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dynamic - How to compute values only if input fields are filled?


In the following code how can I keep the DynamicModule from computing values if the input values are not all filled? If I don't specify default values it will show the code used to compute the values instead of blank values.


Thanks.


backingAgreementValue[w_, σ_, 
  a_] := σ /Sqrt[2 π] Exp[ -(w - a)^2/(2 σ^2)] + 
  w CDF[NormalDistribution[0, 1], (w - a)/σ]

DynamicModule[{w = 40, σ = 600, a = 0, n = 100, horseCut = 0.5,
   backingValue, expectation, backerValue, horseValue, 
  horseExpectationPercentage, values}, 

 Deploy[Style[
   Panel[Grid[
     Transpose[{{Style["Mean", Blue], 
        Style["Standard Deviation", Blue], Style["Threshold", Blue], 
        Style["N", Blue], Style["Player Cut", Blue], 
        Style["Backing Agreement Value", Orange], 
        Style["Expectation", Orange], Style["Player Value", Orange], 
        Style["Player Expectation Percentage", Orange], 
        Style["Backer Value", Darker[Magenta]]}, {InputField[
         Dynamic[w], Number], InputField[Dynamic[σ], Number], 

        InputField[Dynamic[a], Number], 
        InputField[Dynamic[n], Number], 
        InputField[Dynamic[horseCut], Number],
        Dynamic[
         Style[backingValue = 
           backingAgreementValue[expectation, σ Sqrt[n], a] // 
            N, If[backingValue >= 0, Darker[Green], Red]]],
        Dynamic[
         Style[expectation = w n // N, 
          If[expectation >= 0, Darker[Green], Red]]],

        Dynamic[
         Style[horseValue = horseCut  backingValue // N, 
          If[horseValue >= 0, Darker[Green], Red]]],
        Dynamic[
         Style[horseExpectationPercentage = 
           horseCut backingValue/expectation // N, 
          If[horseExpectationPercentage >= 0.5, Darker[Green], Red]]],
        Dynamic[
         Style[backerValue = (expectation -  
              backingValue + (1 - horseCut) backingValue) // N, 

          If[backerValue >= 0, Darker[Green], Red]]]}}], 
     Alignment -> Left], ImageMargins -> 10], 
   DefaultOptions -> {InputField -> {ContinuousAction -> True, 
       FieldSize -> {{5, 30}, {1, Infinity}}}}]], 
 SaveDefinitions -> True]

Answer



Wrap a test around the visible code. If the test passes then display the calculation otherwise display a e.g. spacer:


If[NumberQ[σ]&&NumberQ[n]& ...etc,
Dynamic[Style[
  backingValue = 

   backingAgreementValue[expectation, σ Sqrt[n], a] // N, 
  If[backingValue >= 0, Darker[Green], Red]]],
Spacer[0]
]

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