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<== Date ==> <== Thread ==>

Subject: Re: access to array elements
From: Jeff Hill <[email protected]>
To: Nick Rees <[email protected]>
Cc: [email protected]
Date: Wed, 07 Aug 1996 11:05:48 -0600
Nick Rees wrote:
> 
> >   Would there be any interest in supporting an array element syntax in some
> > or all of the CA clients? For example, dm might support xxx[0] as the first
> > value of xxx and xxx[1] as the second. This would allow arrays to be used for
> > things like (X,Y) positions.
> 
> My only comment is that I see this as something which may be better
> handled at the server end, rather than the client end. William may have
> meant this when he referred to `all of the CA clients'. Basically all
> dm knows about variables is that it asks the world if anyone knows anything
> about a particular string - putting a knowledge of string syntax into
> the clients might be dangerous.

I like Williams idea. Problems will result if it isnt
implemented correctly. The following mappings should allow
what William is asking for without causing an impact on ca clients
such as dm.

PV name string	native count	offset (new)		comment
		(as seen by dm)	(client lib private)
--------------	------------	------------		-------
PVXYZ[2-4]	<= 3		2			db may not have elements 2-4
							so native count may be less
						
PVXYZ[2]	<= 1		2			db may not have element 2
							so native count may be less

PVXYZ		(db specified)	0


When a client such as dm asks for elements n1 through n2 the following 
transformation on the request would occur in the ca client lib prior
to sending the request to the database:

first db element in request to db = offset + n1
last db element in request to db = offset + n2

The trailing brackets would of course be stripped prior to sending
the PV name to the IOC. 

Of course all of this is contingent on fixing current array addressing 
limitations in the ca client/db api.

Jeff

-- 

______________________________________________________________________
Jeffrey O. Hill			Internet	[email protected]
LANL MS H820			Voice		505 665 1831
Los Alamos, NM 87545 USA 	FAX		505 665 5107


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Re: access to array elements Nick Rees

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