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Re: Behavior of Longest

• To: mathgroup at smc.vnet.net
• Subject: [mg107946] Re: [mg107909] Behavior of Longest
• From: dh <dh at metrohm.com>
• Date: Wed, 3 Mar 2010 05:53:40 -0500 (EST)
• References: <201003021254.HAA14195@smc.vnet.net> <34c814851003020532g600c12d8x288278f72647a54@mail.gmail.com>

Hi Leonid,
thank you for the reply.
Maybe I have not fully understood "Longest".
My problem is that I assumed that you can match something between one 
element and the whole "level". E.g. in {a,1,2,b} it should be possible 
to match 1,2. In a "regex" like notation you would write something like: 
_Integer{2}.
However, it looks like in Mathematica you have to match either one element or 
the whole "level". Is this correct?
Daniel



On 02.03.2010 14:32, Leonid Shifrin wrote:
> Hi Daniel,
>
> I don't think it's a bug. By using the form you did, you gave the
> pattern-matcher no choice,
> since there are no other placeholders (in the case of Shortest) to
> absorb the rest of the elements.
>
> In[502]:= {1, 2} /. {___, Longest[x__Integer], ___} :> f[x]
>
> Out[502]= f[1, 2]
>
> In[504]:= {1, 2} /. {Longest[x__Integer]} :> f[x]
>
> Out[504]= f[1, 2]
>
> I have an impression that the intended use is in a list, like in your
> second example or in the following:
>
> In[505]:= {1, 2} /. {Shortest[x__Integer]} :> f[x]
>
> Out[505]= f[1, 2]
>
> In[506]:= {1, 2} /. {___, Shortest[x__Integer], ___} :> f[x]
>
> Out[506]= f[1]
>
> In[507]:= {1, 2} /. {___, Shortest[x__Integer]} :> f[x]
>
> Out[507]= f[2]
>
> In the last two cases, there are placeholders to absorb "unused"
> elements, and thus the pattern-matcher has a freedom it needs for
> Shortest. In fact, in your first example you really did not apply rules
> at level 0  (entire expression) - rather, they were applied at level 1,
> two times: for a sequence <1>, and for a sequence <2>, separately:
>
> In[509]:= Replace[{1, 2},
>   Longest[x__Integer] :> (Print["Rule matched"]; f[x]), {0}]
>
> Out[509]= {1, 2}
>
> In[510]:= Replace[{1, 2},
>   Longest[x__Integer] :> (Print["Rule matched"]; f[x]), {1}]
>
> During evaluation of In[510]:= Rule matched
>
> During evaluation of In[510]:= Rule matched
>
> Out[510]= {f[1], f[2]}
>
> Regards,
> Leonid
>
>
>
> On Tue, Mar 2, 2010 at 3:54 PM, dh <dh at metrohm.com
> <mailto:dh at metrohm.com>> wrote:
>
>     Hello,
>     can somebody give an explanation for the following behavior of Longest
>     (Mathematica version 7.0.1):
>     {1, 2} /. Longest[x__Integer]:>{x}    gives {{1}, {2}}
>
>     the same as Shortest:
>     {1, 2} /. Shortest[x__Integer]:>{x}  gives {{1}, {2}}
>
>     However, it works like expected in the following case:
>     {a, 1, 2, b} /. {x1___, Longest[x__Integer], x2___} :> {x1, {x}, x2}
>     gives: {a, {1, 2}, b}
>
>     looks like a bug to me.
>
>     Daniel
>
>     --
>
>     Daniel Huber
>     Metrohm Ltd.
>     Oberdorfstr. 68
>     CH-9100 Herisau
>     Tel. +41 71 353 8585, Fax +41 71 353 8907
>     E-Mail:<mailto:dh at metrohm.com <mailto:dh at metrohm.com>>
>     Internet:<http://www.metrohm.com>
>
>
>


-- 
Daniel Huber
Metrohm AG
International Headquarters
Oberdorfstr. 68, CH-9101 Herisau / Switzerland
Phone +41 71 353 8606, Fax +41 71 353 89 01
Mail <mailto:dh at metrohm.com>
Web <http://www.metrohm.com

• References:
  • Behavior of Longest
    • From: dh <dh@metrohm.com>