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From: hirchert@ux1.cso.uiuc.edu (Kurt Hirchert)
Newsgroups: comp.lang.fortran
Subject: Re: slow write
Message-ID: <1990Mar22.200839.19016@ux1.cso.uiuc.edu>
Date: 22 Mar 90 20:08:39 GMT
References: <3673.2607e3bb@vax5.cit.cornell.edu>
Distribution: comp
Organization: University of Illinois at Urbana
Lines: 42

In article <3673.2607e3bb@vax5.cit.cornell.edu> h3dy@vax5.cit.cornell.edu writes:
>I have a large array (characters) that I read in, process,
>then write out to a file.  Prior to run time, I dont know
>the final number of elements - it is smaller than the original.
>
>      write (10) array
>
>is fast, but if the array is much smaller, I don't want to
>do this.
>
>      write (10) (array(i),i=1,n)
>
>but an implied do is very very slow!  How can I get the
>combine the advantages whilst avoiding the disadvantages?
>My array is about 15000000 characters long initially.

1. The implied DO is slow only on some machine.  Some compilers are smart
   enough to recognize that the implied DO is transferring consecutive
   storage and use the same kind of fast I/O as for the whole array
   transfer.

2. On machines where the compiler is not that good, the following trick will
   usually work:  Replace the WRITE statement above with
         CALL WRITCH(10, ARRAY, N)
   where the subroutine WRITCH is defined as follows:
         SUBROUTINE WRITCH (IUNIT, ARRAY, N)
         CHARACTER ARRAY(N)
         WRITE(IUNIT) ARRAY
         END
   The subroutine uses the whole array notation (and thus the fast I/O call),
   but the size of the array is determined at run time.  The added procedure
   call overhead makes this slower than just a write of a whole array of
   equivalent size (and thus slower than the implied DO if the compiler does
   a good job of optimization), but faster than transferring each character
   separately (which is typically what you get if the compiler _doesn't_
   do a good job of optimizing the implied DO).

   [If the original ARRAY is declared something other than CHARACTER, adjust
   the declaration in the subroutine accordingly.]
-- 
Kurt W. Hirchert     hirchert@ncsa.uiuc.edu
National Center for Supercomputing Applications