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MathGroup Archive 2006

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Re: DownValues and Cases

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70785] Re: [mg70759] DownValues and Cases
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Fri, 27 Oct 2006 00:28:39 -0400 (EDT)
  • References: <3282293.2074281161897563457.JavaMail.root@vms073.mailsrvcs.net> <B866860F-D756-42CE-A711-88777642D815@mimuw.edu.pl>

Of course I really meant:

h = {HoldPattern[f[x_]] :> 1, HoldPattern[g[x_]] :> 2, b :> 3};

in which case you get:


Cases[h, (x_ :> _) :> Level[x, -1]]


{{x, _, x_, 1}, {x, _, x_, 2}, {}}

Of course one can construct even more artificial examples where even  
my method will not produce the wanted result, namely, when the list h  
already contains something of the form Hold[f[a]]:>b, which we do not  
wish to pick up. One could modify it for such cases but then one  
could again construct examples where the new method would fail. I  
doubt that a completely universal and fail proof method could be  
found to deal with all such cases.

Andrzej

On 27 Oct 2006, at 11:43, Andrzej Kozlowski wrote:

> That will, of course, solve the problem you asked about, concerning  
> DownValues. But it would not solve the more general problem with  
> HoldPattern (admittedly one that is not very likely to occur  
> naturally) . Suppose we have the definitions
>
> f[x_] := 1; g[x_] := 2;
>
> and consider the list
>
> h = {HoldPattern[f[x_]] :> 1, HoldPattern[g[x_]] :> 2, b -> 3};
>
> Suppose we are only interesting in extracting information about f,  
> in the way you originally wanted. This will do it:
>
>
>
> Cases[h /. HoldPattern -> Hold, (Hold[f[x_]] :> y_) ->
>    {x, y}, Infinity]
>
>
> {{x_, 1}}
>
>
>
> but this will do something rather different:
>
>
>
> Cases[h, (x_ :> _) :> Level[x, -1]]
>
>
> {{x, _, x_, 1}, {x, _, x_, 2}}
>
>
>
> That's what I meant when I wrote "the most accurate way" - the way  
> that will allow you to pick up exactly the pattern you want when  
> similar ones are present.
>
> Andrzej Kozlowski
>
>
> On 27 Oct 2006, at 06:19, Bruce Colletti wrote:
>
>> Andrezj
>>
>> Your reply, and that of another, has gripped and led me the  
>> statement below (crafted with Wolfram Tech Support's help):
>>
>> Cases[h, (x_ :> _) :> Level[x, -1]]
>>
>> This is likewise appealing...but does it suffer a flaw not found  
>> in your solutions?
>>
>> Thanks.
>>
>> Bruce
>>
>> =====================
>> From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
To: mathgroup at smc.vnet.net
>> Subject: [mg70785] Re: [mg70759] DownValues and Cases
>>
>>
>> On 26 Oct 2006, at 22:54, Andrzej Kozlowski wrote:
>>
>>>
>>> On 26 Oct 2006, at 15:39, Bruce Colletti wrote:
>>>
>>>> Re Mathematica 5.2 under WinXP.
>>>>
>>>> The results of the first two Cases statements leads me to
>>>> anticipate {{1,3.5}, {2,5}, {_,0}} as the output of the third.
>>>>
>>>> The output of the fourth Cases statement confirms this expectation.
>>>>
>>>> Unfortunately, the output of the third statement is the empty list.
>>>>
>>>> Why so?  I've reviewed a related mid-July MathGroup thread, but
>>>> don't see the answer (if it's there at all).
>>>>
>>>> Thankx.
>>>>
>>>> Bruce
>>>>
>>>> --------------------------
>>>>
>>>> f at 1=3.5;
>>>> f@2=5;
>>>> f[_]:=0;
>>>> h=DownValues@f
>>>>
>>>> Out[4]={HoldPattern[f[1]] :> 3.5, HoldPattern[f[2]] :> 5,
>>>> HoldPattern[f[_]] :> 0}
>>>>
>>>> Cases[h,HoldPattern[f[x_]] -> x,Infinity]
>>>> Out[5]={1,2,_}
>>>>
>>>> Cases[h,(_ :> y_) -> y,Infinity]
>>>> Out[6]={3.5,5,0}
>>>>
>>>> Cases[h,(HoldPattern[f[x_]] :> y_) -> {x,y},Infinity]
>>>> Out[7]={}
>>>>
>>>> Cases[{a :> 4,b :> 5, c :> 6},(x_ :> y_) -> {x,y}]
>>>> Out[8]={{a,4},{b,5},{c,6}}
>>>>
>>>
>>> This is rather tricky. The problem is how to force the
>>> PatternMatcher to interpret HoldPattenr literally rather than as
>>> the pattern to be held. Note that:
>>>
>>> Cases[h, (Verbatim[HoldPattern[f[2]]] :> y_) -> {x, y},
>>>   Infinity]
>>>
>>> {{x, 5}}
>>>
>>> works, but:
>>>
>>>
>>> Cases[h, (Verbatim[HoldPattern[f[x_]]] :> y_) -> {x, y},
>>>   Infinity]
>>>
>>> {}
>>>
>>> doesn't, because now x_ is also interpreted literally rather than
>>> as a pattern. So you have got to somehow to avoid using HoldPattern
>>> directly in your pattern (this is now rally getting confusing), for
>>> example like this:
>>>
>>>
>>> Cases[h, (p_ :> y_) /; Head[p] === HoldPattern :>
>>>    {p[[1,1]], y}, Infinity]
>>>
>>> {{1, 3.5}, {2, 5}, {_, 0}}
>>>
>>> There are probably more elegant ways but none comes to my mind just
>>> now.
>>>
>>> Andrzej Kozlowski
>>
>>
>> A more compact way to do the above is:
>>
>> Cases[h, (p_HoldPattern :> y_) :> {p[[1, 1]], y}, Infinity]
>>
>>
>> But, it seems to me that the most accurate way to match expressions
>> with Head HoldPattern, as you wanted to do, is to first replace
>> HoldPattern by something like Hold or Unevaluated:
>>
>>
>> Cases[h /. HoldPattern -> Hold,
>>    (Hold[f[x_]] :> y_) -> {x, y}, Infinity]
>>
>>
>> {{1, 3.5}, {2, 5}, {_, 0}}
>>
>>
>> Andrzej Kozlowski
>>
>


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