how do i program this to families

[Anonymous]

Distance is A perameter that depends on the peramater (A*2)

C is also a perameter

i want to make it so that distance B is never smaller than 4000 mm

and if it is than the (2*A) is decreased so that B is 4000 mm

if B is more than 4000 than nothing should be done

how do i program this in revit families

[barthbradley]

"C" can't be a parameter.  

Can you give us some real world examples/numerical values to work with.  

[constantin.stroescu]

I'm not quite sure this solve your problem.

I've made a second parameter b=B in order to apply two formulas - one being yours , the other a IF parameter.

[SteveKStafford]

The issue with constraints is leaving Revit an option for flexing the family without the dreaded over constrained warning. You have to decide which values are variables the user can provide and what Revit is free to calculate. In my example I've decided that the user can provide C (overall length) and A which is half of what is left over after you subtract the value for B*2. The IF statement subtracts 2 x A from C then divides by 2; it decides if that result is less than 4000. If it is then it forces 4000 and if it isn't it just uses the value of the original calculation.

Edit: Looks like Constantin beat me to it...

[barthbradley]