RE: Assigning to a sublist
- To: mathgroup at smc.vnet.net
- Subject: [mg35178] RE: [mg35161] Assigning to a sublist
- From: "David Park" <djmp at earthlink.net>
- Date: Fri, 28 Jun 2002 02:31:16 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Bob,
This is a case where zz[[3]][[2]] is not the same as zz[[3,2]].
zz = {{2, {{1, 1}}}, {10, {{3, 1}}}, {5, {{2, 1}}}};
zz[[3,2]] = Append[zz[[3]][[2]], {1, 1, 1, 1, 1}]
{{2, 1}, {1, 1, 1, 1, 1}}
The Head of zz[[3,2]] is zz, a symbol.
The Head of zz[[3]][[2]] is zz[[3]], which gets evaluated to a List
expression and is not a symbol.
You should be able to use the first form, perhaps by Applying Sequence to
the list of subpositions.
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
> From: Bob Harris [mailto:nitlion at mindspring.com]
To: mathgroup at smc.vnet.net
>
>
> 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
>
>