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MathGroup Archive 2007

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Re: Re: problem with Pick

  • To: mathgroup at smc.vnet.net
  • Subject: [mg78320] Re: [mg78221] Re: [mg78194] problem with Pick
  • From: DrMajorBob <drmajorbob at bigfoot.com>
  • Date: Thu, 28 Jun 2007 04:26:48 -0400 (EDT)
  • References: <acbec1a40706230431p4f1db9a9i4320680dda027396@mail.gmail.com> <200706251106.HAA05048@smc.vnet.net> <467FE85C.6030700@wolfram.com> <acbec1a40706251652h78eedc03o95629f4a0e732ad1@mail.gmail.com> <8010218.1182851582213.JavaMail.root@m35>
  • Reply-to: drmajorbob at bigfoot.com

> This could be explained better in the documentation.

You think?

Bobby

On Tue, 26 Jun 2007 03:30:10 -0500, Oyvind Tafjord <tafjord at wolfram.com>

wrote:

> Chris Chiasson wrote:
>> On 6/25/07, Oyvind Tafjord <tafjord at wolfram.com> wrote:
>>> In this case, sel={{},{1}}. sel[[]] matches patt, and so list[[]] is
>>> picked out.
>>>
>>> This behavior, in which all of list gets picked, does produce a fair
>>> amount
>>> of confusion, unfortunately. It often shows up in connection with  
>>> Except:
>>>
>>> In[1]:= Pick[{a, b, c}, {1, 2, 3}, Except[2]]
>>> Out[1]= {a, b, c}
>>>
>>> In[2]:= Pick[{a, b, c}, {1, 2, 3}, Except[2, _Integer]]
>>> Out[2]= {a, c}
>>
>> Thank you for the reply.
>>
>> You seem to be implying that the above behavior is intended, even
>> though it violates the documentation. I believe it is unintended, due
>> to the following output, which seems to indicate that the matches are
>> supposed to be checked on the first level:
>>
>> In[1]:= Pick[{a,b,c},SparseArray@{1,2,3},Except[2]]
>> Pick[{a,b,c},{1,2,3},Except[2]]
>>
>> Out[1]= {a,c}
>>
>> Out[2]= {a,b,c}
>>
>> If I am wrong, then this SparseArray behavior is a bug, the
>> documentation is wrong, and I suggest that an optional level argument
>> be added to Pick.
>>
>
> In what way does this behavior violate the documentation? It uses the
> statement you quoted:
>
> Pick[list,sel,patt] picks out those list[[i1,i2,...]] for which
> sel[[i1,i2,...]] matches patt.
>
> with the 0-length version of i1,i2,... I agree this is often what you 
> don't
> want though.
>
> The SparseArray case is treated differently, in this case only the
> individual elements are treated as "selectors", for optimization and
> usefulness. This could be explained better in the documentation.
>
> E.g., compare
>
> In[20]:= Pick[{{a, b, c}, {d, e, f}},
>   SparseArray@{{3, 4, 5}, {1, 2, 3}}, 4]
>
> Out[20]= {{b}, {}}
>
> In[21]:= Pick[{{a, b, c}, {d, e, f}},
>   SparseArray@{{3, 4, 5}, {1, 2, 3}}, {3, 4, 5}]
>
> Out[21]= {{}, {}}
>
> In[22]:= Pick[{{a, b, c}, {d, e, f}}, {{3, 4, 5}, {1, 2, 3}}, {3, 4, 5}]
>
> Out[22]= {{a, b, c}, {}}
>
> Oyvind Tafjord
> Wolfram Research
>
>



-- 
DrMajorBob at bigfoot.com


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