inst/examples/VanderpolApp.R

In AlfonsoRReyes/rODE: Ordinary Differential Equation (ODE) Solvers Written in R Using S4 Classes

# ++++++++++++++++++++++++++++++++++++++++++++++++++++      example: VanderPol.R
# Solution of the Van der Pol equation
#


setClass("VanderPol", slots = c(
    mu = "numeric"
),
contains = c("ODE")
)

setMethod("initialize", "VanderPol", function(.Object, ...) {
    .Object@mu <- 1.0                 # gravitation constant times combined mass
    .Object@state <- vector("numeric", 3)  # y1, y2, t
    return(.Object)
})


setMethod("getState", "VanderPol", function(object, ...) {
    # Gets the state variables.
    return(object@state)
})


setMethod("getRate", "VanderPol", function(object, state, ...) {
    # Computes the rate using the given state.
    object@rate[1] <- state[2]
    object@rate[2] <-  object@mu* (1 - state[1]^2) * state[2] - state[1]
    object@rate[3] <- 1

    object@rate
})

# constructor
VanderPol <- function(y1, y2) {
    VanderPol <- new("VanderPol")
    VanderPol@state[1] = y1
    VanderPol@state[2] = y2
    VanderPol@state[3] = 0
    return(VanderPol)
}

# run the application
VanderpolApp <- function(verbose = FALSE) {
    # set the orbit into a predefined state.
    y1 <- 2; y2 <- 0; dt <- 0.1;
    rigid_body <- VanderPol(y1, y2)
    solver <- RK45(rigid_body)
    rowVector <- vector("list")
    i <- 1
    while (rigid_body@state[3] <= 20) {
        rowVector[[i]] <- list(t =  rigid_body@state[3],
                               y1 = rigid_body@state[1],
                               y2 = rigid_body@state[2])
        solver <- step(solver)
        rigid_body <- solver@ode
        i <-  i + 1
    }
    DT <- data.table::rbindlist(rowVector)
    return(DT)

}
# show solution
solution <- VanderpolApp()
plot(solution)