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Re: copying a variable

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  • Subject: [mg92343] Re: copying a variable
  • From: ramiro <ramiro.barrantes at>
  • Date: Sat, 27 Sep 2008 22:24:17 -0400 (EDT)
  • References: <gbl303$m03$> <> <> <>

Hi Szabolcs,

I just tried with expressions and it looks great.  I think I was just 
treating the DownValues as a kind of hash table (inspired a bit by the 
mathematica paclets)   I think this is an important step in defining how 
I will work with tree (I work with evolutionary trees in biology).  So 
here are two questions as I'm afraid I'm missing mathematica tricks:

1) If I want to find out the ancestor of a node, would it be best to a) 
just traverse the tree every time (maybe the speed won't be an issue), 
b) do some combination of Position, etc.  to figure it out; c) keep a 
separate record of definitions (like the one I had) of who's the 
ancestor and the children of who.

2) What if I want to do an operation on a tree, I was thinking about 
reading the tree and then making "definitions" of children, ancestors, 
etc.  (just because I am going to use those many times), then making the 
operation somehow and outputting the tree again as an expression. e.g.

   definition = "read all the tree data, ancestors, children, root, 
etc." based on treeX
   .... reroot the tree

   make a new expression based on rerooted tree
   output expression
Thanks for helping. Any suggestions or comments much appreciated.


 Horvát wrote:
> Hello Ramiro,
> Have you considered representing trees simply by Mathematica
> expressions instead of making DownValue definitions like this?  All
> Mathematica expressions are already trees.  Creating data structures
> using definitions is not suited to Mathematica, and is generally a
> very bad idea.
> Just represent your data structure with an expression.  Do you see any
> disadvantage in doing this?
> On Sat, Sep 27, 2008 at 14:19, ramiro <ramiro.barrantes at> wrote:
>> Hi,
>> Thanks for answering, let me try to explain again in a bit more detail.  I
>> have a tree object, which contains definitions about ancestors, children,
>> root,  especial values, etc.  Let's call that tree, and I put data in it as
>> definitions:
>> tree["children", nodeX] = children of node X
>> tree["ancestor", nodeY] = ancestor of node Y
>> tree["root"] = root of tree
>> However, I would like to do operations on this tree that change its
>> topology, and return a new tree, for example reroot
>> reroot[treeX,newRoot] = "a new tree just like treeX but with the root in
>> newRoot"
>> Therefore, I would like to have a "copy" of treeX with all its definitions
>> inside this reRoot function, and thus  I would return a new tree very
>> similar to the old one, with a new root, but the definitions of the old tree
>> would remain unchanged..
>> The example I wrote was just meant to illustrate the problem, but I hope the
>> context I just mentioned might help explain the problem better.
>> Thanks,
>> ramiro
>> Szabolcs Horvát wrote:
>>> ramiro wrote:
>>>> Hello,
>>>> This must be very simple.  Consider the following code:
>>>> k[1] = 2;
>>>> sub[x_] := With[{z = x}, z[1] = 4; z]
>>>> r = sub[k];
>>>> r[1]
>>>> k[1]
>>>>  The output is
>>>> 4
>>>> 4
>>>> I would like to have the assignment inside the function (on z) don't
>>>> affect the parameter (x).  I would like to copy x so that whatever
>>>> assignment I do on z doesn't affect it, in other words, I would like to
>>>> return _a copy_ of x. So that the output should be:
>>>> 4
>>>> 2
>>> I don't understand your question fully.  What is the purpose of having two
>>> symbols with the same definition?  Why couldn't you just do r[1] = 4 instead
>>> of r = sub[k]?
>>> Are you trying to transfer the other definitions associated with k to r,
>>> so that if k[2]=Pi was defined then after evaluating r = sub[k], r[2]=Pi
>>> would be automatically defined too?
>>> I suspect that there might be better solutions to your *actual* problem
>>> than copying definitions ...   But if you really want to do this, you could
>>> try something like DownValues[r] = DownValues[k] /. k :> r for copying ...
>>> Be warned that this is very bug prone and is very likely to cause trouble
>>> with non-trivial definitions.

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