Hello,

I am experimenting with index reduction and conditionals in OM. I understand that during continuous integration, the same if-branch is evaluated and that a new if-branch may only be selected at zero-crossings. However I wonder how an if-branch is selected at inital time. In particular I simulated the following model:

model mode

  Real x(start=0),y(start=3);

equation

  der(x) = y - 3;

  0 = if x < 0 then y else x;

end mode;

Since x = 0 at initial time, the DAE at initial time should be

  der(x) = y - 3;

  0 = x;

and I would expect OM to do index reduction.

However I found that OM is using the if-equation to solve for y:

Variables (2)

========================================

1:  y:VARIABLE(start = 3.0 ) .mode, .Real type: Real 

2:  x:STATE(1)(start = 0.0 ) .mode, .Real type: Real 



Equations (2, 2)

========================================

1/1 (1): der(x) = -3.0 + y

2/2 (1): 0.0 = if x < 0.0 then y else x



Matching

========================================

2 variables and equations

var 1 is solved in eqn 2

var 2 is solved in eqn 1

and I get the following simulation plot:


So how come OM initially selected the first if-branch without checking the conditional?

I use OpenModelica Compiler 1.9.0 beta4+dev (r15987).

Thanks in advance for your insights,
Antoine