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MathGroup Archive 2002

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Re: Assigning to a sublist

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35168] Re: Assigning to a sublist
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Fri, 28 Jun 2002 02:30:54 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <afe4lv$h2h$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

with

zz = {{2, {{1, 1}}}, {10, {{3, 1}}}, {5, {{2, 1}}}}

use

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

Part[Part[zz,3],2] *must* be evaluated and can't be assigned
But Part[zz,3,2] can be used for assigments.

Regards
 Jens

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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