is there a better/simpler way for me to fix this error?</p>