org.opensourcephysics.numerics

## Interface StateEvent

• All Superinterfaces:
MultiVarFunction

```public interface StateEvent
extends MultiVarFunction```
StateEvent defines an event consisting in that a given numeric value changes sign from positive to negative. This numeric value usually consists in an expression of the variables of an ODE. Every valid model state must (in principle) provide a value f(t)>=+tolerance at every instant of time, where tolerance is a (small positive) number prescribed by the user. However, values in the range (-tolerance,+tolerance) are still accepted. The event really happens when the value at the next instant of time satisfies f(t+dt)<=-tolerance. Then, finding the moment when the event takes place corresponds to finding a root of f(t). More precisely, and because of numerical approximation, a root is considered to be found when -tolerance=+tolerance) or, if this is not true, at least the state at the next instant t+dt will not trigger an event (i.e. f(t+dt)>-tolerance). (If this rule is violated an infinite loop may be caused.)
Author:
Francisco Esquembre (Feb 2004, corrected May 2005)
• ### Method Summary

Methods
Modifier and Type Method and Description
`boolean` `action()`
What to do when the event has taken place.
`double` `evaluate(double[] state)`
Returns the value f(t) for a given state of the model.
`double` `getTolerance()`
Returns the tolerance for the event.
• ### Method Detail

• #### getTolerance

`double getTolerance()`
Returns the tolerance for the event.
Returns:
the tolerance
• #### evaluate

`double evaluate(double[] state)`
Returns the value f(t) for a given state of the model. The values returned by this method will be used by the solver to find the exact time at which the event took place.
Specified by:
`evaluate` in interface `MultiVarFunction`
Parameters:
`state` - The current state of the ODE
Returns:
the value for this state
• #### action

`boolean action()`
What to do when the event has taken place. The return value tells the solver wether it should stop the computation at the exact moment of the event or continue solving the ODE for the rest of the prescribed dt.
Returns:
true if the solver should return at this instant of time, false otherwise.