Fortran 77: How to use array output from function or subroutine

Question

I've only been learning Fortran 77 (and its syntax) the last few days, and tried finding answers throughout the site and textbooks already and come up still confused, so I'd appreciate any help. I'm sorry if the formatting on this post is off; this is my first post, and I'm crunched for time.

I'm creating a program to multiply matrices. I want to create a subroutine or a function that will take two matrices as inputs (two 2x2 arrays), and return the multiplied matrix (one 2x2 array). I can't figure out how to get either a subroutine or a function to return an array of fixed size, or how to use the array once it's returned.

I tried using a function, which compiled on its own. But when I tried calling the function from my main program, I couldn't call it on its own:

 MATMULT(MAT0, MAT0, MAT0)                                        
 1
 Error: Unclassifiable statement at (1) 

or assign it to another variable (I tried different REALs and arrays):

  BLAH = MATMULT(MAT0, MAT0, MAT0)                                 
                1
  Error: Return type mismatch of function 'matmult' at (1) (INTEGER(4)/REAL(4))
  MATRIX1.f:26.22:

  BLAH = MATMULT(MAT0, MAT0, MAT0)                                 
                 1
  Warning: Type mismatch in argument 'x' at (1); passed INTEGER(4) to REAL(4)

  BLAH = MATMULT(MAT0, MAT0, MAT0)                                 
                 1
  Warning: Rank mismatch in argument 'x' at (1) (scalar and rank-2)

Since arrays are passed by reference, I'm really not sure what the function is returning, so how can I use the output matrix, if that is indeed the function's output?

I also tried using a subroutine, but (in addition to still not knowing what it's returning or where) then I get a "Two main PROGRAMs" error - so the compiler isn't differentiating between the main program and the subroutine. This might be a problem with my syntax on subroutines? I tried a few different things, but here's my most recent iteration of the code (I'm just trying to get the array-passing to work, so there's not actual matrix multiplication in here yet):

  PROGRAM MATRIX1

  INTEGER N 
  REAL A, B, MAT0(2,2), MATF(2,2), X(2,2), Y(2,2), Z(2,2)
  REAL BLAH


  PRINT *, " ENTER THE VALUE OF A: "
  READ *, A
  PRINT *, " ENTER THE VALUE OF B: "
  READ *, B
  PRINT *, " ENTER THE NUMBER OF MULTIPLICATIONS: "
  READ *, N

C Creates the initial matrix
MAT0(1,1) = 1.0 - A MAT0(1,2) = A MAT0(2,1) = B MAT0(2,2) = 1.0 - B

  PRINT *, "M = ", MAT0

  CALL MATMULT(MAT0, MAT0, MAT0)
  PRINT *, "FINAL "


  STOP
  END PROGRAM

  REAL SUBBROUTINE MATMULT(X, Y, Z)

      END SUBROUTINE

Or (edited to add some of the recommended changes) with a function:

  PROGRAM MATRIX1

  INTEGER N 
  REAL A, B, MAT0(2,2), MATF(2,2), X(2,2), Y(2,2), Z(2,2)
  REAL MATMULT(2,2)


  PRINT *, " ENTER THE VALUE OF A: "
  READ *, A
  PRINT *, " ENTER THE VALUE OF B: "
  READ *, B
  PRINT *, " ENTER THE NUMBER OF MULTIPLICATIONS: "
  READ *, N

C Creates the initial matrix
MAT0(1,1) = 1.0 - A MAT0(1,2) = A MAT0(2,1) = B MAT0(2,2) = 1.0 - B

  PRINT *, "M = ", MAT0

  Z = MATMULT(X, Y)


  STOP
  END PROGRAM

  FUNCTION MATMULT(X, Y)
      REAL X(2,2), Y(2,2), Z(2,2), MATMULT(2,2)
      RETURN
      END

I'm still getting errors:

  Z = MATMULT(X, Y)                                                
              1
  Warning: Legacy Extension: REAL array index at (1)
  MATRIX1.f:28.19:

  Z = MATMULT(X, Y)                                                
              1
  Error: Array index at (1) is an array of rank 2

Answer 1

In this era there is no reason for use FORTRAN 77. Fortran 90/95/2003 is easier to use, more powerful, and better assists the programer in finding mistakes. gfortran is an excellent open-source compiler.

Here is an example in Fortran 95 of a similar program/function: a function that implements a vector-cross product. The example shows the function and program in the same file (there are other approaches) with the function receiving two vectors and returning one vector. Example: Computing the cross product of two vectors in Fortran 90

Answer 2

There are two ways to do this:

1) Subroutine. Declare a subroutine like you did. However, specify input and output parameters:

REAL, INTENT(IN) :: X(2,2), Y(2,2)
REAL, INTENT(OUT) :: Z(2,2)

and call the subroutine using call keyword: CALL MATMULT

2) Function. Also specify input parameter, but now return value of the function:

! Insert calculation
MALMULT = ... 

Don't forget to declare it in proper way: FUNCTION MATMULT(x,Y)

Call it: Z = MATMULT(X,Y)

Answer 3

You mention several problems in your question. Certainly your work will be enhanced using more recent Fortran dialects, as given in the answers by M. S. B. and Ilmirus. And you really should be using implicit none (even for Fortran 77 a lot of compilers support that - and lower case - as an extension).

Coming to the conceptual part, I must say that you need to continue your learning. However, if you want a function which returns the matrix product then you will have something like

function matmult(x, y)
  real x(2,2), z(2,2), matmult(2,2)
  ... calculation of matmult
end function matmult

Followed by

z = matmult(x,y)

in the main program, where z is real z(2,2), rather than the scalar blah you have in your example, and is your result.

Coming to your subroutine, there are indeed problems: real subroutine matmult(x,y,z) requires not the real. Then calling this with call matmult(mat0, mat0, mat0) where you intend to update mat0 with a result is asking for aliasing problems. call matmult(x,y,z) will "store the result" where the subroutine calculations put it. Using intent as given by Ilmirus is a good guide to the behaviour desired by the programmer.

Finally, if you just want to get the result of multiplying A and B as matrices and storing in C: use C = matmul(A,B) with a later-than-F77 compiler.

However, in Fortran 77 one doesn't return array results, so the subroutine approach is the one to go for. In response to your edit:

  REAL MATMULT(2,2)

is declaring matmult an array, not a function, so references to matmult(X,Y) give rise to your error.