Generate list of strings from a list of assigned variables

Question

How do I generate a list of strings from a list of assigned variables?

E.g. convert

var1 = 10;
var2 = 11;
var3 = 17;
var4 = 5;

compvar = {var1, var2, var3, var4}; (*all variables assigned*)

into

compvarstr = {"var1", "var2", "var3", "var4"};

Using ToString obviously converts the variables assignments into strings e.g.

compvarstr = ToString[#] & /@ compvar

gives,

 {"10", "11", "17", "5"}

I'm after the unassigned variable names as strings e.g.

 {"var1", "var2", "var3", "var4"};

Apologies if this is a duplicate - I had a bit of a look and nothing seemed to answer it.

Answer 1

You must introduce some form of holding in you definition of compvar as otherwise, assuming it is defined after var1, var2, etc., there is no information to retrieve:

var1 = 10;
var2 = 11;
var3 = 17;
var4 = 5;

compvar = {var1, var2, var3, var4};

Definition[compvar]

compvar = {10, 11, 17, 5}

You could use Hold but then you would need to ReleaseHold (or similar) every time you used compvar. Instead I suggest you use SetDelayed and then recover the definition using my step function from:

• How do I evaluate only one step of an expression?

It returns an expression wrapped in HoldForm:

compvar := {var1, var2, var3, var4};

step[compvar] // InputForm

HoldForm[{var1, var2, var3, var4}]

To convert to a list of strings:

Cases[step[compvar], s_Symbol :> SymbolName @ Unevaluated @ s, {2}]

{"var1", "var2", "var3", "var4"}

Or:

StringSplit[ToString @ step[compvar], ("{" | "," | " " | "}") ..]

{"var1", "var2", "var3", "var4"}

The first method will return Symbols (as strings) only while the second will convert other expressions as well.

---

Incidentally if you do not need to store your Symbols in a List you could use a more direct form:

compHeld = Hold[var1, var2, var3, var4];

List @@ SymbolName /@ Unevaluated /@ compHeld

{"var1", "var2", "var3", "var4"}

Answer 2

Here's a way:

var1 = 10;
var2 = 11;
var3 = 17;
var4 = 5;
compvar := {var1, var2, var3, var4}
compvar; (*all variables assigned*)

ClearAll[f];
SetAttributes[f, {HoldAll}];
f[x_, y__] := Flatten@{f[x], f[y]}
f[x_] := SymbolName@Unevaluated@x

OwnValues[compvar] /. {HoldPattern[y_] :> {x__}} :> f[x]

(*
 {"var1", "var2", "var3", "var4"}
*)

Answer 3

one way is to make a replacement rule seperately and use that.

Clear[var1, var2, var3, var4];
vars = {var1, var2, var3, var4};
values = {var1 -> 10, var2 -> 11, var3 -> 17, var4 -> 5};
compvar = vars /. values

compvarstr = ToString[#] & /@ vars
FullForm[compvarstr]

Otherwise, the way you had it:

 var1 = 10; var2 = 11; var3 = 17; var4 = 5;
 compvar = {var1, var2, var3, var4}; (*all variables assigned*)

Now the var1 name itself is replaced by 10 right away by the evaluator. Hence compvar will always be {10, 11, 17, 5} and the name of the variables is not known inside compvar since their value is used.

Answer 4

Better late the never, right? I created this answer while thinking about one of recent questions that was a duplicate of this one.

I kind of like this way, it is compact and without #&@ :)

ClearAll[VNL];
SetAttributes[VNL, HoldFirst];

VNL[list_] :=   Thread[
                       Hold[list] /. OwnValues[list]
                       ] /.    Hold[s_] :> (SymbolName[Unevaluated[s]])

Let's borrow belisarius' variables :)

var1 = 10;
var2 = 11;
var3 = 17;
var4 = 5;
compvar := {var1, var2, var3, var4}

VNL[ compvar ]

  {"var1", "var2", "var3", "var4"}

Answer 5

Here is another alternative, in which the values of the variables are temporarily cleared using Block and an injector pattern.

ClearAll[getSymbolNames];
SetAttributes[getSymbolNames, HoldAll];
getSymbolNames[list_Symbol] := 
 Hold[list] /. OwnValues[list] /. 
  Hold[{vars__Symbol}] :> Block[{vars}, SymbolName /@ {vars}]

With some planning one might initialized the list of variables before the values to var1 etc. are assigned; otherwise, use SetDelayed as in the other answers.

Clear[var1, var2, var3, var4];
compvar0 = {var1, var2, var3, var4};

var1 = 10;
var2 = 11;
var3 = 17;
var4 = 5;

compvar := {var1, var2, var3, var4};

getSymbolNames[compvar0]
(*  {"var1", "var2", "var3", "var4"}  *)

getSymbolNames[compvar]
(*  {"var1", "var2", "var3", "var4"}  *)

Answer 6

Redefine your data in an associative way

data=<|
"var1"->10,
"var2"->11,
"var3"->17,
"var4"->5
|>

because it looks like you are simply looking for the Keys and the Values of your data

compvar = Values@data
compvarstr = Keys@data

{10, 11, 17, 5}

{"var1", "var2", "var3", "var4"}