Pass a real array if a complex array is expected

Question

I have a Fortran subroutine that expects a complex array like

subroutine foo(cnumbers, n)
    integer :: n
    complex :: cnumbers(n)
    ...
end subroutine foo

and later I want to call it like

real :: rnumbers(40)
...
call foo(rnumbers, 20)

However, I get the compiler error:

error #6633: The type of the actual argument differs from the type of the dummy argument.

Of course, this is comprehensible since a real array is not a complex array. But there must be a way to make it work.

Because if the subroutine foo and the call of foo are in different modules and are written down in different Fortran files, then the compiler does not complain, and everything works fine.

Does someone know how to make it work? How to pass a real array if a complex array is expected?

You should show more code on the calling side. If rnumbers is not a dummy argument you can just equivalence it with a complex array. — Vladimir F Героям слава
– Vladimir F Героям слава, Commented May 2, 2016 at 8:28
No, rnumbers is not dummy, its an actual variable that I create with real :: rnumbers(40). By the way, I am using the Intel Fortran compiler 15.0.3. — thyme
– thyme, Commented May 2, 2016 at 8:37
if your subroutine was external (no explicit interface) your compiler would not detect the type issue and it would work fine. — agentp
– agentp, Commented May 2, 2016 at 15:37

Community · Accepted Answer · 2017-05-23 12:30:24Z

2

You can either use transfer(rnumbers, ...) to convert the type (a temporary array is likely to be created) or use equivalence to avoid it

    real :: rnumbers(40)
    complex :: cnumbers(20)
    equivalence (rnumbers, cnumbers)

    set the value of rnumbers

    call foo(cnumbers, 20)

If you need allocatable arrays the equivalence will not work.

You can also use an external subroutine and lie the compiler about the interface and just pass the real array instead of the complex one. It is not standard conforming, but it is sometimes used. See also Gfortran complex actual to real dummy argument

edited May 23, 2017 at 12:30

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answered May 2, 2016 at 8:52

Vladimir F Героям слава

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5 Comments

thyme Over a year ago

Thank you for the answer. But unfortunately I deal with allocatable arrays. Is there no workaround with pointers or so? I gues the solution with transfer is to expensive when it comes to very large arrays.

Vladimir F Героям слава Over a year ago

@thyme That's exactly why I requested that you add more of the calling code! Why you simply can't do that? I sort of assumed that the code you posted is incomplete and added the disclaimer... The only thing you did is that you just replied "rnumbers is not dummy, its an actual variable that I create with real :: rnumbers(40)" even if it is not true at all...

thyme Over a year ago

Ok, sorry, I was too naive. I thought to make the example as simple as possible...

jlokimlin Over a year ago

See stackoverflow.com/questions/30160456/…

Vladimir F Героям слава Over a year ago

Some recent relevant discussion and proposals fortran-lang.discourse.group/t/… github.com/j3-fortran/fortran_proposals/pull/325

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