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From: jerry@violet.berkeley.edu (Jerry Berkman)
Newsgroups: comp.lang.fortran
Subject: Re: Dimensioning arrays at run-time, best way?
Message-ID: <1990Sep22.184727.4648@agate.berkeley.edu>
Date: 22 Sep 90 18:47:27 GMT
References: <614@keele.keele.ac.uk> <1990Sep17.150155.10220@alchemy.chem.utoronto.ca>
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Organization: University of California, Berkeley
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In article <614@keele.keele.ac.uk> phd11@.uk.ac.keele (Zipzoid) writes:
>what is the best way of dimensioning an array whose maximum
>memory requirement (size) would be read in at run-time?
>
On a Cray, you can use automatic arrays:

      PROGRAM EXAMPLE
      read *, n
      call sub1(n)
      END

      SUBROUTINE SUB1(N)
      real X(N)

C     Do the work in this subroutine.
C     Of course you can't use common blocks so you'll have to pass X
C      around as an argument.
      END

Since X's bounds are a dummy argument, the Cray CFT77 compiler
generates code to allocate memory for the array when the subroutine is
entered, and deallocate it when the subroutine is exited.
This is a Fortran 90 feature which is not common, but is probably
supported on some other systems.

As others have noted, VMS type Fortran systems have an extension that
allows passing values, so a similar trick can be used by calling
malloc() or something equivalent and passing the value returned as
an array.

On 4.3 BSD VAX UNIX, there is a routine called falloc(), which stands
for Fortran alloc().  This allocates the space but you must use ofsets
to a common block to refer to it, less convenient than the above methods.

For those new to this, dynamic allocation has been used with Fortran
since before I can remember.  Almost every big package such as SPSS
uses dynamic allocation.  I'm glad it will be in Fortran 90.

	- Jerry Berkman, U.C.Berkeley, (415)642-4804
	  jerry@violet.berkeley.edu

disclaimer: opinions are my own, not my employers, etc.