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 > >