Re: Protecting from evaluation
- To: mathgroup at smc.vnet.net
- Subject: [mg14622] Re: [mg14588] Protecting from evaluation
- From: Carl Woll <carlw at fermi.phys.washington.edu>
- Date: Wed, 4 Nov 1998 13:46:51 -0500
- Sender: owner-wri-mathgroup at wolfram.com
Hi William,
The problem with your first function is that ToString evaluates its
argument before turning it into a string. Thus, if
v=20;
ToString[v]
will return 20. If you really want to use ToString in this way, it is
better to create your own function, as in
ClearAll[HoldToString]
SetAttributes[HoldToString, {HoldAll}] HoldToString[x_] :=
ToString[Unevaluated[x]]
However, what is wrong with the function HoldForm? For example,
HoldForm[v]
will display v, while it's FullForm is still HoldForm[v]. Thus,
{HoldForm[v],v}
will return
{v,20}
For the second part of your question, say you have a list
li = {v,w}
and then you set
v=20;
w=25;
If I understand your question, you want to be able to display the
unevaluated form of the first element of li. One idea is to use
HoldForm again, as in
heldlist=Thread[HoldForm[li]];
which will have the FullForm
List[HoldForm[v],HoldForm[w]]
Then, if you want to print v and the value of v, you could do
{heldlist[[1]], ReleaseHold[heldlist[[1]]]}
or some suitable variant.
Carl Woll
Dept of Physics
U of Washington
On Mon, 2 Nov 1998, William B. Marks wrote:
> To display variable names along with their values I can put them in
> quotes first using
>
> quote[var_] := ToString[var]
> SetAttributes[quote,HoldAllComplete].
>
> Then this works
> quote[Unevaluated @ cVWGL]
> cVWGL
> but this fails
> quote[cVWGL]
> 20
> but since this fails
> Unevaluated /@ {cVThGL, cVWGL}
> {Unevaluated[900], Unevaluated[20.]}
>
> I can't see a way to protect members of a list of variables except by
> putting quotes around them by hand. Any suggestions?
>
>