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From: u3369429@murdu.OZ (Michael Bednarek)
Newsgroups: comp.os.vms
Subject: Re: VMS weirdness (aka The DCL "&" operator)
Message-ID: <1325@murdu.OZ>
Date: Fri, 13-Nov-87 23:52:45 EST
Article-I.D.: murdu.1325
Posted: Fri Nov 13 23:52:45 1987
Date-Received: Tue, 17-Nov-87 03:48:58 EST
References: <8711081040.AA04841@ucbvax.Berkeley.EDU> <1789@charon.unm.edu> <8711120058.AA11607@ucbvax.Berkeley.EDU>
Followup-To: comp.os.vms
Organization: I.A.E.S.R., Melbourne University
Lines: 67
Keywords: symbol apostrophe ampersand substitution DCL_compiler

In article <1789@charon.unm.edu> sdowdy@charon.UUCP (Stephen Dowdy) writes:
>in response to <8711081040.AA04841@ucbvax.Berkeley.EDU>
>               SERAFINI%TOM@IO.ARC.NASA.GOV
>The "&" symbol is used by DCL as an "indirect substitute variable" directive.
>It is similar in function to the "'" directive, but carries the substitution
>one step further.  It uses the contents of a symbol to refer to a symbol
>that should be operated on.
>
>An example:
>	$	a := "hello"
>	$	b := "a"
>	$	write sys$output "  ''b'"
>	  a
>	$	write sys$output "  ''&b'"
>	  hello
 
Wrong. I tried it and it doesn't work. It would work, however, if:
 $ b:="A"

Anyway, this is not a particularly well chosen example, as
 $ write sys$output "   ",'b
produces the same result.

It still surprises me that your example, if uppercase A is used, works.
Quoted from VAX/VMS DCL Dictionary section 6.2.2:
"You cannot use ampersands to request substitution within characters strings
 enclosed in quotation marks."
 
Your enclosed ALIAS procedure uses the same format:
>$	Write Sys$Output "[ALIAS] ""''P1'"" = ""''&P1'"""
and it works, even if DCL's uppercasing is inhibited, e.g.:
 $ @alias "say" ! gives:
[ALIAS] "say" = "Write Sys$Output"

Again, the use of `&' is not really mandatory in ALIAS, it could be re-written:
$	Write Sys$Output "[ALIAS] """,P1,""" = """,'P1,""""

So, where are ampersands really necessary? In subscripted variables!
F'rinstance VMS_SHAR needs a character array `File(500)*255' (Fortran syntax)
to store all filenames to be packaged. It sets up a counter, say `n', and for
each filename encountered, stores it in "File'n". Then to run CHECKSUM on each
file: "$ Checksum &File'n"
DCL will first substitute "'n", giving e.g. "1", resulting in "&File1", which
will be substituted by its definition, the filename. The obvious advantage
over other HLLs is the lacking necessity of declaring the size of the array
beforehand. (And you thought a DCL compiler would be possible? It isn't.)

>Unfortunately, this works only for variables that are defined using ":=", not
>using "=".  (I personally think using "=" is generally more useful.)
??? Which VMS version are you running?
 
>$! Title:	Alias.Com
>$! Purpose:	To assign or return the value of a (or all) symbol(s)
>$!			An "act-alike" to Unix "alias"

What's wrong with `$ Show Symbol <symbol>' and '$ <symbol>=[=]"<definition>"'?
To simulate `alias', you need at least PIPE by
KENW@NOAH.ARC.CDN (Ken Wallewein).


Michael Bednarek
Institute of Applied Economic and Social Research (IAESR)
Melbourne University, Parkville 3052, AUSTRALIA, Phone : +61 3 344 5744
Domain: u3369429@{murdu.oz.au | ucsvc.dn.mu.oz.au}  or  mb@munnari.oz.au
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