Re: copying a variable

*To*: mathgroup at smc.vnet.net*Subject*: [mg92343] Re: copying a variable*From*: ramiro <ramiro.barrantes at uvm.edu>*Date*: Sat, 27 Sep 2008 22:24:17 -0400 (EDT)*References*: <gbl303$m03$1@smc.vnet.net> <48DE179F.5090201@gmail.com> <48DE24D3.3010109@uvm.edu> <f831b3d60809270720j486e8eccxc0098ae2ce9c49c4@mail.gmail.com>

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. reroot[treeX_]:=Module[{definitions}, 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. Ramiro HorvÃ¡t 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 uvm.edu> 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 HorvÃ¡t 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. >>> >>