# Calculated properties

Hi

please, can someone out there explain how a calculated property really works ?

A few examples of the various very interesting features will be greatly appreciated!

Panther 2010

Okay, I try to explain what those CalculatedProperties are good for, based on my own experience with Vassal so far.

Roughly spoken a âCalculatedPropertyâ (CP from now on) as similar to a âDynamicPropertyâ (DP form now on) which is set to âSet value directlyâ, but with one important distinctions: It doesnât need a trigger to change. It will change immediately whenever a property of the associated expression changes. The expression of a DP will be calculated only once, for every trigger it receives. A CP will always be recalculated. I think a CP best works togehter with the âGetProperty()â function you can put into expressions. I can try to demonstrate this with an example I am using for my module.

I have a deck of cards. Each card has two different properties (triangle and circle) where each property can have a value between 0 and 5. I now want a label between the deck which counts the number of cards in the deck where the triangle property equals 1. This can be done with the deck option âPerform counting of property expressionsâ. And I then can use a TextLabel with the name of the property as its value. However now I want a button which changes the content of the label. When I press the button, the label shouldnât print the number of cards where the triangle property is 1, but where the circle property is 1. Thatâs where the CP with the âGetProperty()â function comes into play. My CP is specifyed as:

Value = GetProperty(\$DeckName\$ + â_â \$Property\$)

And the Textlabel I mentioned before now has the content: \$Value\$. My button now changes the property \$Property\$ to âCircleâ which immediately leads to a reevalutation of \$Value\$ and the new value can be seen in the label. Of course you could also use a DP instead and set the property \$Value\$ within the DP, but the power of the CP is its flexibility. I combine different CP within a prototype, which each calculate different stuff, depending on the properties given. I need this type of calculation in different places of the module but based on different concrete property values. So I use this CP in a prototype and add marker traits to the pieces where I need the calculation results. This way I can easily use expressions within a gamepieces without worrying abouts where they come from.

I hope I somehow could make clear, what I mean and what the benefits of CP are. I think they are bessed used in prototypes that perform typical calculations you will need in different places of your module. If you want I could make a sample module including the example I talked about. Itâs actually a little more complex than I explained, but I wanted to keep it as short as possible.

By Calculated Property, I am assuming you mean the Dynamic Property trait? A Dynamic Property is like a cell in a spreadsheet with a formula in it. Whenever you use the value of the Dynamic Property, it recalculates its value based on the formula and the values of the other properties it references.

I think the previous comments about âDynamic Propertiesâ where actually about Global Properties which are holders for values that you can set specifically by trigger.

No, I think you mixed up the both terms. Both traits are available for game pieces, âDynamic Propertiesâ, which change on triggers and âCalculated Propertiesâ, which are, as you described, similar to spread sheet cells. âGlobal Propertiesâ behave similar to âDynamic Propertiesâ as they can only change on triggers.

Well, there you go, I am out of touch. Considering I wrote those traits, itâs a bit embarrassing.You are of course correct.

Hi Sebaestschjin

Well, I will try to explain a bit better what Iâ m trying to do.

A) In a DP or a GP you can increment the value by a fixed amount (+1, -2 , etc.). I need to use a CP because I want to increment the value of a given property by the value of another property.

I tried :

Property name : Prop1
Expression : Prop1 + Prop2

but this didnât work.

Then I tried :

Property name : Swap
Expression : Prop1+Prop2

followed by : Prop1 = Swap

and this seems to work, at least for now, and only in a test prototype.

B) I noticed the IF feature of CP and tried to use it, without any success. I tried in several ways, but it looks like I always made a sort of syntax error (the âred crossâ never disappears"âŚ).

What I want do is this :

If Prop1 = true then Prop2 = X else Prop2 = Y.

It looks very simple, but Iâm not able to do this working in no way, at least till now.

Any suggestion and/or working examples would be very appreciated.

Thank you again!

Panther 2010

Hi Panter2010

To A)
DP can also increment by another given property, not only by a fixed value. The only problem is that you have to take care how you expression looks like. If you simply type

``\$Prop\$``

Vassall will handle this as a string, so it will not use the actual property value. You have tu sorround it with curly braces:

``{\$Prop\$}``

This will be done automatically if you use the expression generator by clicking on the calculator icon. But sometimes I quickly wanted to enter an expression and I often forgot the curly braces which wonât work. So maybe this is also the case for you?

Your example of a CP with
Name: Prop1
Expression: Prop1 + Prop2
wonât work, because it will create an infinite loop while evaluating the expression of Prop1. Prop1 will be recalculated whenever the properties of its expression changes. Since Prop1 itself is part of its own expression this is always the case.

So you donât need a CP for this case. In principal a DP and a CP are very similar. The only difference is that a DP can only change on a Trigger, so either a click by the user on a menu, or some other key command from somewhere else. A CP is always recalculated. As Brent Easton already said, its behaviour is very similar to a spread sheet cell with a formular.

To B)
The If-Then-Else syntax is as follows:

``If(<expression>,<then-case>,<else-case>)``

Where the names in <> are actual expressions. So in your case this would look like this:

``````Name:Prop2
Expression: { If(\$Prop1\$ == true, X, Y) }``````

You have to use == instead of = for an equality check and again the curly braces are needed. What I am not sure about is how Vassal actually handles boolean values. It works in this example but to on the safe side I always use string comparison.

I attached a dummy module which uses all the mechanics I described above, if you want to have a look at it.