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MathGroup Archive 2012

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Re: Why does the order of down values come back?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124800] Re: Why does the order of down values come back?
  • From: David Reiss <dbreiss at gmail.com>
  • Date: Tue, 7 Feb 2012 04:04:52 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jgo0e9$a2k$1@smc.vnet.net>

Sorry for my previous suggestion...  What appears to be happening is
that DownValues (even if you set Sort->False) will continue to order
the rules based on which seems to be more general with the special
cases first and the more general patterns afterwards.

On Feb 6, 2:45 am, Shizu <slivo.v... at msa.hinet.net> wrote:
> In[]:= f[0] := 0;f[1] := 1;f[n_] := f[n - 1] + f[n - 2]
>
> In[]:= DownValues[f]
> Out[]:= {HoldPattern[f[0]] :> 0, HoldPattern[f[1]] :> 1, HoldPattern[f[n_]] :> f[n - 1] + f[n - 2]}
>
> In[]:= DownValues[f] = Reverse[DownValues[f]]
> Out[]:= {HoldPattern[f[n_]] :> f[n - 1] + f[n - 2], HoldPattern[f[1]] :=
> 1, HoldPattern[f[0]] :> 0}
>
> In[]:= DownValues[f]
> Out[]:= {HoldPattern[f[0]] :> 0, HoldPattern[f[1]] :> 1, HoldPattern[f[n_]] :> f[n - 1] + f[n - 2]}
>
> =========================
> My question is:
>
>     Why does the order of down values comes back after reordering?
>
> Thanks.




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