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MathGroup Archive 2002

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Re: Assigning to a sublist

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35166] Re: [mg35161] Assigning to a sublist
  • From: Andrzej Kozlowski <andrzej at lineone.net>
  • Date: Fri, 28 Jun 2002 02:30:51 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Use:


zz[[3,2]] = Append[zz[[3]][[2]], {1, 1, 1, 1, 1}]

  instead of

zz[[3]][[2]] = Append[zz[[3]][[2]], {1, 1, 1, 1, 1}]


The latter does not work, because, as the error message tell's you the 
in assignments of the form
Part[something,...]= ..., the "something" must be a symbol. The former 
assignment above has the FullForm
Part[zz,3,2] so it satisfies this condition while the latter has the 
form Part[Part[zz, 3], 2]  so it doesn't.

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/





On Thursday, June 27, 2002, at 01:23  PM, Bob Harris wrote:

> Howdy,
>
> I'm trying to assign a new value to an entry in a sublist (of another 
> list),
> and I can't understand why it won't work.
>
> For example, I do the following:
>
>   In[1]:= zz = {{2, {{1, 1}}}, {10, {{3, 1}}}, {5, {{2, 1}}}}
>   Out[1]= {{2, {{1, 1}}}, {10, {{3, 1}}}, {5, {{2, 1}}}}
>
>   In[2]:= zz[[3]][[2]]
>   Out[2]= {{2, 1}}
>
>   In[3]:= Append[zz[[3]][[2]], {1, 1, 1, 1, 1}]
>   Out[3]= {{2, 1}, {1, 1, 1, 1, 1}}
>
>   In[4]:= zz[[3]][[2]] = Append[zz[[3]][[2]], {1, 1, 1, 1, 1}]
>   Set::"setps : zz[[3]] in assignment of part is not a symbol."
>   Out[4]= {{2, 1}, {1, 1, 1, 1, 1}}
>
> For some reason it doesn't like the assignment.  What confuses me is 
> that is
> zz[[3]][[2]] were just a variable, it would work.  Further, if it were 
> just
> an entry at the *top* level of a list, it would work, as this example 
> shows:
>
>   In[5]:= yy = zz[[3]]
>   Out[5]= {5, {{2, 1}}}
>
>   In[6]:= yy[[2]]
>   Out[6]= {{2, 1}}
>
>   In[7]:= yy[[2]] = Append[yy[[2]], {1, 1, 1, 1, 1}]
>   Out[7]= {{2, 1}, {1, 1, 1, 1, 1}}
>
> So it seems like the issue is just that deeply nested things don't 
> behave
> like things that are not as deeply nested.  Am I right about that?  How 
> can
> I modify an entry in a sublist?
>
> Thanks,
> Bob H
>
>
>
>



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