Rank mismatch in function result

Question

I am learning to program in fortran and was making a basic program. In it I am trying to return an array from a function in my program, which is resulting in this error: Interface mismatch in global procedure 'test1' at (1): Rank mismatch in function result (0/1).

Below is a reconstruction of the error:

program Test
    implicit none
    integer :: a
    interface
        function test1(n) result (m)
            integer :: n
        end function test1
    end interface

    a = test1(4)

end program Test

function test1(n) result (m)
    implicit none
    integer :: n, m(2)
    m(1) = n**2
end function test1

The error is after the interface statement at function test1(n) result (m).

Since I am new to fortran, I assume I have missed something basic so any help is appreciated.

Answer 1

The interface and implementation of test1 have mismatched arguments, and a is mismatched to the result of test1. If you declare a to be an array of length 2 and add a declaration of m to your interface to also be an array of length 2, then everything should work:

program Test
   implicit none
   integer :: a(2)
   interface
       function test1(n) result (m)
           integer :: n, m(2)
       end function test1
   end interface

   a = test1(4)
   write(*,*) a

end program Test

function test1(n) result (m)
   implicit none
   integer :: n, m(2)
   m(1) = n**2
end function test1

Declaring functions

Declaring functions in the global scope and calling them through an interface will lead to a lot of excess code writing and maintenance. There are two better ways to declare functions; as internal functions, or in a module.

If you're writing a short program, you can get away with putting your functions directly inside your program, using contains. This gets rid of the need for an interface entirely:

program Test
   implicit none
   integer :: a(2)

   a = test1(4)
contains
   function test1(n) result (m)
      integer :: n, m(2)
      m(1) = n**2
   end function test1
end program Test

note that I've dropped the implicit none from the function, as it's now inherited from Test.

If your code gets a bit bigger, you will want to declare your functions in separate modules, as:

module test_module
   implicit none
contains
function test1(n) result (m)
   integer :: n, m(2)
   m(1) = n**2
end function test1
end module

program Test
   use test_module
   implicit none
   integer :: a(2)

   a = test1(4)
end program Test