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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124813] Re: Why does the order of down values come back?
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Wed, 8 Feb 2012 05:28:23 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <jgo0e9$a2k$1@smc.vnet.net> <201202070905.EAA23510@smc.vnet.net>
  • Reply-to: drmajorbob at yahoo.com

I suspect the unsorted version is irrelevant, however, in that when  
f[something] is evaluated, rules will be tested from most specific to  
least.

Bobby

On Tue, 07 Feb 2012 03:05:34 -0600, Oleksandr Rasputinov  
<oleksandr_rasputinov at ymail.com> wrote:

> 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]
>


-- 
DrMajorBob at yahoo.com



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