Assign value to the symbol returned by function

Here are two methods that are essentially the same.

Clear[p, flag, k];
p = 2;
k = 1;
flag = True;
Evaluate[If[flag, p =.; p, k =.; k]] = 0;
p  {* 0 *}

If you want to use a function, maybe this method:

Clear[func, p, flag, k];

func[flag_?BooleanQ, str1_String, str2_String] := If[flag,
  Clear[str1]; ToExpression[str1],
  Clear[str2]; ToExpression[str2]]

p = 2;
k = 1;
flag = True;
Evaluate[func[flag, "p", "k"]] = 0;
p  (* 0 *)

The salient points are (1) the symbol must be cleared or unset and (2) use Evaluate on the left side of the equals sign.

Here is a fairly concise but still silly way to do it.

Clear[p, flag, k];
p = 2;
k = 1;
flag = True;
ToExpression[If[flag, "p", "k"] <> "=0"];
p

0

However, I would rather write

Clear[p, flag, k];
p = 2;
k = 1;
flag = True;
If[flag, p = 0, k = 0];
p

and, I believe, so would everybody else. it is the standard practice and is both more concise and efficient then any playing games to limit the code to one written Set expression.

At first I wasn’t sure how to realize this, but upon seeing m_goldberg’s example with Set inside of If, it hit me:

Clear[p, flag, k];
p = 2;
k = 1;
flag = True;
If[flag, Set[p,#], Set[k,#]]&[0];
p

And this, of course, returns 0 if flag = True and 2 if flag = False.

It might be easier to see it this way:

Clear[p, flag, k];
p = 2;
k = 1;
flag = True;
If[flag, p=#, k=#]&[0];
p

which also works like the above. You could streamline this a bit more & wrap it into a function, but I think this answers your question & gives it to you in a form that is loose enough for you to modify to your desires! Nothing fancy, just anonymous functions.