deferred length character as subroutine argument [Fortran]

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From: ralf.schaa on 14 May 2010 23:35

I am struggling to get something like the following to work - is it
possible at all?
cheers
-ralf

program main
character( : ), allocatable :: theStr
call GetStr( theStr )

contains

subroutine GetStr( theStr )
character( : ), allocatable, intent( out ) :: theStr

! open a file etc.

! read the string from the file
! dynamic allocation of 'theStr' ???
read( funit, * ) theStr

end subroutine GetStr
end program main

From: Jim Xia on 14 May 2010 23:45

>
> program main
> character( : ), allocatable :: theStr
> call GetStr( theStr )
>
> contains
>
> subroutine GetStr( theStr )
> character( : ), allocatable, intent( out ) :: theStr
>
> ! open a file etc.
>
> ! read the string from the file
> ! dynamic allocation of 'theStr' ???
> read( funit, * ) theStr

The answer is NO. READ statement never does an dynamic allocation
automatically on its own. You have to allocate theStr before reading
it.

Cheers,

Jim

From: Richard Maine on 14 May 2010 23:48

ralf.schaa <ralf.schaa(a)gmail.com> wrote:

> I am struggling to get something like the following to work - is it
> possible at all?

Yes, but......

1. It requires f2003 for multiple reasons. F95+TR isn't enough, as the
TR doesn't include allocatable character length.

2. Yes, you can dynamically allocate theStr, but a read statement does
*NOT* do so. People have asked for functionalities somewhat like that,
but they don't exist.

You can allocate with an ALLOCATE statement or with the f2003
allocate-on-assignment. You cannot do it with a READ statement.

For example, one might modify the code shown as follows.

> program main
> character( : ), allocatable :: theStr
> call GetStr( theStr )
>
> contains
>
> subroutine GetStr( theStr )
> character( : ), allocatable, intent( out ) :: theStr
character(1000) :: some_temp_str
>
> ! open a file etc.
>
> ! read the string from the file
> ! dynamic allocation of 'theStr' ???
read( funit, * ) some_temp_str
theStr = trim(some_temp_str)
>
> end subroutine GetStr
> end program main

This assumes that 1000 (or some other suitable size) is large enough for
any string that will be encountered. I have no error checking. It also
assumes that the dynamic allocation is just to get the string length
"right" rather than to save space; if it is to save space then the
some_temp_str defeats the purpose. My crude version also assumes that
trim gets what is desired (in particular, that quoting was not used to
get a value with significant trailing blanks).

One could write more complicated versions with fewer restrictions by
using non-advancing I/O to read the string in pieces, but that's
significantly messier (and I'm not going to try to code it on the fly in
a usenet posting).

--
Richard Maine | Good judgment comes from experience;
email: last name at domain . net | experience comes from bad judgment.
domain: summertriangle | -- Mark Twain

From: ralf.schaa on 14 May 2010 23:59

Thanks Jim and Richard,

I changed my code following Richard's advice and it works fine.

Cheers
-Ralf

From: robin on 15 May 2010 11:25

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