Making a multiple choice button

Question

I've written the following code to create a multiple choice button that takes an arbitrary number of answer options and will assign a variable true/false depending on whether or not the correct choice is selected:

mchoice[question_, answers_, correct_, var_] :=
 DynamicModule[{x},
  Column[{Dynamic[var = (x == correct);], (*problem line *)
    Dynamic@
     ActionMenu[question, 
      MapThread[#1 :> (x = #2) &, {answers, Range@Length@answers}]],
    PaneSelector[
     MapThread[#1 -> #2 &, {Range@Length@answers, answers}], 
     Dynamic[x]]
    }]
  ]

The code is executed as below and gives the button:

Clear[avar]
mchoice["argon", {"solid", "liquid", "gas"}, 3, avar]

The correctness of the selection is stored in avar which I can evaluate at the end of the notebook to "grade" the quiz, if you will. The problem is the presence of Null above the button. With the code as written, I cannot seem to create a button that both updates the value of avar and doesn't print the answer. For example, if I substitute Dynamic[var = (x == correct)] for the problem line, then the student will know if the chosen answer is correct. If I use Dynamic[var = (x == correct)]; then the result is visually correct but avar will not update when the student selects an answer. I can get the desired result by using Dynamic[var = (x == correct);] /. {Null ->""} but that feels like I'm cheating (yes, I'm getting the desired result, but no, I'm not using Dynamic correctly).

I'm interested in either of the following:

1. A better solution to my (* problem line *)
2. Alternative methods for creating a multiple choice button.

Answer 1

You can use DynamicWrapper[] :

 mchoice[question_, answers_, correct_, var_] := DynamicModule[{x},
  DynamicWrapper[
    Column[{
     ActionMenu[question, MapThread[#1 :> (x = #2) &, {answers, Range@Length@answers}]], 
     PaneSelector[MapThread[#1 -> #2 &, {Range@Length@answers, answers}], Dynamic[x]]
     }],
   var = (x == correct)]]

Answer 2

This is not an answer but an extended comment on andre's answer.

I up-voted the answer because it's basically a good one. However, I want to point out two problems with the answer as posted.

1. The following doesn't work because y has a value when mchoice is called.

   y = 42; Dynamic @ y
   mchoice["argon", {"solid", "liquid", "gas"}, 3, y]
   

2. The following doesn't work because x is shadowed by the local x of the DynamicModule.

   Clear@x; Dynamic @ x
   mchoice["argon", {"solid", "liquid", "gas"}, 3, x]
   

I offer the following modification:

SetAttributes[mchoice, HoldFirst]
  mchoice[var_Symbol, question_, answers_, correct_] := 
  DynamicModule[{x$$x}, 
     DynamicWrapper[
       Column[{
         ActionMenu[
           question, 
           MapThread[#1 :> (x$$x = #2) &, {answers, Range @ Length @ answers}]], 
         PaneSelector[
           MapThread[#1 -> #2 &, 
           {Range @ Length @ answers, answers}], 
           Dynamic @ x$$x]}], 
       Unevaluated @ var = (x$$x == correct)]]

This isn't foolproof -- the identifier x$$x can't be passed in as var -- but it's safer than andre's version. Also, I have changed the order of the formal arguments, which is not desirable. But it is the best I can come up with.

Edit

The reason I changed the order of the formal arguments was to be able to use the HoldFirst attribute.

Unique could be used rather than a 'funny' local variable, but I personally find it a bit awkward. It would go like this:

Unique[]

$3

Dynamic @ $3
mchoice[$3, "argon", {"solid", "liquid", "gas"}, 3]

---

The shadowing of global x by the dynamic module's local is peculiar. It appears to happen only because x is dynamic in the module. Consider

a = 42; Dynamic @ a

SetAttributes[f, HoldFirst]
f[u_] := (DynamicModule[{a}, 
  a = RandomInteger[100];
  Unevaluated@u = a];)
f[a]

which works; however,

f[u_] := (DynamicModule[{a}, 
  a = RandomInteger[100];
  Unevaluated @ u = Dynamic @ a];)
f[a]

fails in the same way mchoice["argon", {"solid", "liquid", "gas"}, 3, x] fails. It appears that wrapping the variable in question with Dynamic globalizes it.

Answer 3

If I understand correctly fix to your code would be to put checking procedure into ActionMenu:

 MapThread[#1 :> (x = #2; var = (x == correct);) &, {answers, Range@Length@answers}]