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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124802] Re: Why does the order of down values come back?
  • From: "Oleksandr Rasputinov" <oleksandr_rasputinov at ymail.com>
  • Date: Tue, 7 Feb 2012 04:05:34 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jgo0e9$a2k$1@smc.vnet.net>

On Mon, 06 Feb 2012 07:45:13 -0000, Shizu <slivo.vitz 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.
>

Because DownValues sorts its output by default. The actual downvalues are  
not, however, sorted; the order is as you set them. To see their actual  
order, use the (undocumented) option:

DownValues[f, Sort -> False]



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