How to use If statement inside DynamicModule?

Question

I believe I am missing an important piece of information. I am trying to execute a simple If statement inside a DynamicModule. The variable tested in the If statement is a Dynamic variable, in this case, a Number. As it can be seen from the elementary (but wrong) few lines of code the If statement does not work because the tested variable "active" is not a Number (NumberQ=False).

DynamicModule[{active = 0},

 If[active == 0, color = Green, color = Red];

(*--- Output and Dynamic Input ---*)

 Column[
  {

   Panel[Style["Result", color, Bold]],

   Panel[Row[{"Setting@active= ", Setting@active, "  ", 
      "NumberQ[Setting@active]= ", NumberQ[Setting@active]}]],

   Panel[Row[{"Dynamic[active]= ", Dynamic[active], "   ", 
      "NumberQ[Dynamic[active]]= ", NumberQ[Dynamic[active]]}]],

   Button["Set active=True", active = 1;, Method -> "Queued", 
    Background -> Green],

   Button["Set active=False", active = 0;, Method -> "Queued", 
    Background -> Red]

   }]

]

I assume the issue is trivial ... but as I said, I think I am missing a basic piece of knowledge here. Any assistance is most appreciated. Many thanks in advance.

Answer 1

You're actually really close. DynamicModule is just a scoping construct that localizes variables: it does not actually make your output dynamic. To do that you need (surprise, surprise) Dynamic. Just wrapping Dynamic around the whole contents of your DynamicModule will make your code work:

DynamicModule[{active = 0},

 Dynamic[
  If[active === 0, color = Green, color = Red];

(*--- Output and Dynamic Input ---*)

 Column[
  {

   Panel[Style["Result", color, Bold]],

   Panel[Row[{"Setting@active= ", Setting@active, "  ", 
      "NumberQ[Setting@active]= ", NumberQ[Setting@active]}]],

   Panel[Row[{"Dynamic[active]= ", Dynamic[active], "   ", 
      "NumberQ[Dynamic[active]]= ", NumberQ[Dynamic[active]]}]],

   Button["Set active=True", active = 1;, Method -> "Queued", 
    Background -> Green],

   Button["Set active=False", active = 0;, Method -> "Queued", 
    Background -> Red]

   }]
  ]

]

As a final word of wisdom: when you're using If in Mathematica, please use === to test for equality instead of ==. The reason for this is that == stands for mathematical equality and not syntactic equality. Sometimes both will work, but == will not always evaluate to True or False and what that happens, If will not evaluate either (unless you use the 4th argument of If).