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MathGroup Archive 2001

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Re: Re: Extracting the name of a variable

  • To: mathgroup at smc.vnet.net
  • Subject: [mg26960] Re: [mg26943] Re: Extracting the name of a variable
  • From: "Carl K. Woll" <carlw at u.washington.edu>
  • Date: Tue, 30 Jan 2001 03:38:14 -0500 (EST)
  • References: <94tl4s$lck@smc.vnet.net> <200101280100.UAA24760@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Mike,

Depending on how your list1 was defined, there is a way to extract the
symbol names that make up the list. First, a definition for
extractNamesFromList:

In[27]:=
SetAttributes[extractNamesFromList,HoldAll]
SetAttributes[SymbolName,HoldAll]
extractNamesFromList[a_]:=SymbolName/@Unevaluated[a]

For your example, we had

In[36]:=
q=4;
z=5;
list1:={q,z,w}

Note that the definition of list1 uses a SetDelayed(:=) instead of Set. This
is important, as the use of Set would mean that Mathematica would store the
information {4,5,w} for list1, and not {q,z,w}. Now, the question is, how
does one get at the information that Mathematica knows about list1? The
answer is given by the functions DownValues, UpValues, and OwnValues (there
are others). So, in this case we want to use OwnValues:

In[39]:=
OwnValues[list1]

Out[39]=
{HoldPattern[list1] :> {q, z, w}}

So, OwnValues of list1 contains the information you want. All we need to do
is extract it using the function Extract. Before I do that, note that
extractNamesFromList does what you want when given {q,z,w}.

In[46]:=
extractNamesFromList[{q,z,w}]//FullForm

Out[46]//FullForm=
List["q", "z", "w"]

So, to extract the symbol names from list1, we can do

In[47]:=
Extract[OwnValues[list1],{1,2},extractNamesFromList]//FullForm

Out[47]//FullForm=
List["q", "z", "w"]

Extract takes the part of OwnValues[list1] that contains your list of
variables, and wraps that list with extractNamesFromList before evaluation
takes place. So, it is possible to solve your problem.

Carl Woll
Physics Dept
U of Washington

----- Original Message -----
From: "Mike Yukish" <may106 at psu.edu>
To: mathgroup at smc.vnet.net
Subject: [mg26960] [mg26943] Re: Extracting the name of a variable


> "Ersek, Ted R" wrote:
>
> > Mike Yukish asked how one can take a variable which has a value assigned
and
> > convert it to a string without letting it evaluate.
> >
> > I provide the solution in the section for (ToString) on my web site.
The
> > URL is
> > http://www.verbeia.com/mathematica/tips/Tricks.html
>
> Many thanks to all who replied. I posed my problem as a subpart of a more
> complex problem, which still defeats me. I had hoped to Map[ ] the
function
> extractName over a list, and extract the names of all the variables in the
list.
> Ted has identified its difficulties, which I offer up here.
>
> Problem: Create a function that takes a list of variables, and extracts
the
> symbol name of each. This is an added level over the initial problem.
>
> q=4
> z=5
>
> list1={q,z,w}
>
> extractNamesFromList[list1]
>
> out[]:=
>
> {"q","z","w"}
>
> Ted Ersek and Bob Hanlon have both shown me how to create a function that
will
> work with
>
> extractNamesFromList[{q,z,w}]
>
> out[]:=
> {"q","z","w"}
>
>
> but  extractNamesFromList[list1]
> out[]:=
> "list1"
>
> So passing a predeclared list is a different story, and Ted pointed that
out. I
> think I can live with the restriction, though.
>
>
>
>



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