Manipulate Evaluation Order Problem

Question

I seem to be getting some unintended results from a nested Manipulate that I have not been able to resolve. I boiled down the problem I'm having to a simplified version. I can not seem to get the evaluation order of Manipulate correct.

Manipulate[
 Manipulate[
  Print[{Parity, n}];
  {Parity, n},
  {n, Select[Range[1, 10, 1], Parity]}
 ],
 {Parity, {OddQ, EvenQ}}
]

The problem isn't what it shows in the window, it is with what the Print statement outputs. For instance, if it is in the state EvenQ,2 and I click on OddQ, the Print output will read:

{OddQ,2}

Despite the fact that it shows {OddQ,1} in the window.

Why is this, and how can I fix it? In this boiled-down example it doesn't matter, but in my real life one, an error is issued every time I switch the outer manipulate.

Answer 1

I can't help you with why your code doesn't work, but this seems to work. In this solution n is reset to either 1 or 2 when Parity is changed before Print is called.

Manipulate[Manipulate[
  If[Not[Parity[n]], n = Mod[n + 1, 2, 1]];
  Print[{Parity, n}];
  {Parity, n},
  {n, Select[Range[1, 10, 1], Parity]}], {Parity, {OddQ, EvenQ}}]

Answer 2

I don't think you should be nesting Manipulate objects. The fact that you are dealing with Dynamic objects means that things often don't work quite as they do otherwise.

See: Why won't this work? Dynamic in a Select

Here is an example of a scoping irregularity that is easily produced with Block and Manipulate:

Parity = "You shouldn't see this.";

Block[{Parity = EvenQ},
 Manipulate[
  {Parity, n},
  {n, Select[Range@10, Parity]}
 ]
]

I recommend that you use Manipulate only in simple cases, turning to manual construction with DynamicModule, Control, etc. when you need something complicated.