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From: sjc@key.COM (Steve Correll)
Newsgroups: comp.lang.fortran
Subject: Re: slow write
Message-ID: <1550@key.COM>
Date: 22 Mar 90 20:07:26 GMT
References: <3673.2607e3bb@vax5.cit.cornell.edu>
Distribution: comp
Organization: Key Computer Labs, Fremont, CA
Lines: 46

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!

Because the implied-do notation permits you to skip around within an array,
selecting non-contiguous elements, a compiler must, in the general case,
write the elements out one at a time. Some compilers recognize the special
(but frequent) case where the implied-do elements are contiguous in memory,
and can thereby economize by writing them out in one big block. Apparently
yours doesn't.

Try putting the write statement inside a subroutine and passing the array as
"character*(*)" or (for a non-character array) using an adjustable-array
declaration:

	subroutine s(c, a, n)
	character*(*) c
	real a(n)
	write(10) c
	write(11) a
	end

	...
	call s(c(1:<desired length>), a, <desired length>)
	end

There's no guarantee, but this does make it easier for the compiler to
recognize that the elements are contiguous and perform each WRITE statement
as a single operation.

Incidentally, this illustrates one reason why users may be disappointed in the
first generation of Fortran 90 compilers. If experience is a guide, it will be
some time before most compilers mature enough to recognize all the special,
optimizable cases of the new array constructs.
-- 
...{sun,pyramid}!pacbell!key!sjc 				Steve Correll