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Re: modify elements of arb.dimensionality matrices

• To: mathgroup at smc.vnet.net
• Subject: [mg77278] Re: modify elements of arb.dimensionality matrices
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Wed, 6 Jun 2007 07:10:47 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <f43ibj$3nf$1@smc.vnet.net>

alexxx.magni at gmail.com wrote:
> I was able to write an interface to deal with matrices having
> dimension defined at runtime (see
> http://groups.google.it/group/comp.soft-sys.math.mathematica/browse_frm/thread/f286f5bdafba4195/c85eaca115f18a46?lnk=st&q=+%22Alessandro+Magni%22&rnum=2&hl=it#c85eaca115f18a46).
> 
> My last problem - more a performance problem - is modify access to
> those matrices:
> 
> reading access  at a given index location i (where i is the location
> {x,y,z,...}) is made with:
> 
>> Extract[h, i]
> 
> while to increment h[[]] at i I use:
> 
>> h = ReplacePart[h, i ->  Extract[h, i] + 1]
> 
> but I really miss the possibility to write h[[x,y]]=h[[x,y]]+1, and
> I'm user there are serious performance issues with my solution.
> 
> The nearest I came to a solution was with:
> 
>> h[[##]] & @@ i = "x"
> 
> but I immediately get:
> 
>> Set::write: Tag Apply in (s[[##1]]&)@@{1,2,1} is Protected. >>
> 
> Do you have any idea on this?
> 
> thank you!
> 
> Alessandro Magni

Set[h[[##]], "x"] & @@ i

should work.

Regards,
Jean-Marc