As part of some data anaylsis, I am using the function Trace to give me the name of an array of data, which I then use to label my plots.
eg. my data array is called r1 so I run:
ToString@((Trace[r1])[[1]])
which simply returns the string "r1".
All fine and dandy.
The problem arises when I try to do this for a list of data arrays, I write:
Trace[#]&/@{r1,r2}
which I expect to give me the list {"r1","r2"}. Instead I get {{},{}}.
What am I doing wrong?
Any help will be much appreciated!
Since List evaluates its arguments, the construct
Trace /@ { stuff }
will usually give you empty lists because the stuff has been evaluated before Trace sees it (and Mathematica is smart enough not to re-evaluate expressions which haven't changed). There is no more evaluation to be done, so Trace shows nothing.
Personally I would not use Trace for this purpose. You could instead define a function to extract symbol names using SymbolName:
SetAttributes[name, {Listable, HoldFirst}];
name[x_] := SymbolName[Unevaluated @ x]
I've made name Listable so that it automatically threads over lists.
E.g.
r1 = {1, 2, 3};
r2 = {2, 4, 6};
name[r1]
(* "r1" *)
name[{r1, r2}]
(* {"r1", "r2"} *)
{r1, r2}is evaluated first, so the symbols are replaced by their values beforeTracesees them. – Simon Woods Jul 24 '14 at 12:07Trace[#][[1]] & /@ {Unevaluated@r1, Unevaluated@r2}– Öskå Jul 24 '14 at 12:22(I'm not sure the etiquette here for asking follow-up but related questions).
Is there a way to prevent r1, r2 from being evaluated where I still pass the list {r1,r2} (as opposed to) {Unevaluated@r1, Unevaluated@r2}? I assumed something like:
might work, but it still seems to be evaluating the list {r1,r2} first.
– Mashy Jul 24 '14 at 12:48