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MathGroup Archive 2006

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Re: DownValues question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70093] Re: DownValues question
  • From: dimmechan at yahoo.com
  • Date: Tue, 3 Oct 2006 06:16:39 -0400 (EDT)
  • References: <efld9n$d49$1@smc.vnet.net><efq5mt$2bi$1@smc.vnet.net>

Yes, I was wrong.
Thanks for your advice.

Regards
Dimitris

Jens-Peer Kuska wrote:
> Hi,
>
> you are sure that
> DownValues[f] = Reverse[DownValues[f]]
>
> "(re)assign a new value to DownValues"
> and not to DownValues[f] ??
>
> Regards
>    Jens
>
>
> dimmechan at yahoo.com wrote:
> > One more question. (I apologise for the number of questions).
> >
> > The following rule is not allowed
> >
> > x + y = z
> > Set::write : Tag Plus in x + y is Protected.
> > z
> >
> > since Plus has the attribute Protected.
> >
> > If you want to avoid the drastic way of Unprotect Plus and add the rule
> >
> > you can use
> >
> > x /: x + y = z
> > z
> >
> > Suppose now the following
> >
> > Clear[f]
> > f[x_, 2] := foo
> > f[2, x_] := foofoo
> >
> > Then
> >
> > DownValues[f]
> > {HoldPattern[f[x_, 2]] :> foo, HoldPattern[f[2, x_]] :> foofoo}
> >
> > If you want, you can change the order that assigned down values are
> > stored by
> >
> > DownValues[f] = Reverse[DownValues[f]]
> > {HoldPattern[f[2, x_]] :> foofoo, HoldPattern[f[x_, 2]] :> foo}
> >
> > However
> >
> > Attributes[DownValues]
> > {HoldAll, Protected}
> >
> > Using Set we attach a value to the head of the left-hand side.
> > So, how we can (re)assign a new value to DownValues if its Head
> > has the attribute Protected?
> > 
> > Thanks in advance for any help.
> >


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