Re: problem with Pick

• To: mathgroup at smc.vnet.net
• Subject: [mg78209] Re: [mg78194] problem with Pick
• From: Oyvind Tafjord <tafjord at wolfram.com>
• Date: Tue, 26 Jun 2007 04:08:14 -0400 (EDT)
• References: <acbec1a40706230431p4f1db9a9i4320680dda027396@mail.gmail.com> <200706251106.HAA05048@smc.vnet.net>

Chris Chiasson wrote:
> Step to reproduce:
> 1. Evaluate these two inputs with a fresh kernel:
> Pick[{a,b},z[{},{1}],{__}]
> Pick[{a,b},{{},{1}},{__}]
>
> Expected results:
> {b}
> {b}
>
> Actual results:
> {b}
> {a,b}
>
> Relevant Documentation:
> Pick[list,sel,patt] picks out those list[[i1,i2,...]] for which
> sel[[i1,i2,...]] matches patt.

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}

Oyvind Tafjord
Wolfram Research

>
> Relevant Evaluation:
> In[3]:= MatchQ[{},{__}]
> Out[3]= False
>
> In[4]:= {Internal`\$CreationID,\$Version}
> Out[4]= {853857,6.0 for Microsoft Windows (32-bit) (April 27, 2007)}
>
> --
> http://chris.chiasson.name/

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