If I start with a symbol q that I've assigned a value to,

q = 0;

and one, x, that I haven't, I get different answers from ValueQ depending on how I call ValueQ. If I map ValueQ over a list,

Map[ ValueQ, {q,x} ]

Mathematica returns {False, False}. If I apply ValueQ directly, however,

{ValueQ[q], ValueQ[x]}

Mathematica returns a different answer, {True, False}. I've tried

{Names["q"], Names["x"]}

which returns {{q},{}} before evaluating the two ValueQ calls, and {{q},{x}} after the calls. There's obviously some sort of side effect from the call to ValueQ taking place behind the scenes. If I quit the kernel to start with a clean slate each time, however, and reverse the order in which I call the two versions (Map or no Map) I get the same results.

This is troubling behavior. What's going on?

I'm using Mathematica on Linux x86 (64-bit).

  • 1
    $\begingroup$ Please, see Section "Properties & Relations" in Documentation on ValueQ. The effect I suppose is concerned with HoldAll attribute of ValueQ, so that ValueQ/@Unevaluated[{q, x}] gives right answer {True, False}. $\endgroup$ – Alx Dec 23 '16 at 4:57

Map is evaluating its arguments, so you end up with ValueQ[0] (False) instead of ValueQ[q]] (True);

In[3]:= Map[ValueQ, Unevaluated[{q, x}]]

Out[3]= {True, False}
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  • $\begingroup$ That makes sense now. I've seen other functions with HoldAll attribute behave the same way. $\endgroup$ – Rodney Price Dec 24 '16 at 1:35

There're actually 2 issues in your question. One is the evaluation control problem, this has been explained by Brett Champion and Alx. The other issue is:

Why {Names["q"], Names["x"]} returns {{"q"}, {}} before the ValueQ calls?

The answer is: ValueQ isn't relevant at all, the key point is, the symbol x has appeared in the code Map[ValueQ, {q, x}] so it's created.

A simpler way to reproduce the issue is:

(* {} *)
(* {"x"} *)
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